## Monday, February 25, 2008

### Long FL stereo photography - PePax Principle

To neutralize the compression resulting from the use of longer lenses we can increase the stereo base. This leads us to the “PePax Principle”, advocated by H. C. McKay in the '50s. According to this principle, if you increase the focal length of the recording lens you should increase the stereo base proportionally to get a picture that resembles ortho stereo.

This technique is used for wild animal photography and assumes two things: 1) The near object is far away from the camera, and this is the reason for using long FL lenses. 2) The background is limited (there is no infinity and the depth range of the scene is restricted). These two conditions ensure that the stereoscopic deviation remains under control. (As we showed in the previous posting, under constant magnification and narrow depth range, p ~ B/F, so doubling B and F results in the same stereoscopic deviation.)

Two examples are shown here: The bear was photographed in the Cleveland zoo using 135mm lenses. At first, I took the picture with the camera side-to-side (6 inches). I was not happy with the depth compression so I tried increasing the stereo base to follow the PePax principle (10 inches). The results looked better. When I entered this picture in Detroit, someone asked me how could I come so close to the bear with my Realist, an indication that the picture looks like it was photographed closer with shorter FL lenses.

The three bowling pins have been photographed and are shown so that the front pin has the same size (constant magnification). The depth range is also limited. The middle picture is taken at F, B, I. For the top picture the camera is moved twice away from the subject (2I) and the focal length is doubled (2F) but the stereo base remains the same. This picture (2F, B, 2I) shows depth compression, compared to the (F, B, I). In the bottom picture, we have doubled the stereo base (2F, 2B, 2I). This bottom picture compares well with the original one.

So, What's the Catch?

By increasing F and B proportionally we make further objects, photographed with a longer lens, look as if they were photographed closer with a normal lens. This sounds too good to be true. As a matter of fact, it is too good to be true! To understand how the resulting pair differs from the “ortho pair”, compare the top and bottom figures of the bowling pins. Even though the depth is increased and the compression seen at the top is partially eliminated, the bottom picture is still different from the middle (which was recorded from closer). The difference is in the perspective, or the relative sizes of objects at different distances from the camera which has not changed. As a result, distant objects appear larger than nearby objects or nearby objects appear smaller than distant objects. This is direct result of trying to fool the brain into thinking that we are closer to our subject when we are really further away.

### Effect of Focal Length on Stereoscopic Deviation

What happens to the stereoscopic deviation as we increase the focal length? We have mentioned that the focal length acts as a magnification factor and magnifies the stereoscopic deviation, per the formula: p = FB (1/Imin – 1/Imax) (1)

So, according to (1), if we double the focal length, the stereoscopic deviation will be doubled. This assumes that the distances of the near and far objects do not change as we change the focal length. Now, what are the chances of this happening? Pretty slim, I think. Unless if the near and far objects are in the line of sight near the center, as we zoom into the scene we will be moving past near objects, thus keeping the deviation under control. I first noticed this while watching zooming during digital stereo projector. As our projectionist was zooming into the scene, the range of depth changed and the deviation and sense of depth seemed well-balanced and under good control.

Constant Magnification

So far we have examined the effect of the focal length with a fixed distance from the subject. What happens if we change both the focal length and the distance to the subject so we can have the subject fill the frame? “Fill the frame” implies constant magnification. In this case we can either use a short FL lens and come close to the subject or use a long FL lens and stay far back. There are many situations (wild life photography, portraits, etc) where we use long FL lenses in order to stay further from the subject and still fill the frame. In the case of constant magnification, instead of using (1) we should go back to the original formula: p = M B (2)

This formula shows that if the magnification remains the same, the stereoscopic deviation is independent of the focal length. This formula assumes that there is infinity in the picture. But in many real situations not only there will not be any infinity, but the scene will have a rather narrow depth range.

Narrow Depth Range

From (1) we have: dp = F B dI / I**2 (dI is the depth range). Substituting M = F/I, we get:

dp = M**2 B (depth) / F (3)

This formula shows that for constant magnification and a narrow depth range, the stereoscopic deviation varies inversely to the focal length. So, if we want to maintain the same amount of deviation, while we are increasing the focal length, we need to increase the stereo base. This is the basis of the “PePax” principle.

### Effect of Focal Length

In previous postings we examined the effect of the stereo base. Now we will see what happens if we change the focal length of the recording lens or the viewing lens (or viewing distance).

These effects are summarized in the Table here. Note that what matters for these effects is not the absolute value of the focal lengths of the individual lenses, but the relationship between the two.

If no lenses are used (as in the case of mirrors or projection) we can substitute the focal length of the viewing lens with the viewing distance. If the original film chips are magnified (as with prints) then the focal length of the recording lens must be multiplied by the magnification. A better way to treat this subject is to talk about angles. The condition for ortho stereo is that the scene is viewed from the same angle as it is recorded.

Why does this mismatch of recording/viewing affect the perception of depth?

One way to explain it is this: A longer focal length lens (which essentially magnifies the image) makes it look as if the camera was closer to the subject than it really was. When we view this picture we mentally compare it with the one recorded from closer. In this case, to be compatible with the reduced perspective (2d) and deviation (3d), the depth must be reduced. Hence the perceived depth compression.

Some people think that the focal length affects the perspective (relative size of near vs. far objects). This is not correct. Only the distance affects perspective. Let’s say that we record a scene with a wide angle lens, and then, without changing position, switch to a long FL lens and take another picture. If we then enlarge the picture from the wide angle lens to match the size of the objects in the picture from the long FL lens, the two pictures will be identical! The focal length acts only as a magnification factor and this is true both in 2d and 3d. What creates the compression/stretch “illusion” is the mismatch of the viewing distances. If both the wide angle and telephoto lens pictures are viewed from the same distance, then they will result in a different impression.

A Simple Experiment

Here is an experiment anyone can do right now: While viewing a stereo image (for example, freeviewing some of the images in this newsletter) move the image away and see how this affects the perceived depth. You should see the depth increase (“stretch”). By bringing the image closer, the depth should decrease (“squash”). The effect is rather subtle but most people notice it. Another experiment is to move back and forth during stereo projection. There appears to be more depth in the projected image when viewed from further away. Note that these effects are not perceived proportionally.

Seen Also in 2D

This effect is seen in both 2d and 3d images but it is more noticeable in stereo. In 2d it is the change in perspective with distance that creates this impression. Most of us are familiar with races in which, when filmed straight-on with long lenses, it appears that all the runners are in the same line and we are surprised to see how far apart they are when the camera changes angle of view. The reason the runners appear in the same line (depth compressed) is that their sizes are the same (zero perspective). That's the result of filming the race from far away and not the result of using long lenses, but the long lenses help get a larger image. To be more exact we should say that this compression is the result of viewing the image from much closer than it was recorded. We should always have in mind that it is not the recording lens or distance that creates the effect but the mismatch between the recording and viewing distances.

### What is the BEST Stereo Base?

Given the freedom to achieve any stereo base (as it is the case when using a slide bar and a single camera) what is the correct stereo base to use for a given scene?

This has been the subject of some debate. My answer to this is that there is only one “correct” stereo base and this is Bv (B=Bv, equal to the spacing of the eyes). Anything else will result in an impression that alters reality, in which case there is no right or wrong.

If we decide however that we want to alter reality then there are formulas and rules of thumb which guide us into producing stereo pairs with a decent amount of depth (not too little or too much). There are two schools of thought: One advocates having a constant/maximum on-film deviation. It uses the basic stereoscopic formula, plugs the distances Inear, Imax, also F, and maximum-on-film-deviation, usually 1.2mm (for 35mm film) and calculates B.

I find this approach very artificial. The stereo base will change any time the distances of near/far objects change. Imagine that the spacing of our eyes changes as we move around, thus changing the distances of near/far objects. It is crazy!

The other school of thought advocates a constant convergence angle (expressed as ratio: B/I). One example is the well-known rule of thumb the “1/30 rule” which says that the stereo base should be equal to 1/30 the distance of the nearest object (B/I = 1/30). I prefer this approach for my stereo photography, but I understand that the convergnece angle can change, depending on the subject. For example, close-ups and macro photography generally requires a larger convergence (1/20 to 1/10). The reason I like the convergence approach is that it is easier to measure (divide the stereo base by the distance of the near object, or, multibly the ratio with the distance, to get the stereo base) and easier to visualize.

Is "More" Always "Better"?

Related to the this topic is the frequently asked question: “Wider stereo base means more depth. Stereo photography is about depth. So a wider base (and therefore more depth) is always better, right?”

I hope it is clear that the answer is “not necessarily”. More is not always better. Sometimes less can be better. The effect of putting more depth into the scene will result in the scene appearing smaller in size. This can lead to unusual and impressive images, like a “toy model” impression of a building or Grand Canyon. But many times making an object appear larger in size is equally, or more impressive. And many times just reproducing a scene in near-ortho (as seen by the eyes) is best. It all depends on the subject, application, and personal taste.

By all means experiment with different stereo bases but it would be a mistake to assume that more is always better!

### Ortho Hyper Hypo Stereo

By considering the stereo base with respect to the interpupillary distance, we can talk about three special types of stereo images: ortho stereos, hyperstereos, and hypostereos, defined in the Table here.

## Sunday, February 24, 2008

### Space Control

We saw in the previous posting that changing the stereo base changes the distance of the subject from the observer. The amount of movementis proportional to the change of the stereo base. For example, doubling the stereo base will pull the scene twice as close. Reducing the stereo base by half will push the screen twice back.

This leads to the concept of space control. By controlling the distance of an object to the camera and the stereo base, it is possible to alter and control the apparent dimensions and distance of this object.

Those who have mastered space control have produced fascinating illusions by multiple exposures, like the ones shown by Tommy Thomas in the book “The Stereo Realist Manual”. Examples include the miniature girl inside the wine glass or the artist who is painting a picture of a live model (see left).

Here is how this illusion is produced: Start by taking a picture of the artist with the canvas covered by the dark cloth. Then proceed to double-expose the model in the area inside the dark cloth. To achieve this, the model must be photographed from a distance (to reduce the size) with increased stereo base (to be pulled closer). This requires very good planning and lots of patience and luck!

Digital photography can certainly make these kinds of effects easier to produce in the “digital darkroom”, but what do I know? :)

### Stereo Base

In a previous posting we looked at the three variables that affect a stereo image, the stereo base (distance between lenses, B), the focal length of the lenses (F) and the distance from the subject. A conventional stereo camera has fixed lenses so the only way to affect the image is by changing the distance to the subject. Going beyond the stereo camera usually means changing B or F. From these two variables, the stereo base is perhaps the easiest variable to experiment with, since any camera (including a stereo camera) can be used in two successive exposures to record stereo pairs with any desired stereo base.

The effect of changing the stereo base are summarized in this statement: Increasing the stereo base increases the deviations, pulls the scene closer to the observer and makes the objects within the scene appear smaller. Reducing the stereo base decreases the deviations, pushes the scene away from the observer and makes the objects appear larger.

The effect on the deviations is pretty clear from the fundamental stereoscopic deviation: p = FB/I, but how increasing B makes the scene come closer or appear smaller? The figures on the left might will help clarify this.

Why Appear Closer?

Consider the series of pictures of the 3 bowling pins, taken with different stereo bases. These pictures are shown here exactly as recorded (with a slide bar) with no attempt to adjust the stereo window. It is clear that the image is shifting towards the inner edge of the frame with increasing base. From our discussion of the stereo window we know that when the film chips are pushed closer, the scene appears to be moving towards the observer. Stereoscopic viewing of the bowling pin pictures confirms that this is indeed the case. Increasing the stereo base does make the scene appear closer to the observer.

Why Appear Smaller?

This is a direct consequence of the previous effect. The actual size of the objects does not change but when the stereo base increases they appear to be closer, so our brain concludes that they must be smaller.

This is a little more subtle to observe and is more pronounced in stereo projection than freeviewing on this page. But let's try it: Consider the series of letters in the stereogram here. All letters have the same size. The relative shift of the letters (artificial deviation) makes the ones in the center (“Smaller”) appear closer to the observer. Hence they look smaller. The ones in the back (“Larger”) appear the furthest from the observer hence they look larger. It might take a little while for this to be clearly seen. Stereoscopic viewing is absolutely essential to experience this. If instead of parallel, you use cross-freeviewing then the opposite effect will be perceived, i.e. the “Larger” will be forward and smaller and the “Smaller” will be pushed the back and appear larger.

### Stereo Photography Viewing Variables

The three "recording" variables F, B, and I, affect the way the stereo image is recorded on film but they also affect the way the stereo image is perceived, i.e. how it appears during stereoscopic observation.

To understand the stereoscopic impression when we view a stereo image, we also need to know the focal length of the viewing lens, Fv, and the interpupillary distance (eye spacing) of the observer, Bv. Fv and Bv are now our viewing variables.

Finally, even if we know the recording variables and the viewing variables, what we actually perceive also depends on our brain & experience, what we call "peception". So, we can say that:

3d image perceived = (recording variables) + (viewing variables) + (Perception)

There are two conditions that, when satisfied, viewing the stereo image most closely imitates viewing directly the original scene: 1) Stereo base is equal to the interpupillary spacing (B=Bv, approximately 65mm or 2.5") and 2) focal length of the recording lens is equal to the focal length of the viewing lens (or viewing distance), F=Fv. This is known as “ortho stereo”.

Ortho Stereo: B = Bv & F = Fv

General-use stereo cameras are well-suited for this type of stereo photography which explains the choice of lens separation in Realist-format cameras. The focal length of the recording lens is not important as long as it is matched by the viewing lens. Most 35mm film viewer lenses have a FL of 40-50mm. The 35mm FL lens in many stereo cameras is a compromise, offering good depth of field, decent field of view, and near-ortho viewing conditions.

Any deviation from these conditions will result in a visual impression that deviates from reality. We will explore some of these situations in subsequent postings.

### Stereo Photography Recording Variables

This blog is based on my Tutorial “Beyond the Stereo Camera”. You can purchase the entire collection of my stereo Tutorials by going to: http://www.stereotutorials.com/

There are three variables which affect the way images are recorded on film:

1) Focal length (F) of recording lens.
2) Stereo base (B) of stereo system.
3) Distance (I) of the camera to the subject.

These three variables affect three “metric” (measurable) aspects of the recorded image:

1) On-film size of an object (or magnification).
2) Relative sizes of objects at different distances from the camera (this is also known as linear or geometric perspective).
3) Stereoscopic deviation.

These effects are summarized in the Table reproduced here. Note the formulas that express the relationship between the recording variables and the metric aspects of the recorded image:

• Magnification: M = s’/s = f(I-f) ~ f/I, or on film size s’ = s f / I, only depends or object size, focal length and distance.
• Perspective: ds/S = dI/I, only depends on subject distance. (ds is a change in image size due to a change in image distance dI)
• Stereoscopic Deviation: p = FB/I, depends on F, B and I

• The focal length acts as a magnification factor. It magnifies the size of the recorded image without altering the perspective. It also increases the stereoscopic deviations.
• The stereo base is the only variable unique to stereo photography and it only affects the stereoscopic deviations, which is the only metric aspect unique to stereo.
• The distance of the camera to the subject, essentially the only variable available in a standard stereo camera, affects all three aspects of the recorded image. The effects are proportional to the inverse distance (1/I) which we can call “closeness to the subject”. By coming closer to the subject you 1) increase the on-film size of the subject, 2) intensify the perspective (make closer objects appear larger than further objects) and 3) increase the stereoscopic deviations. That's a good argument for getting closer!

### Basic Stereoscopic Equation

Consider an object A at a distance I from the lenses of the stereo camera, which are separated by B (stereo base). An object at infinity is formed at O1 on the left side and at O2 at the right side, while the image of A is A1 and A2. The situation is symmetric so half the stereoscopic deviation (or parallax) is P/2. From similar triangles we have:

B/2 / I = P/2 / I' or P/B = I'/I (1)

From our previous posting we know that the ratio I'/I is the magnification M. So we get:

P = M B (2)

This is the basic stereoscopic equation. I cannot help but make the following analogy: Variables in the image space (with prime ') are related to variables in the object space through the magnification. For example, I' = M I, s' = M s, and here we have P = M B, so we can think of P as B', in other words, the stereoscopic deviation is the “image space” equivalent of the object space stereo base.

If the subject is far away from the lens we can use the low magnification approximation and write (2) as follows:

P = FB / I (3)

Equation (3) gives the parallax with respect to infinity (remember, we measured P from point O which is a point at infinity). If we have a near object at Imin and a far object at Imax, then the stereoscopic deviation equation can be written more generally as:

P = F B (I/min – Imax) (4)

The stereoscopic deviation is proportional to the focal length, the stereo base, and inversely proportional to the distance.

### Basic Lens Equation

I find myself using the basic lens equation quite a bit so I would like to derive some important formulas. Consider a lens of focal length f. The object is at distance I from the lens, while the image is formed at distance I'. The size of the object is s, the size of the image is s'. See the diagram here.

The basic lens equation is: 1/f = 1/I + 1/I' (1)

The magnification by definition is M = S'/S = I'/ I (2)

If we use equation (2) to solve for either I or I' and substitute it in equation (1), we obtain these two useful formulas:

M = f/x (3) and M = x'/f (4)

From (3) and (4) we can write (1) as: f**2 = x x'

If the subject is far away from the lens (low magnification) then I >> f and I = x, I' = f, so the magnification is approximately equal to M = f/I. This is the low magnification approximation.

At high magnifications I gets close to f, and I' gets very large, so I' = x' and M = I'/f. this is the high magnification approximation.

An interesting situation occurs at M = 1, then x = x' = f, and the subject is at distance 2f from the lens and the image is formed at distance 2f from the lens. In this case the total distance from the object to the film plane is the smallest possible (4f).

## Monday, February 11, 2008

### Close ups with the HORSEMAN 3D Camera

The Hosreman 3D camera with its shorter stereo base (Spacing of lenses, B = 34mm) is well suited for close ups. As a matter of fact, I have only used it for close ups at the near focus (0.7m) and also with +1 and +2 lenses (at near focus again). For most indoor pictures I use a flash in auto mode (the camera does not have TTL flash) at f8. As I said in an earlier post, the results are just great! I am very pleased.

Here are some comments/specifications of interest to me for close ups (for complete technical specs, see the info above – from the instruction manual, click to enlarge – and also the official Horseman 3D web page):

• The Horseman is based on the Hasselblad Xpan II and it has been modified by installing the lens plate, containing two 38mm f/2.8 lenses. So F = 38mm. Minimum aperture is f16.

• The spacing of the lenses (stereo base) is B = 34mm.

• The filter threading is 62mm and covers both lenses. This makes it very convenient to use filters. You only need one 62mm filter and this will cover both lenses.

• What is unusual about this camera is that it has only one (metal-blade focal plane) shutter, long enough (65mm) to cover both lenses. As you can see from the picture here, there is a bar that separates the two images. The bar has a notch in the bottom of the right side. This notch will show up in the top of the left image and helps differentiate the left from the right image.

• I measured the size of the film gate openings as carefully as possible and concluded that each film gate opening is 29.5 x 24mm. The size of the bar at the center is 5mm. This makes the total width of the shutter area 29.5x2+5 = 64mm.

• Measurements on the film show that each image is 30.5x34.5 mm. The separation between a stereo pair is 3.8mm and the separation from pair to pair is 1.7mm. Based on these measurements, the total distance from the left side of the left image to the right side of the right image is 30.5x2+3.8 = 64.8, while separation from stereo pair to stereo pair is 64.8+1.7 = 66.5. I measured the sprocket holes above the image and each image is about 7 sprocket holes wide, while from pair to pair we have exactly 14 sprocket holes. Given that spacing of sprocket holes is 4.75mm, 14x4.75 = 66.5mm. It appears that the camera advances 14 sprocket holes between each shot, using a sensor located in the sprocket hole path.

• The camera gives 20 stereo pairs in a 36 exposure film. Another unusual feature is that when you load the film, the camera advances the entire film to the right side, and then it releases it back to the canister as you take picture after picture. One advantage of this system is that if you accidentally open the back, you will not ruin the pictures taken so far (because they are already inside the canister).

There are two questions in my mind, based on the comments above:

1. How is it possible that the film opening is 29.5mm wide, while the image is 30.5mm wide? I suspect that this is happening because the film is sitting a bit further than the film openings. The light comes from the lenses as a cone and the cone is being cut a bit higher than the film openings, thus a larger area is being cut.

2. Does the camera have a built-in stereo window? From the measurements it appears that the spacing of the film gates is 34.5mm. Considering that the spacing of the lenses is 34mm, this will create a stereo window with 0.5mm parallax, which, for this camera, it places it at 2.5m (8 ft).

The table here summarizes the important metrics of the Horseman 3D camera, when used for close ups, focused at the near distance, with or without close up filters. Because I am not sure about the built in window, I am using x as the “built in window parallax”. This could be 0.5mm as my measurements indicate, or it can be zero if there is no built-in window.

To mount my Horseman 3D slides I use either 7p (28mm) mounts or 5p (21mm) mounts. I use the narrower 5p mounts for composition purposes because most portraits are framed better in square or vertical mounts. If you must use 7p mounts then you can tolerate 31.5-28 = 3.5mm of image loss. As you can see from this table, you can mount +1 shots in 7p mounts even without a built-in window, but for +2 shots you are forced to use 5p mounts.

As I said earlier, I use the camera mostly for close ups. It nicely bridges the gap between a standard stereo camera and a macro stereo camera. Here is how I carry the camera with me: I stack the two close ups filters (+1 closer to the lenses, then +2) with a metal cover over the lens. This way I always have with me the two close up filters. If I want to use the +1 filter, I remove the cap and +2 filters together. I also carry a measuring tape, marked with the three near distances and I use this to quickly frame my subject. Even though the camera has a rangefinder window, this does not work when using close-up filters, and it is also faster to use a frame even for the near shots without filters. Finally, I carry a flash with f8 auto mode.

Pictures (mainly portraits of people or shot of my kitty) work well with the camera focused at the near distance at f8 with a flash. Even though the convergence is 1/18, stronger than 1/30, the pictures look very natural. With the close up filters some care is needed to select the subject and it is better to use f16.

Update (2/26/08): The camera certainly has a built-in stereo window and it appears to be around 0.5mm as I suspected when I wrote the original post. This helps a bit by reducing the image loss in close ups, but it is not of much practical value for me. But it is good to know that this is something that the designing engineers took into account.

## Sunday, February 10, 2008

### The Macrolist

The Macrolist is a specialized macro stereo camera, a replica of the Macro Realist Stereo camera. It was designed and built by David Burder in England. David is a world-renowned stereo personality who has been involved in many stereo projects, including unique modifications of the Nimslo stereo camera. I estimate that 30-50 Macrolists were built. This makes this camera a rare stereo collectible, in addition to being a very practical camera to use for those interested in macro stereo photography.

Technical Specifications:

• The Macrolist is based on an original Stereo Realist body with a custom lens and shutter assembly.
• Image size is standard 5p ("Stereo Realist format") or 23x24mm. Unlike the Realist-format, the images are side by side (not interlocked). You still get about 29 pairs from a 36 exposure roll of standard 35mm film. The film advance is the original film advance systems of the Stereo Realist camera.
• The lenses are air spaced triplets, effective focal length 35mm, coated optical glass elements. Effective aperture is f/40 and it is fixed. This camera gives a greater depth of field than the original Macro Realist, which is considered an advantage.
• Lens spacing (stereo base) is 16 mm. Field size is approximately 50-65mm and the magnification about 1:2.
• The shutter is a Copal shutter with a full range of shutter speeds. Recommended exposure for the macro pictures is via electronic flash. Focusing is simply done by placing your subject between the two prongs of the camera.
• Compared to other macro stereo systems, the Macrolist is rather “strong”, as you can see from the convergence of 9 degrees so some care is needed to keep stereo deviation in check.

### Close-ups with a Stereo Realist – Closer than 2.5 ft

The Realist can focus as close as 2.5ft. If you want to focus closer then you have to either fool around with the focusing wheel or attach close-up (supplementary) lenses.

The limit of 2.5 feet is imposed by the rotation of the focusing wheel, which is limited by a small screw. This screw can be removed (see picture) allowing the Realist to focus even closer. It has been reported that the “second round” of the focusing wheel corresponds to focusing distances from 20 inches (at INF) to 14 inches (at 2.5ft) but testing should be done to get the exact focusing distance. This testing can be done by using a ground glass and a magnifier and observing a test image.

If you decide to experiment with this method make sure you don’t lose track of which round the wheel is at. Also, because the focus plane is pressuring the film, you should turn focusing to infinity before advancing the film.

We discussed close-up lenses in previous postings. With a little imagination you can attach close up filters (or any filters) to your Stereo Realist camera. For example, I have used a film cap with a 20mm hole, with the filter attached to it, over the Realist lenses (see picture). Some people have used the Realist Film ID unit for macro pictures. This unit is essentially a strong close-up lens with the advantage that it attaches to the Realist lenses and offers a holder right at the near sharp focus distance. You must find some way to shift or rotate the subject for stereo relief and you need to move the unit from one lens to the other without disturbing the setup. Sounds like a challenge.

Here is a Table that summarizes key variables when using the +1 and +2 close up lenses with the Realist focused at 2.5 ft.. In-between magnifications can be achieved by focusing the camera further. For example if you use the +1 lens and focus the camera at 10ft, then the actual focus point is at 30 inches (2.5ft) which is the same as the near focus without lenses.

The drawback of these extreme close-up settings is realized when we look at the image loss. Using the +2 lens at 2.5ft focus we lose 8mm of image. This leaves us with 23.5mm – 8mm = 15.5mm of useable image width. There is no slide mount that can mask this (4p/Half Frame/Nimslo mounts have 16mm openings)

One way around this problem is to use prismatic close-up lenses. These have the shape of a wedge and must be aligned over the lenses. In addition to changing the focus, they also shift the image to reduce/eliminate image loss. The drawback is that they introduce aberrations and distortions.

I am familiar with the “Stereo Angle-Lens”, a prismatic close-up lens manufactured by Photo-Liz Inc in Long Beach NY. As seen from the copy of the instructions reproduced here, these filters come in 3 different strengths. Of interest is the “neutral” filter which is a simple prism. This is used to push the window back in close ups without filters or also to push the window forward when taking hyperstereos!

I have tried these filters for an extreme close-up portrait of my daughter. The results were interesting, but the resulting image has too much stretch and distortion for my taste, even when viewed in the viewer.

The bottom line is that for extreme close ups with the Stereo Realist, you need to reduce the stereo base. Some people have used the Realist and a slide bar for close ups. This procedure will reduce the stereo base in a stereo pair, without wasting any film: Cover the right lens, and take the first picture. Do not advance the film. Shift the camera slightly to the left (to reduce the spacing of the lenses). Cover the left lens and take the 2nd picture. I have seen a portrait taken with this method! (The model must stay still for along time). This method might have novelty value, but if you are going to use a slide bar, it is much more convenient to use an SLR camera, not the Stereo Realist.

### Close-ups with a Stereo Realist – 2.5 to 7 ft

In the range of 2.5 to 7 ft, all you have to do is point the camera to your subject, focus and shoot. Sounds simple, however, if you want to do this successfully, you have to take certain precautions: First you have to limit your background. Second, you have to crop the final images for the correct stereo window.

I see this in our stereo club. Some people take pictures with objects from 4 ft to infinity. This is too much for me. Such pictures cannot be viewed comfortably in projection. It is also impossible to mount them properly using standard stereo mounts. (Note: Some people attempt to mount such pictures using a technique called “double depth”. This technique eliminates “window violation” while keeping this excessive amount of depth. Personally, I prefer to keep the background a stereoscopic deviation within limits.)

How do you block the background? By altering the composition. For example, put your subject closer to a wall or other natural barrier, use a flash (background is dark), shoot a picture of a dog or cat looking down, etc.)

We can construct a Table which shows the maxium object distance for a certain near distance. I have done this in the previous posting for 4 focus distances. For example, if the near object is at 4 ft (1.2m), the far object should be no further than 10 ft (3m).

As it turns out, you don’t need this Table because it is built into the Stereo Realist’s Depth of Field (DOF) scale. If you line up the near object distance with one of the f8 marks, the other f8 mark will show you the far object distance. For example, in the picture here one f8 mark is at 3ft (near distance). The other is at 5 ft (far distance). (Note: If your Stereo Realist does not have a DOF scale - earlier models did not come with one - I have these for sale in my web page)

I credit Charles Piper for pointing this out. As it turns out, both the stereoscopic deviation and the depth of field have an inverse relationship to the distance. So we can use the DOF information to derive stereoscopic deviation information. There is nothing mystical about this. It is a coincidence that allows us to kill two birds with one stone.

The f8 pointers correspond to 1.2mm, which is generally accepted at the maximum allowable on-film stereoscopic deviation. Charles Piper followed a more conservative approach and used the f5.6 pointers. These correspond to 0.8mm deviation and it is safer for stereo projection (in general, “less is better” when it comes to stereo projection).

When it comes to stereo slide mounting, the image loss will often dictate what image size to use. The openings of the Realist film gate are 23.5mm wide. The 3 relevant RBT slide mount sizes are 21.5mm (5p), 19.5mm (5p close up), and 16mm (4p/Nimslo). So these sizes can accommodate an image loss of 2mm, 4mm and 7.5mm. Considering that film cutter is not always perfect, we should give a margin of 0.5mm, so the three slide mounts will cover an image loss of 1.5, 3.5, and 7mm. These correspond to a near object distance of 940mm (3ft), 556mm (1.8ft, 22in), 333mm (13in).

### Close-ups with a Stereo Realist - Theory

The Stereo Realist is designed to take pictures with the near object at 7 feet (2.1m). For “normal” stereo photography, the following advice applies: “Do not let the near objects come closer than 7ft to the camera”. So, before taking a picture, make sure that there is nothing closer than 7ft to the camera.

You can of course take pictures of objects closer than 7ft, and the camera will focus as close as 2.5ft. This is the area of close-up Realist photography. One advantage of the Stereo Realist for Close-ups is that the viewfinder is centrally located and parallax-free. So the framing is always accurate and you don’t need a focusing frame or other aids. You will appreciate this if you try close ups with other stereo cameras. One disadvantage is that distance of the lenses (Stereo Base, 70mm) is too wide for close-ups.

There are two precautions not only for Realist but all close-up & macro photography: As you get closer and closer to your subject, you have to block distant objects. If you don’t, then your stereo pair will have too much deviation and it will be impossible to project and possibly difficult to view in the viewer.

Also, you have to be prepared to make certain adjustments (cropping) in mounting to set the proper stereo window. For the slide film user, this means that you might have to use close-up or half-frame mounts, instead of standard (5p) realist-format mounts.

Without any aid, the Stereo Realist camera will focus as close as 2.5 ft (0.76m), which is surprisingly close (the near focus of most 35mm lenses is 0.9m, or 1m). This near focus can be extended in two ways: The first way is by using a close-up (supplementary) lens. This is a universal way, and can be used with any camera/lens. A +1 lens will bring the near focus to 0.43m (1.4 ft). The second way is Realist-specific. You can remove the little screw that stops the focus wheel at 2.5ft and focus even closer. We will discuss these practical issues in the next posting.

Here are the basic metrics of the Stereo Realist:
• Focal Length, F = 35mm
• Spacing of lenses (Stereo Base), B = 70mm
• Near focusing distance, Inear = 2.5ft (0.76m)
• Internal cropping = 1.2mm

• This last item needs an explanation: The distance between the film gates is 71.2mm, which is 1.2 mm longer than the spacing of the lenses. This shift is done on purpose to block out a small part of the image (left edge of left image and right edge of right image, remember the images are reversed inside the camera) and set the stereo window at 7 ft.

The Table below gives basic measurements for the Stereo Realist close up photography. I have elected to list the following near object distances: 1) 7 ft, that’s where close up photography starts, 2) 4 ft 3) 2.5 ft (near unaided focus), 4) 1.4 ft (focusing with the Realist at 2.5 ft and +1 close up filter).

I have tabulated the following quantities of interest (all values are in mm):
• Inear = near object and where the camera is focused.
• Ifar = the recommended maximum distance of the far object, in order to keep the total stereoscopic deviation to 1.2mm.
• Back Plane Extension, BPE: The Realist focus by moving the back plane away from the lenses. This value shows you how much the plane is moved back. We have called this quantity x' in our formula derivations here. Mostly a curiosity item.
• Magnification M: This is expressed as a ratio, for example 1/21 at 2.5ft. This can show you the field of view. Considering that the height of the film gate is 25mm or about 1 inch, a magnification of 1/25 means that you can get 25 inches of your subject matter inside the picture. For comparison, a portrait usually requires 12 to 16 inches so the Realist is not really capable for a tight portrait even at the closest focusing distance.
• Convergence: This is the ratio of the stereo base over the near distance, B/Inear. This ratio is 1/30 at 7ft. Values from 1/10 to 1/30 are generally acceptable.
• Image Loss: This is the amount of film that needs to be cropped in order to fix the stereo window. It takes into account the internal cropping of 1.2mm so at 7ft there is no image loss. This quantity is of practical mounting value for slide film users.

### Limit to Stereo with Translation – Converge or not Converge?

As the magnification increases, translation cannot be used effectively any more. Consider for example M = 5 (5 times magnification).

I = 1.2F, I' = 6F, Cropping = 5B

How much should I translate? My rule of thumb B = I/20. Assuming F = 100mm, B = 6mm. But cropping = 30mm. If we use 35mm film, all we have is 36mm to record the image. If we crop 30mm, we have very little image to work with. We need to work this the other way around. Let’s say we need to maintain half the film width. Then cropping = 18mm, B = 1.8mm. We can only translate by 3.6mm, not 6mm we originally had planned. That might not be enough and result to a flat image.

In practical terms, we are forced to converge the lenses or use tilt instead of translation for magnifications of 10x or higher. I mentioned earlier that you should avoid convergence because it leads to keystone distortion. As it turns out, at high magnifications the working distance becomes equal to F and the extension equal to MF. Because of the large extension, perspective is minimized, which means that we can use tilt with no problem. (I will try to explain this “perspective is minimized” in a subsequent post.)

The same is true of very long focal length lenses are used. Perspective is minimized and convergence is not only acceptable, but also the only practical way to record the image.

### Image Loss In Macro 3d Photography

This is an important issue that can cause problems for a beginner. Every time you take a stereo picture by shifting a camera/lens parallel, the “stereo window” is placed at infinity. This creates two wide bands on the left side of the left image and the right side of the right image, which not only do not contribute anything but they also do not help to properly locate the image in space. To restore the proper location of the stereo window, some cropping is required.

Consider the picture shown here. This is a close up of a 10 inch doll taken by shifting a digital camera. Can you see these wide bands? A rule for proper placement of the stereo window (3L rule) says that “The Left eye should see Less on the Left side”. In this case clearly there is more to see on the left side of the left image, not less. We need to crop out these bands. By doing this, the entire picture will now properly be behind the stereo window, as you see in the corrected pair.

How do you get rid of these bands? It is easy to do it in digital photography or when making prints (cut off part the left side of the left print and the right side of the right print). But what do you do if shoot slide film? In this case you use the stereo mount to crop the edges, by shifting the film chips away from each other until the bands are hidden behind the mount.

The amount of cropping needed depends on the stereo base (B) and the magnification (M). It is given by basic stereoscopic formula: Cropping = P = M B. See this blog for derivation.

Consider some special cases: As infinity (M=0) no cropping is needed. At low magnifications (regular stereo camera distances) M = F/I, and the formula becomes Cropping = FB/I, where I is the distance of the subject. This is about 1.2mm for most stereo cameras.

At M = 1 we have the interesting situation where a shift of B in the object space creates the same shift B in the image plane, so cropping = B. In a previous slide bar example we showed that for M=1, a good value for B is 5mm. The final image will require cropping by 5mm. So our 36mm length of 35mm film image area is now reduced to a useable 31mm length. This is important for slide film stereo photographers because it means that you cannot use 33mm or 31.5mm (all available by RBT) to mount this stereo pair and the next available size is 30mm. If you want to preserve more of the stereo image, you might want to consider less stereo base, just to reduce image loss. You might need to work this the other way around. Let's say that you want to mount your image in a 31.5mm mount. You cannot crop more than 4.5mm. To have some room in mounting, you put a limit of 4mm cropping. This corresponds to 4mm shift. So, instead of 5mm, you shift by 4mm only, the choice being dicated by cropping considerations only.

Most stereo cameras from the '50s are constructed with a built-in stereo window. This is achieved by shifting the film gates with respect to the lenses. For example, the Realist lenses are separated by 70mm while the film gates are separated by 71.2mm. This shift creates a window at 7ft from the camera, which saves film and makes automatic mounting easier (by just centering the chips in the mount we get a window at 7ft.)

Consider the schematic here (click at it to enlarge it). In the camera on the left, the film gates are centered under the lenses. The stereo window is at infinity and you will notice that at any distance from the camera the left eye sees more on the left side instead of less, which means that this point is in front of the stereo window. The correct stereo window can be set by trimming parts of the final image. The camera on the right creates a stereo window by separating the film gates wider than the lenses. The left eye now sees less in the left side for objects past the stereo window. In this case, if the film chips are centered, the window is set automatically and less film trimming is required for close-ups.

## Saturday, February 9, 2008

### Stereo Close-Up with a Slide Bar

This is a great way to get started in macro stereo photography. For best results and convenience, I recommend using an SLR camera (with TTL flash capability if you plan to use a flash), a macro lens, and a slide bar. Here is why:

1) An SLR camera allows you to see exactly what you are photographing. 2) Flash is often used in macro pictures and in close-ups it is difficult to determine flash settings. TTL capability means that the light output from the flash is measured through the lens, and this is an accurate and convenient way to use flash. 3) A macro lens might be a bit more expensive than ordinary lenses but it is built for optimum optical and mechanical performance in close ups. My personal preference is a 50mm macro lens. 4) A slide bar (on a tripod) keeps the camera well aligned between the two shots.

The basic idea is to set up the shot, take the first picture, shift the camera, and take the second picture. As with all single-camera stereo photography, the subject must remain stationary between the two exposures. This limits the use of this technique, but there are still plenty of areas (tabletops, etc) to use it. All you need is some imagination. As an example of stereo photography with a slide bar and plenty of humor and creativity, consider Stan White’s View-Master set “Beyond the Third Dimension”.

The most common question/concern during slide bar stereo is: “How much should I shift the camera?” This shift is what we usually call “stereo base” (B).

One school of thought uses the formulas given in a previous post and aim to achieve a constant on-film-deviation (this is P, or Parallax or stereoscopic deviation). I personally find this solution very artificial because for a subject with lots of depth it calls for a small stereo base B, while for a flat subject it calls for a large and some times excessive stereo base. Why would B change with the depth range of the subject? There is no physical justification for this.

Instead, I recommend looking at the “convergence angle”, that is, the ratio B/I of stereo base (B) to object distance (I). For macro stereo photography this ratio is usually between 1/10 and 1/30. I recommend 1/20 as a good starting point. If you are beginner, you can bracket stereo base, just like photographers bracket exposure, but you will soon find that the choice of stereo base is not critical. You will get a good, natural-looking 3d close-ups for a wide range of stereo bases.

So, here is what you do: 1) Measure the distance of the subject to the lens. 2) Divide this distance by 20 and use this figure as your stereo base B. 3) Slide the camera parallel by B. I do not recommend converging the lens to the subject. This creates “keystone distortion”.

Note: At magnifications greater than 0.5x where you usually know the magnification (can be read from the lens’ barrel for example) you can use this formula: I = F (1 + 1/M) to calculate I. For example, when using a 50mm lens at M=1, I = 100mm, B = 100/20 = 5mm.

### Macro Photography with Close-Up Lenses

When extension is not an option (because the lens is fixed to the camera body, for example) an alternative way to focus closer is to use close-up (or supplementary) lenses.

Close up lenses are marked in diopters. A diopter is 1/F, where F is the focal length of the close-up lens expressed in meters. So a +1 lens has a focal length of 1 meter (40 inches), a +2 lens has a focal length of 0.5 meters (20 inches) etc.

Assume that the lens of focal length F is focused on an object at distance s. The image is formed at a distance d behind the lens. From the fundamental lens equation we get: 1/F = 1/s + 1/d. (1)

Now, let’s insert a close-up lens with focal length f, in front of this lens. Without touching the focus of our lens (so d does not change), now we have to place our object closer to the lens, at a distance s’, in order to have it in focus. The combined strength of the two lenses now is F + f, and the fundamental lens formula now becomes: 1/F + 1/f = 1/d + 1/s; (2)

Subtracting (1) from (2) we get: 1/f = 1/s' – 1/s, or 1/s' = 1/s + 1/f (3)

This formula can be used to calculate the sharp focus distance (s’) when using a supplementary lens (f), while our original lens is focused at distance (s). What is interesting is that the focal length of the original lens is not involved. So, we can construct tables that universally apply for all lenses. Another interesting observation is that if our original lens is focused at infinity (1/s = 0) then the sharp focus distance is equal to the focal length of the supplementary lens.

Based on (3) we can construct this useful table for lens whose close focus is 1m (about 3 ft, common close focus distance in many lenses):

Close Up Lens / Infinity Focus / Close Focus
--------------------------------------------
None infinity 1m
+1 1m 0.5m
+2 0.5m 0.33m
+4 0.25m 0.20m

Two more comments: 1) Close-up lenses can be stacked. In this case, their powers are added. If we stack a +1 and +2 lens, we get a combined strength of +3. 2) It is recommended that the aperture of the lens is kept closed (f8 or smaller) when using supplementary lenses, both to get a decent depth of field and also to keep the aberrations down.

When using close-up lenses with rangefinder cameras, a focal frame is useful for framing the subject. An example is shown in the picture here. This is taken from Rodolf Kingslake’s book “Optics in Photography”. I highly recommend this book for anyone interested in this topic.

### Macro Photography with Extension

When an object is at infinity, the image of this object is formed at a distance equal to the focal length (f) behind the lens. As the camera is focused close and closer, the image is formed further and further behind, at a distance x' from f.

Most camera lenses are designed to operate satisfactorily with an object lying between infinity and about 10 focal lengths. The maximum lens focusing distance is then about 1/10 of the focal length, representing a maximum magnification of 0.1. One exception are macro lenses who are designed to work at higher magnifications, usually 0.5 or 1.0.

To make a lens focus closer than its minimum focusing distance, one can use extension tubes or bellows. This is of course possible only if the lens can be removed from the camera. The basic equations in work here are the fundamental lens formula and the definition of magnification. I will write these as follows (see also this blog):

1/f = 1/I + 1/I'
and:
M = s'/s = I'/I = f/x = x'/f

I am using this notation: f = focal length, x' = lens extension (distance of lens from film plane), I = Subject distance (distance of subject from lens), also known as working distance because it shows you how much room to you have to work with your subject, M is the magnification.

An interesting situation exists at M=1. In this case, I = I' = 2F, i.e. the extension is equal to the working distance and it is equal to two times the focal length. For example, if you are using a 50mm macro lens at M=1, then the subject is at 100mm in front of the lens and the image is formed 100mm behind the lens. As it turns out, the total distance from object to film plane (4F or 200mm) is the smallest possible. As you increase the magnification beyond 1, the image moves quickly behind the lens and a bellows system will be needed. For best optical performance, it is recommended that the lens is reversed when M > 1

Another note of practical importance: At magnifications close to 1 or higher, focusing is not done by changing the distance of the lens to the film plane, because this changes the magnification. Focusing is done but my moving the lens and camera closer to the object. So, in macro stereo photography, first you set the magnification by fixing the extension I', and then you focus by changing the working distance I. For stereo photography with a slide bar, you might need a stereo bar with two axes. The vertical axis is used for stereo translation. The horizontal axis is used for focusing.

### Macro Stereo Photography

I have been interested in close-up/macro stereo photography, since I got my first real camera, a Minolta X-700 in 1988. Instead of the standard 50mm lens, I elected to buy the 50mm Macro lens for a lot more money (the lens cost as much as the camera body, if not more). That was a lot of money and I had to think long and hard before making the decision. But we only live once, so I decided to go for it. This is one of the best decisions I have ever made. I used this lens extensively in my research and for personal 3d photography. At some point I had acquired all Minolta macro photography equipment including Bellows, and microscope lenses. Even though I eventually sold all the Minolta equipment, I kept the macro lens (see picture) and at least one X-700 body.

Two methods are used go get a lens to focus closer than its minimum focusing distance: 1. Extension, 2. Close up lenses. We will look at these methods in subsequent postings.

Regarding the math behind 3d macro photography, you only need to use two formulas:

1. The fundamental lens formula: 1/f = 1/I + 1/I', where f = focal length of the lens, I = distance of object from film plane, I' = distance of image from film plane. See also this blog.

2. The basic stereoscopic formula: P = FB/I = FB (1/Imin – 1/Imax), where P is the stereoscopic deviation, F is the focal length of the recording lens, B is the stereo base, I is the distance of the subject from the camera (Imin is the minimum distance, Imax is the maximum distance, Imax – Imin is the range of depth in the scene.) See also this blog.

## Thursday, February 7, 2008

### Cords for single/twin camera Release

Yesterday I spent a good part of the day creating remote cords to fire the Contax/Yashica camera (2d or 3d). The focus was on a cord for twin cameras. I have been through this with two different camera systems in the past, once with my Minolta X-700 cameras and once with my Pentax ZX-M cameras.

I took some time to think about it. With the wireless infrared remote (WIR), I had a cord with a Contax/Yashica plug in one end, and a simple 2.5mm Mono plug in the other. So I decided to base my connections in 2.5mm mono plugs and jack inlines.

The picture above shows the following (from left to right):

1. Original Contax cord from WIR (plus is at right angles)
2. Cord I built using the C/Y plug and a 2.5mm MP.
3. Similar cord but with a Minolta plug for my Horseman.
4. Same but with a plug for my RBT S1
5. At bottom: Connector cord with two inline jacks on one end and a 2.5MP on the other.

I also created 3 switches from the left over material, to fire the cameras: One from the Minolta release, one from the Contax release, and one from a Hama release.

Cords 1 and 2 can be used with the connector cord to fire twin cameras (see picture of the connection). They can also be used to fire single cameras, when attached to one of 3 switches or in the WIR.

Cords 3 and 4 can be used to fire the Horeseman 3D and the RBT S1 cameras, again either alone or when attached to the WIR.

### Horseman / Hasselblad XPan II Cable Release

Here is something that not too many people know: You can use a Minolta remote release cord to fire the Hasselblad XPan II camera and also the Horseman 3D camera (which uses the same body as the HXP II)

As expected, the HXPII cable is rather expensive (\$100 or so) but you can find Minolta cords for a bout \$20. So one advantage of using the Minolta cord is the price. Another advantage of this connection is that you can use the Minolta Infrared wireless device to fire the Horseman 3D or Hasselblad XPan II cameras. That's neat!

### Link of Lenses in Ochotta Cameras

I am familiar with the lens linking done by RBT and also by the service center used by 3D Concepts. The links in the lenses of the Yashica stereo cameras, built by Fritz Ochotta, is different. It is simpler and in my opinion very effective. As you can see from the pictures above, each lens is fitted with a metal ring. The ring is secured with screws at the top and bottom. At the top there is piece with a round end. A metal link is connecting the two balls.

This system is really simple and it works well. It is easy to adjust. You can easily remove the link bars and put them back in place. You can even remove the entire system and restore the lens to its original state (note: the rubber focusing rings have been removed from the lenses).

Depending on the focusing range of the lenses, it is quite possible that the link imposes a minimum focusing. In the 85mm lenses I can remove the bar, turn the lenses to close focusing, and then re-attach the bar, this time at the bottom.

As it turns out, these contax lenses are well matched so the aperture linking links the same apertures in each lens. Are the links needed? For a wide angle lens (say, 28mm or wider), one can get along without links. Just remember to put the same aperture in each lens and focus near infinity. For longer lenses, focusing is more critical so I think it is important to link the focusing. The aperture linking is a matter of convenience and it protects you from accidentally changing the aperture in one lens only, and ending up with different exposures.

I would like to be able to link other pairs of lenses so I am looking for help in getting the materials required to create more links.

### Tribute to the Horseman 3D Camera

The Horseman 3D camera has and is giving me amazing stereo images. And I am not the only one who thinks so… Every picture from this camera that I have entered in our club competitions has won an award. Honestly, I think this is the best stereo investment I have ever made (After the RBT S1 stereo camera of course :)).

Here is what I wrote in photo-3d in November 2006 when I got the camera, before I even mounted my first roll:

“The workmanship in this camera beats anything that RBT has ever produced, hands down. This is a REAL camera. It feels solid, sophisticated, attractive, yet easy to use. Such quality comes of course at (an astronomical) price, but we only live once. Knowing myself, my photography, and my preferences, I am sure I will love the results. I have always been attracted to the grand effect of hypostereos, and close ups. This is a specialty camera, but it is the kind of specialty that I enjoy. I think it will acquire a permanent location in my camera bag.”

I stand behind these comments 100% a year later. But I want to emphasize on thing: Because I love this camera, this does not mean that you will love it too. I have only used it at the nearest focusing point (0.7m without close ups lenses, I have also used +1 and +2 CU lenses, see another posting on using Close up lenses with this camera) This camera is for close ups mainly, not for general photography. If you like portraits and close ups of animals or similar subjects, this is a great camera.

Here is what I wrote in photo-3d last January:

“I just came back from our Stereo Club meeting. The competition topic was "Portraits". A portrait of our new kitty took first place with a perfect score 27 (3 x 9). A simple shot... snapshot with on-camera flash, but it really jumps out (my reaction when I first saw this picture in the viewer was "wow!") Plus, this slide won the "popular vote" (people's choice) [This slide eventually won the “Slide of the Year”]

Official Horseman 3D page from the manufacturer: http://www.horsemanusa.com/3D/index.html
Comprehensive review in Shutterbug: http://shutterbug.com/equipmentreviews/35mm_cameras/0507horseman/
Manual (in English, I edited it to keep English only) that I have placed in the internet: http://info3d.home.att.net/Horseman3D.pdf
If you are thinking of buying one, I recommend 3d Concepts (that where I got mine) for expert and knowledgeable service: http://www.stereoscopy.com/3d-concepts/index.html

Enjoy stereo!

### February 2008 - OSPS Club meeting

Tuesday February 5th we had our usual club meeting. It was an interesting meeting as usual. A couple of guests, including Marty, a beginner in stereo, just shot his first roll with a Stereo Realist, and another gentleman, who is doing a very interesting work, extracting stereo pairs from still video frames of a sunken submarine (I will have to write more about this later).

John Waldsmith talked about our May 3-4 Stereoscopic Weekend. Our President, John B., talked about the Hannah Montana 3D movie and give it mixed reviews.

Three people showed slides during the Open Projector: I went first with slides that I took at the Buckeye Trail, a couple of days after my 50K race (I talked about it in the meeting), close-ups with my Horseman camera, and the first slides from my Ochotta stereo attachment, plus a slide bar stereo pair (White House). Our kitty was my model in some of the Horseman camera, plus the 2 macro stereos that I showed. My impression from the viewer is that the Ochotta stereos look flat, but they project well. I was surprised that a member said that the Ochotta stereos looked more natural than the Horseman stereos, which he thought had too much stretch. I don’t completely agree. Close ups at the near focusing of the Horseman look fine. Maybe things look stretched when I use +1 and +2 close up filters.

Gary B. showed beautiful slides from Sicily. The sharpness and depth was fantastic and the mounting was perfect! Chuck W. followed, with more slides from Italy.

After the break, we moved on with the competition. The subject was “Motion”. My entries:

1. I had taken several slide bar stereos of some family athletic trophies. I entered one of two runners (my trophies from the Kent 10K races where I have placed in the top 5, from a field of 25 runners!) I labeled this one “Spirit of the Marathon”, from the movie I saw recently).
2. I remember I had a picture of Tony and the kids jumping in the water, from our 2005 vacation in Minnesota. It took me a while to find it. I called it “1, 2, 3, Jump!”.
3. For my third entry I had something else in mind, but when I could not find #2, I projected one of my fireworks from last year (with twin Pentax cameras, 4 ft apart, and 20mm lenses). I figured that they qualify as “Fire in Motion”.

In my opinion, I would rate the slides as follows: 1) 1, 2, 3, Jump!, 2) Spirit of the Marathon, 3) Fire in Motion. The judges have different opinions. they gave the “Fire in Motion” first place! The “Spirit of the Marathon” got 3rd place. The “jump” did not place! The reason I did not think the Fireworks would do well is that they had very little depth. But the composition was nice and the slide had good impact.

Chuck W. asked why I was not getting any ghosting with the Fireworks slide. The answer is that the slide has little depth and it is mounted close to the window. To get ghosting you must have a decent separation on screen, which means that the high contrast subject is far away or mounted sufficiently behind the screen. Mine was close to the screen.

There were several good slides and, overall, another enjoyable meeting!