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.

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