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.

No comments: