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:
- 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!