How Beamsplitters / Beamspreaders Work


A beamsplitter involves a system of mirrors and/or prisms which fit in front of the normal lens of an SLR, and "split" the film into a left and right version. A model using mirrors operates like this:

Currently manufactured beamsplitters includes the Franka beamsplitter, and the Pentax beamsplitter.

In most cases, the beamsplitter itself is used with an attachment as a slide viewer. The images may also be free-viewed.

Beamsplitters generally introduce vignetting around the image frames, and are sensitive to the f/stop used. The more wide-open the lens (numerically smaller f/stop), the less this is a problem. But those lens settings work against good 3D, in which you generally want to have as much depth-of-field as possible. In particular, the center band between the two photos gets larger as the camera lens is stopped down. As a rule of thumb, generally f/stops should be kept at f/8 - f/5.6 or more; going down to f/11 will generally give unacceptable results.

Pros and Cons of beamsplitters

Beamsplitters have strengths and weaknesses. Some of these are:



Here are some example photos (contains 29K of JPEG files), showing the effects of f/stop, and beamsplitter keystoning.

Beam spreaders

A beamsplitter (such as is used in the Loreo camera) is a similar concept, but replaces the single lens in a beamsplitter with two lenses. This has the effect of "spreading" out the stereo base. For example, the Loreo camera uses two small lenses which each take a half-frame image (i.e. each one exposes half of a 24x36mm film frame, for two 18x24 side-by-side images). Without a beamspreader, this would result in a stereo image with a stereo base (lens separation) of 18mm. The beam spreader mirror arrangement simply widens out the stereo base to a more normal value (i.e. 65mm).

Rocky Mountain
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