How Beamsplitters / Beamspreaders Work

"Beamsplitters"

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

Advantages:

• Depending on the unit, can be considerably lower cost than a dedicated 3D camera. Some units actually cost more than a dedicated 3D camera, however.
• You can use your existing SLR with all it's features (i.e autofocus, autoexposure, faster shutter speeds than '50s 3D cameras, etc.)
• Stereo alignment is automatic - no need to cut and mount slides or worry about the stereo window.

Drawbacks:

• The dark band between the two images may fool auto-exposure systems.
• Beamsplitters introduce keystone distortion because the outer mirrors are not exactly at a 45 degree angle. This is not a problem when the beamsplitter itself is used as a viewer, but can be if, for example, a print is made and viewed by free-viewing. Many users do not find this objectionable, but you might.
• It is important to keep the beamsplitter level with the camera. Some models stay in place better than others.
• If the front element of your SLR lens rotates as focus is adjusted, a beamsplitter may be a lot of trouble to use.
• The "tall thin" format of most beamsplitters can be compositionally challenging.

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

Beam spreaders