## Sunday, June 26, 2011

### Depth Limit of 3D Scenes

Back in the days when the stereo Realist format ruled the 3D world (it was not that long ago), slide mounting for stereo projection was considered the norm. Even if you never intended to show your slides on a big silver screen, you kinda took the pictures (with your clunky old stereo camera) and mounted the slides with the stereo projection principles in mind.

When mounting stereo slides for stereo projection, you had the choice between three types of mask. The windows/apertures in each type of mask have the same center but the size of the windows are different (thus changing the position of the stereo window).

Here are the three types of mask available:
- DISTANT MOUNTS: Cover subject matters from 7 feet to infinity.
- MEDIUM MOUNTS: From 4 to 20 feet.
- CLOSE-UP MOUNTS: From 2.5 to 6 feet.

The main reason to have those masks is that there is no need for projector adjustment when projecting a series of slides because the centers never change (to be honest, I don't fully understand this but it's not really the point here). The other reason (and that's the one we're interested in) is that, when projected, the distances between corresponding points (on the left and right images) are always from 0 to 2.5 inches apart, which guarantees comfortable viewing for the audience. Those masks basically limit the depth, the distance between the foreground and the background in your pictures. In general, when a stereo pair is projected onto a screen, you don't want to have points separated by more than 2.5 inches (the distance between our eyes) because, otherwise, it forces our eyes to diverge (this is referred to as divergence and you really don't want it). If you consider the distant mounts, the points at 7 feet are positioned at the stereo window (the silver screen) and the points at infinity are 2.5 inches apart, which is perfect for comfortable viewing. Same idea for the medium and close-up mounts. Remember that convergence is usually never a problem for our eyes but divergence always is.

These principles are in my opinion still valid today with 3D digital cameras like the Fuji Finepix 3D W3 camera. For example, I don't think it's a very good idea to take a picture with very near objects and distant ones as the convergence range is likely to be too large.

Of course, rules are always meant to be broken.