I-A distance and 1/30 rule
 

I've read in a few places including "3D movie making" , that a good rule of thumb for I-A distance is 1/30 the distance to the nearest close object. That turns out to be about 2.5" at 6'. However the same places sometimes amend that to say that for theatrical showings use 1/60 . I'm curious about this as whenever its brought up the 1/30 rule is what's quoted first as if its the "old traditional" advice. Yet until recently theatrical was the only format for 3D production so wouldn't it have been a 1/60 rule?

I ask because in my limited experience I've never gotten my I-A near that, more oftem 25- 35+ mm and sometimes much less, ( expecting some large trade show projection and then on TV monitors),

Leonard Levy

1/30th works for small screen delivery such as on a PC or the web but for screens larger than 14" you are better off going much more conservative, say 1/60th. Better still calculate your maximum disparity as a percentage for the largest screen your production will be shown on (using 65mm as the greatest disparity on the screen) and use that. If you can't do that then 4% is a sensible figure that works for most TV applications and 2% for small projection screens.

That's exactly what I'm doing Alister , but I'm curious how it came to be called a 1/30 rule since until recently there was no small screen 3D at all.

The rule comes from 3D photography. Photographic prints are small, such as 8x10", or even 4x5".

Aha, That makes sense.

I still teach it to absolute beginners because on a "normal" lens it gives you a 3% positive parallax on infinity objects, which is fine for the small screen. This was the general base calculation in 3D photography with cameras like the Kodak Stereo since most photos taken with slide film in the 50's were outdoors. Therefore a depth budget from approximately 2 meters (65mmx30) to infinity was achievable with the 65mm I.A. Those slides were viewable on special viewers so your eyes could look around the scene and converge at will. I actually have a collection of over 1000 Kodak 3D slides spanning 1953-1983 and they are fascinating to look at. They came with the camera when I bought it and I (with some help from Chris Hurd) have already started to find the same locations and take "now" photographs with the same camera.

Hi Tim,
you mentioned:
"Those slides were viewable on special viewers so your eyes could look around the scene and converge at will."
Curious what was the "special"-ness and can it be applied or intergrated into stereoptical video
viewing?
Carlton

I should have said "proprietary" instead of special, even though the 3D slide format was somewhat standardized.

Modern HMDs function in the same manner.