P3D Perspective & stereo pairs on the web

[bercov@xxxxxxxxxxx (John Bercovitz)]

I think it would be very helpful for most viewers if stereo pair 
makers who put pairs up on their sites would label the pairs with 
the correct viewing distance.  That way, newbies and otherwise 
sensitive individuals would not be put off by faulty perspective.  
I'm fairly used to hunting for the correct perspective point as 
are others on this list but some people don't even know what a 
perspective point is yet, so let's help them out.

Fortunately, this is not a difficult thing to do.  Generally 
you'll want to make the on-screen images in your pair about 2.5" 
(63.5 mm) wide and view them from a distance of about 17.5" (445 mm).  
These are comfort levels for viewing parallel and close up.  All 
you need to do, then, is write some words under the pair that says 
the pair should be viewed from a distance which is 7 (=17.5/2.5) 
times the width of either image as it appears on your screen.  
This will take care of the variety of sizes at which the images 
may appear on an individual's screen and it also will take care of 
an individual's downloading and resizing your images.

If you want to put up pairs you take with your standard 5P or 7P 
camera, plan ahead for the shot: You need to compose differently.  
A standard camera has a lens focal length which is about 1.5 times 
(=35/23) the height of the image it produces.  On the computer 
screen, we'll be viewing from 7 times the image width.  If the 
images are roughly square, that means we'll be viewing from four 
or five times the distance we _should_ be viewing from unless we 
crop and enlarge.  So...  We crop and enlarge.  How much?  Well, 
that's where composition comes in:  

Compose the entire shot within a 5 mm square (about 1/5th of the  
image measured linearly.)

Resolution will be shot to hell?  Don't worry, by the time the 
image makes it to a computer screen you won't be able to tell if 
it was taken with a Schneiss lens or a pinhole camera.

If you insist on using full frame and a slide bar, then use a lens 
of around 170 mm (=7*24 mm negative size) and blow a 24 mm wide 
rectangle from the negative up to 63.5 mm width for the screen.

John B


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