[tech-3d] Why consider ratio of Viwer FL to Camera FL?

["Michael K. Davis" <zilch0@xxxxxxxxxxxx>]

Hi Boris,

>Date: Mon, 20 Nov 2000 17:33:48 -0500
>From: boris@xxxxxxxxxxxx
>Subject: Re: Stereo Base Calculation With a $20.00 Handheld

[snip]

>I have a question.  Why does a viewing device focal length (i.e. a viewing
>geometry) even come into play?  On film deviation is the product simply of
>camera optics, stereobase, and near and far point.  Why the viewer F.L.?
>Granted, the viewer optics affect the perceived depth, but it certainly
>does not change OFD.
>
>(And for my application, in shooting for anaglyphs, there is no viewer F.L.
>There may be a print size, and a viewer's naked eye distance to print -
>that would form the equivalent to a viewer's focal length.  But to shoot
>for a specific size anaglyph print, and therein a specific deviation on
>paper, I want to predict and set up a shot simply for a particular
>deviation on film.  Viewer optics are not otherwise considered.)
>
>Boris

If you already know an on-film deviation that gives you the look you like,
using whatever display mechanism you employ, then you need not be concerned
about compensating a mismatch between viewer focal length and camera focal
length.  

I include this ratio of vFL to cFL as a factor in calculating base because
just as sitting in the front row of a 3D theater causes squash not
experienced in the middle row, my camera lenses, being of shorter focal
length than my viewer lenses cause that same squash.  Increasing the base
proportionately compensates this.  

I have a closeup 2D picture of my dog's face shot with an 18mm lens (on a
35mm body).  His nose is nearly the width of his forehead.  Viewed at a
distance of ten feet, he looks comical in this picture.  But if you view
the print at a distance equal to the camera focal length times the
enlargement factor, (about 8 inches for this 11x14 print), his proportions
appear to be normal.  

Mike


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