Re: Paired shots from different focal lengths

[P3D John Bercovitz <bercov@xxxxxxxxxx>]

> On the surface this is true BUT we also jiggled the images 
> to the same image size and DEPTH OF FIELD IS EQUAL AT THE SAME 
> F/STOP AT THE SAME MAGNIFICATION REGARDLESS OF FOCAL LENGTH.

I think I see the source of my confusion with this statement.  I 
think the implication of the statement is that enlarging a photo 
changes its depth of field.  In the case of stereo photos, where 
you always try to view from the center of perspective, you can
see that enlarging a photo 2X and then viewing it from 2X as far
away, results in no net change in anything.  (Neglecting enlarger
resolution losses.)

In short, it is true that you will get the same depth of field
from the same vantage point regardless of lens focal length as
long as the _linear_ aperture is the same.  It is also true that
you will have the same depth of field with any lens focal length
as long as the magnification is the same (you move the camera
to make sure this is true) and as long as the _relative_ aperture 
(f/number) used is the same.  These statements are for viewing
from the center of perspective (or the same (proportional to
the correct) incorrect distance).  There are certain caveats on 
these statements in macro work.

John B


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