P3D Re: Mark Shields Ortho Q

[michaelk@xxxxxxxxxxx (Michael Kersenbrock)]

> It was just stereobase equals subject distance divided by focal length.

So if one takes a picture of things at infinity,  infinity
is divided by the focal length and we have a stereobase of infinity?
Huh?

I thought the stereobase (when reading McKay, etc) basically is the
lens spacing (or equivalent in the case of beamspltters where it'd
be the distance between the half-frames' reflections).  So far as 
I know it has nothing to do with focal length. Doubling the focal
length and doubling the stereobase (inter lens distance) can
partially compensate for each other, but they aren't equivalent
by any means.

Perspective is determined by where the image is taken, regardless
of lens focal length.  If one enlarges a section of a wide angle
lens image to "fit" a telephoto taken one, they'll produce the same
image (in terms of perspective).  If one shifts where the image
is taken (to provide a perspective-shift) then the amount of shift
for a given subject the constant distance also is constant regardless
of the lens focal length.

The reason one sometimes uses a wider stereobase (hyper) for telephoto
shots is that for a distant subject, the parallax is very little and
it'll look flat -- so it's done hyper (large stereo base) to provide
some 3D effect beyond what it'd look like by eye in real life.  Only
down side is that it tends to make the subject look like "toys" when
viewed, more or less.  It isn't because of the lens being a "long"
lens, but because the subject is far away.

Mike K.


> I chose that because it was simple and straightforward. I'm basically
> of the "shoot happy" school, but I just got the Nimslo and am not
> really comfortable with it yet--besides, I just ordered some macro,
> closeup, and distance lenses for it, so I have to sort it all out!
> I'm also fascinated with the FED and trying to determine what difference
> it might make in my shooting--it has a 63.5mm stereobase and 38mm lens
> as compared to Kodak/Realist 70mm and 35mm.
> 
> Mark
> 
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