Re: front surface mirrors, distance

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

P3D SHIELDS write:

>I'm thinking of starting out with the Porter adapter, which at $25.95 
>seems the best bargain. I suspect at that price it doesn't have 
>front-surface mirrors.

I have to second George's recommendation that you look into a '50s stereo
camera such as a Kodak or Realist, which can be had in the $80-$100 range
for a "user" in working condition.

I have experience with the cheap mirror attachments, and they have a LOT
of drawbacks.  First is attachment.  The Reel-3D version uses a plastic
threaded ring.  Depending on how the threads mesh with the step-up or
step-down ring (if you need one) and/or the filter threads on your lens,
you may have a very difficult time getting and/or keeping the device
level (horizontal) on the camera.  Given that it's plastic and there's
some flex, it can pop off when the camera gets jostled in a crowd (this
has happened several times to us).  A better-made unit with metal rings
should eliminate that problem.

Second problem is aperture.  Smaller apertures (which are desirable for
maximum depth-of-field in stereo) will cause the black band between
the images to be unacceptably large.  Thus you will have to limit yourself
to no more than f/8 or preferably f/5.6, which will greatly limit your
depth-of-field.

Third problem is convergence.  While looking through the viewfinder
and seeing the double image, you will have to adjust the mirrors to
center your subject in both images.  This is notoriously hard to do,
and there's only friction to keep the mirrors in place.  Add focusing
and shutter speed to the equation and you will find yourself having
to make three or four adjustments for practically every shot.

Fourth problem is compositional.  These devices produce an image pair which
is taller than it is wide.  This will restrict the range of subjects
you can photograph without a lot of extraneous sky or ground etc.

Fifth problem is resolution.  The second-surfaced mirrors noticeably
soften the image.   If you enjoy really sharp pictures, you will not
get them from one of these cheaper units.

For not a lot more money you can get a stereo camera which will not
have any of these built-in problems and will produce sharp, high-quality
image pairs which can be projected, viewed or printed.

	-Greg W.


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