Sorry for the long post, I'm sure I'm not covering new territory for most,
but it's been a learning experience for me and I'd appreciate your
comments/corrections.

I said

>but the view in the antique viewer seems much better. And I don't know why,

any ideas?

George was right when he said:

>>You should make an effort to answer this question for yourself.

So I made the effort, here's what I found and what I think it means.

I found unexpected things so I measured a couple of times.
All measurements are rough, still. I didn't use the focus knob on the Zeiss
viewer, it was racked all the way in. Disclaimer (and foreshadowing) - I
think the plastic viewer is a fine viewer and a bargain (but).

Plastic Viewer:
~ 30mm diameter lenses (what I can see of them).
~ 70mm focal length. John Thurston's web site says 75mm, but it looks to be
70mm to me. Maybe the viewers vary? Small "sweet spot". I noticed I have to
look through the centers to get everything looking right.

Zeiss Ikon viewer:
~ 40mm diameter lenses (what I can see of them).
~ 75mm focal length.
Noticeably larger sweet spot.

Sputnik:
75mm focal length (assuming I understand the markings on the lense barrel).
I could only measure from the outside, because I have a fresh load of film
in it. I took the pictures I used for the comparison with this camera.

Comparison:
In summary, the plastic viewer is fine until I compare it to the Zeiss; then
the plastic viewer looks bad. I think I'm looking at the difference between
an "ortho" system and one that's not. I think this is because the focal
length of the Spud and the Zeiss match and the fl between the Spud and the
plastic viewer don't.

I don't notice any obvious colour fringing or distortions (I contradict
myself later) for either viewer so long as I look through the "sweet spot".
I can't say with certainty but it seems the Zeiss lenses are at least as
good as the ones in the plastic viewer.

My guess is the image is "wrong" in the plastic viewer because of the
magnification. I'm also thinking the apparent magnification of the view
supports my finding the focal length shorter on the plastic viewer.

It seems the "depth cues" (I'm borrowing and may not understand that term)
are missing when viewing with the plastic viewer. It looks like "parts of
the space between things" are missing. It's as if some of the air was pumped
out of the scene and things are crowding each other. Wish I could express it
better. I think the best words would be "natural" and "not".

The apparent “wrongness” may be due to “squash.”

Viewer lenses longer than taking lenses gives the impression of “stretch,” a
perceived lengthening of the Z-axis of the scene. The result of the opposite
condition is “squash,” a perceived compression of the Z-axis. Both
conditions are more noticeable when the subject’s shape and distance are
known.

Systems with lower resolution than MF3D have stretch in the system...
Usually not so much with MF3D systems. This is accepted and even welcomed in
the lo-res systems.

Folk do seem to be more sensitive to squash than stretch, I think, me
included. But I’m surprised that a 5mm difference in 70-75mm is much of a
problem. I don’t think it would be for me.

Regards,
        LeRoy

On 2/6/07 6:00 PM, "Neima, Matt" wrote:

My guess is the image is "wrong" in the plastic viewer because of the
magnification. I'm also thinking the apparent magnification of the view
supports my finding the focal length shorter on the plastic viewer.

It seems the "depth cues" (I'm borrowing and may not understand that term)
are missing when viewing with the plastic viewer. It looks like "parts of
the space between things" are missing. It's as if some of the air was pumped
out of the scene and things are crowding each other. Wish I could express it
better. I think the best words would be "natural" and "not".

> Comparison:
> In summary, the plastic viewer is fine until I compare it to the Zeiss;
> then
> the plastic viewer looks bad. I think I'm looking at the difference
> between
> an "ortho" system and one that's not. I think this is because the focal
> length of the Spud and the Zeiss match and the fl between the Spud and the
> plastic viewer don't.

We are looking at a small difference... 70 vs 75mm. It could be a factor,
but not very strong to the point that one looks natural and the other looks
unnatural.

Does your Zeiss viewer have interocular adjustment? You should also measure
the separation of the lenses in each viewer and compare it with the
separation of the aperture openings in your slide mounts and infinity
separation in your stereo images. This can be a factor too.

George Themelis

>> Comparison:
>> In summary, the plastic viewer is fine until I compare it to
>> the Zeiss; then
>> the plastic viewer looks bad. I think I'm looking at the
>> difference between
>> an "ortho" system and one that's not. I think this is
>> because the focal length of the Spud and the Zeiss match and
>> the fl between the Spud and the plastic viewer don't.
>
> We are looking at a small difference... 70 vs 75mm. It could
> be a factor, but not very strong to the point that one looks
> natural and the other looks unnatural.
> Does your Zeiss viewer have interocular adjustment? You
> should also measure the separation of the lenses in each
> viewer and compare it with the separation of the aperture
> openings in your slide mounts and infinity separation in your
> stereo images. This can be a factor too.
> George Themelis

I agree that such a small difference in focal length shouldn't
make a dramatic difference. Another variable that might affect
how the viewers compare would be "eye relief", related to how
close your eye needs to get to the lens, and whether it is able
to get close enough. As an eyeglass wearer, I find this to be a
problem with the plastic MF viewer which is also nonfocusing so
I can't skip the glasses. This affects how much of the image you
are able to see easily and the aparent magnification, and how
immersive the view feels. Just a thought. -Linda

>>In summary, the plastic viewer is fine until I compare it
>>to the Zeiss; then the plastic viewer looks bad. I think I'm
>>looking at the difference between an "ortho" system and
>>one that's not.

My guess is that the, "plastic viewer" is not in sharp focus.


Best regards,

DON