Re: 30 mm viewing lenses, where to get

[Paul Talbot <ptww@xxxxxxxxx>]

This topic has a mixed heritage!

Lincoln Kamm wrote on P3D:

> >> I just got a medium format camera and a 30mm lens for
> >> it.  For medium format, that 30mm is a full frame fish
> >> eye lens.  So, what I was wondering is, if I am able
> >> to track down some maching lenses (30mm) to view some
> >> slide bar stereos taken with that set up, how will
> >> that look different than with the 78mm lenses of the
> >> saturn slide viewer?

Oliver Dean wrote, in part:

> Forgive a hasty answer:  You probably won't be able to find achromat
> viewer lenses capable of covering medium format (2 1/4' x 2 1/4") at
> only 30 mm focal length for anything less than a fortune, if at all.

I think Oliver is quite correct.

> Viewing a medium format image with a 
> 30 mm focal length lens would require a lens diameter much larger
> than the focal length in order to see all corners of the image, which
> means a maximum lens aperture of less than f/1.0, and such lenses are
> difficult and expensive to design.
> 
> I think that the coverage enabling you to see the
> whole image would still be a problem with a 30 mm effective focal
> length, no matter how it is obtained.

Correct again.  As an experiment, I put two of the 46x78 lenses
of the SaturnSlide back to back.  This gives an effective FL
of roughly 39mm.  Wearing glasses (but with no lens holders to
get in the way), this combination doesn't even cover the vertical
dimension at the center of the image.

I tried removing my glasses, which has two partially offsetting
effects.  I can get my eye closer to the lens, improving
coverage.
But the effective FL decreases (because I'm nearsighted and have
to move closer), so the coverage decreases some.  I haven't run
the numbers through the forumla John B. sent me long ago, but
I'd guess the effective FL without my glasses is a little under
35mm.  With this setup the lenses just barely cover the vertical
dimension of the "6x6" image area (50mm).  Interestingly, I could
not see the entire vertical dimension at once; I had to scan up
and down to check how much coverage I was getting.

This combination might work if you are cropping the image with
the fronts of a 645 portrait mount and a 645 landscape mount.
But that would sort of defeat the purpose, wouldn't it?  Also,
this is still before considering the effect of the lens holders;
my eye was practically touching the lens, which would likely not
be the case with an assembled viewer.

With all that said, I'm not sure Lincoln's original question
has actually been adressed:

> So, what I was wondering is, if I am able
> to track down some maching lenses (30mm) to view some
> slide bar stereos taken with that set up, how will
> that look different than with the 78mm lenses of the
> saturn slide viewer?

Some specific differences I can think of:

Possible advantage:

  - You'll eliminate the stretch effect of the long
    (relative to the shooting lens) viewing lenses.

Definite disadvantages:

  - You'll cut off enormouns portions of the image (this
    is the part discussed so far).

  - You'll blow up the grain to an enormous size.

  - You'll blow up dust, scratches, spots, and dirt in
    the emulsion to an enormous size.

Anybody else have some to add?

Possible alternatives:

If you really want to see what the images look like with an
ortho viewing situation, perhaps you can get closer to your
goal by enlarging them.  If I've learned correctly from listening
to the wise men of this list, a 10x enlargement of an image
taken with 30mm lenses should be viewed from 300mm.  I don't
think a 10x enlargement will be enough to solve the problem,
though.  But I'm wondering whether at some enlargement size
it will become feasible to use a mirror device like the
Free Viewer's Assistant, the View Magic 6x6, or David Lee's
large format viewer to get close enough to view the image
from the ortho seat.

I'm not smart enough to figure out whether this would work or
not.  One of the wise men of the list will have to bail me out
at this point.

Paul Talbot