[photo-3d] Re: Medium Format Viewers (again!)

[Paul Talbot <list_post@xxxxxxxxx>]

I've not been able to reply to all the messages I'd
wanted to of late; here's one that came in while I was
out of town for the NSA-SCR Ft. Worth event; it's been
waiting patiently for me to get around to commenting:

Dylan the Hippy Wabbit wrote:

> I've begun using a pair of Yashica 124Gs to shoot stereo slides

Excellent!  Glad to hear it.

> I was thinking medium format loupes, for their known high quality and
> guaranteed full coverage of a 6x6cm slide. They would also come
> conveniently preset to the correct focus.

That's assuming you have the lens-image distance correct, and your
eyes focus correctly.  You'll still likely need to make the focus
adjustable to accommodate different eyes.  The base of the loupe
may prevent you from moving the lenses close enough to the image
for some people to use your viewer.  Thus you may have to remove
the base.  Was the base what you were counting to ensure proper
placement of the images for correct focus?

> I was already of the opinion
> that I could fix the separation, and the recent discussions have confirmed
> that to my satisfaction.

That discussion assumes you have suitable optics.  Do MF
loupes necessarily have a large viewing sweet spot?  I've
seen ones that definitely don't.  Given how loupes are
intended to be used, the size of the sweet spot may not
be a factor taken into account in their design.

> The downside of that is, of course, cost.  £110 each.  The Widescreen
> Centre in London sometimes have a good, but reasonably priced, model.  The
> Russian factory isn't being forthcoming though.  :-(

Several of us on the medium format 3D list have found those
Russian loupes wholly unsatisfactory for use in a viewer.

> If I go with RMM3Ds siamesed mounts what is the separation?

The apertures are a bit over 62mm apart; with normal MAOFD of
2.7mm for medium format, that places the max far point separation
right around 65mm.  65 to 66 mm works well for lens separation.

> I'd
> anticipated looking through the lenses slightly off-centre like a print
> viewer, but with the size of the slides being so close to typical eye
> separation it may not be necessary.

Anyone whose eyes are not the same distance apart as your fixed
lens spacing will use the lenses slightly off center.  With a
66mm lens spacing, a person whose eyes are set 58mm apart will
be using each lens 4mm off center.  Same for a person with eye
spacing of 74mm; anyone in between those figures would be using
the lenses even closer to center.

Lenses with a small sweet spot are likely to give unpleasant
results when used 4mm off-center (and I've seen such lenses).
With the 46x78 lenses in Alan's kit, a 4mm shift off center
is less than 9% of the lens diameter and only 10% of the width
of the 'clear aperture' of the lens.  Looked at another way,
4mm off-center is still 19mm from the edge of the lens.  That's
why it's important to have good optics with a large sweet spot:
viewing distortions rapidly increase toward the edges of a
lens.  Consider that with a 30mm diameter lens, 4mm off-center
is 11mm from the edge of the lens.  Or, far worse: a 20mm diameter
lens (e.g., Red Button), where 4mm off-center is a mere 6mm from
the edge of the lens!

> Any suggestions where I've Still Not Got It would be appreciated.

I've tried to make the above generic enough for general 3D
discussion purposes.  You should join the medium format 3D
discussion list for more guidance from people who specialize
in medium format stereo.  Info on how to subscribe is at:
http://www.pauck.de/archive/mailinglist/mf3d/info.html

Paul Talbot

 

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