[MF3D.FORUM:1181] Re: How much shift can be necessary to set the window?

[Paul Talbot <ptww@xxxxxxxxx>]

"Michael K. Davis" wrote:
> 
> For folks using 6x6 cameras to produce your chips for 50mm x 50mm RMM
> mounts, for example, do you ever find yourself wishing you had more of a
> margin horizontally, for setting the window?

The Sputnik produces 55x55mm images, and there is plenty of room
to set the window in the 50x50mm mounts.

> I'm considering a larger mount, using 6x7 chromes.

Which mount would that be?

> My camera produces a 55x68mm image.  Ignoring
> any other "gotchas" for the moment, does anyone think a 50x63mm window is
> too large, horizontally?

Yes, I think it is too wide.

> I know that only leaves 2 mm between images, if
> centered at 65mm,

I believe you've forgotten to take into account the deviation.
My guess is you chose 65mm to stay within the constraints of
a normal parallel viewing viewer.  But 65mm is a reasonable
figure for maximum *infinity point* spacing.  To allow for up
to 3mm of on-film deviation, you should plan to space the two
film chips no more than 62mm apart.

The RMM MF stereo mounts have apertures spaced 62mm apart.
Coincidence?  Or simply brilliant design?  I say we give
Joel a little pat on the back.  :-)

> but I'm more interested in getting feedback on whether
> having only 2.5mm all the way around is enough for setting the window in
> most circumstances.

It's probably more than enough in most circumstances.

> If not, how much smaller than 63mm would you design it
> to have enough to handle most situations and still meet the goal of
> enjoying a larger format than 50x50?  6x6 users don't have much more than
> 2.5mm all the way around, right?

Sputnik shooters crop 5mm off the vertical and the horizontal;
of course it is not necessarily 2.5mm from each side.

My investigations of wider apertures has led me to believe that
57mm is about the largest practical aperture width for mounting
in the 80x132 King Inn/RMM standard mount dimension of 80x132.
Of course there is no absolute rule that says you must stick
with the 80x132 size, but if you go to something larger, most
other people won't be able to view your work.  I also don't think
there is much more to be gained.  If you use the 62mm figure above
for maximum aperture spacing, and allow for a border of, say, 4mm
between the images (to help create a nice frame, and for the
structural stability of the mount), you are down to 60mm maximum
aperture (1/2 the 4mm center border coming from each aperture).
So you'll have to go to a lot of trouble to obtain an extra 5%
((60-57)/57) width.  (You could bump that figure up a wee bit
more some by pushing design to the limit, for example using 66mm
spacing and 2.5mm maximum deviation, but that's even more potential
trouble and still gets less than an extra 8% width.)

If you and Bill pool your resources, perhaps you can commission
a new mount with 57mm wide apertures.  :-)  I'd love to have one
available.

Paul Talbot