Why not simply take a normal loupe and grind the shield (or attached side-plates) down so that when it is rocked on the rear surface of the glass (to align with the exit pupil) the focus remains on the front of the glass (or where the front of the glass would be if using an aerial image with cross-hairs or whatever)?
Apologies if this has been mentioned before, I expect it has but I didn't see it.
DIY vacuum film holder:
http://www.deadbread.com/crumbs/vac.html
Best,
Helen
I'm getting ready to move (still) so I've got to keep it short. What you describe above should work. However, I'm interested in the limits of fundamental elements (focusing, CIE color space - nice to see some on your site, diffusion, etc.) for the principle analog references (BW, E6, C41).Originally Posted by Helen Bach
I've been able (arguably) to cross-correlate comparisons of the final end product in pixels regardless of the source (analog or digital) reasonably well. In order to do that
I've tried to introduce a lot of precision (that many here find boring) and identify weak
links. What works best for me is the 30x to 35x loupe while most here seem to prefer a 6x.
IMHO its difficult to get repeatable ultra fine focus behind a fresnel and ground glass, that led to the plenum (as a stand off for the emulsion plane) and so on. Admittedly getting the maximum number of pixels out of a piece of film is often not as important as other factors (which film/filters and lenses, color space, dynamic range, acuity, etc.).
Anyway, the tool set seems to be useful in pinpoint evaluation of an entire frame when using tilts and shifts or evaluating an ariel perspective.
R.
Nice mirror like reflection off the film (at that URL), but (sadly) it is an example of a non-flat surface.
While packing up I found the "Linhof Super Screen" holder for the GG (and fresnel) I use on the 4x5 Speed Graphic.
Back to sorting and packing, (sigh).,.
R.
rvhalejr, will you ever be done moving!
As I believe I mentioned a while ago I machined a vacuum holder for use with old Kodalith graphic arts film for early microelectronics lithography reproduction. I used an optical flat with sodium monochromatic light to assess the flatness of film clamped by vacuum. But the reflection off the first surface of the film can give a decent sense (though not quantitative of course) as shown in Helens' reference.
But the holder I used was machined from type 302 stainless IIRC. There were holes of about 0.032 diameter spread uniformly from edge to edge on maybe 1/4 inch centers for a 4 X 5 format. The back of the plate was hogged out everywhere yielding full vacuum access to all nominally 500 holes. The trick was to fabricate the entire plate then anneal the stainless followed by slight surface grinding to obtain optimum flatness. I have to guess now but I believe the TRO (Total Run Out) over 4 X 5 inches was less than 5 um - say 10 sodium fringes. However errors in film surface flatness depended critically on particles trapped on the platen surface and that often caused TRO to approach 100 um. We finally resorted to a laminar flow clean setup.
Helens notion of a swival loupe, especially for the high mag you want, I may have mentioned earlier. But it will produce only a one dimensional line of sharp focus either on GG or of an aerial image as observed by the viewer.
Nate Potter, Austin TX.
Good Lord willing. Our church is helping us move, but yesterday we almost hit 100 degrees so its still a pretty scary proposition.
Thanks for posting your specifications (w.r.t. the vacuum back) which are much better than mine (I gave up at a hole density of about 200).
I'm going to come clean on the fine focusing yield and scanning thing. Arguably my best scan results are achieved when the film is held perfectly flat (I don't use a drum scanner) but newton's rings usually ruin it.
I found some mounting fluid, plastic and/or glass with nearly the same refractive index as film emulsion (gelatin) and the acetate base. Matching the r.i. on either side of the film would hopefully attenuated newton's rings to extinction.
Anyone with common sense would have probably found it cheaper and easier to get an old tango.
R.
Whew !!! We are finally in the new place (Cheers !!!). The bad news is that some where in 100 or so storage boxes is the Large Format equipment. It will be a number of months before a small studio/lab is set up and R&D started again.
R.
Still trying to unpack boxes.,. To make matters worse I feel the need to fab a universal holder that will slip on as many lenses as possible and hold the Cokin P holder in place.
The threaded ring adapters and goose-neck clamp are rather awkward .,.
R.
Last edited by rvhalejr; 11-Dec-2009 at 18:27. Reason: Default <cr><lf>
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