Mechanic properties of sheet film

[f/λ]

Hello everyone,

this is a rather peculiar topic and probably a bit out of the usual, but here we go.

Whilst one may find entire pages dedicated to the photographic properties of film, I have been unable to obtain a few rather specific details of sheet film (4x5), regarding the more physical aspects.
Maybe somebody familiar with the topic might be able to help out.

1. Film thickness: Has anybody actually measured the overall thickness of sheet film? Ilford, Kodak and Fuji list base thickness, but as I would assume the emulsion layer will add quite a bit to this, these numbers are hardly of any value from a technical POV.

2. Flexibility: It is obviously no problem to spool 35mm and medium format film, but as the base thickness increases with film size, rolling sheet film might be a bit of a problem. As well as that, it might or might not damage the emulsion. Has anybody ever tried bending sheet film and/or rolling it? What would be the smallest radius acheivable without causing creases or damaging the emulsion?

3. Not it gets a bit weird: vacuum. There have been a number of aerial cameras, designed for usage with some ridiculously large roll film. Vacuum plates were common to ensure film flantness and a proper focal plane. Film holders with a vacuum system were also available at some point, but the big G is not really of much help here. Has anybody some data on hole size, pattern, number of holes, pressures used? I imagine there is some kind of optimum curve regarding those parameters, and I would be more than gratefull not having to find it myself.

Cheers and thanks,

f/λ

 

[Photo Engineer]

Sheet film is nominally 7 mils thick (0.007") and this thickness is used to prevent buckling in the center, especially with ULF fims. The emulsion thickness hardly counts at all it is so thin. It is on the order of hundreds of microns for all layers in total. Sheet film will roll and packs were designed to allow it to go around some pretty tight curves. Other than a direct crease, I have never damaged sheet film.

Aerial cameras use sheets of film on "normal" 5 mil or even 2.5 mil support, as the spool and vacuum platen hold the film flat against buckling. Aerial film is more like 35mm in "weight".

PE

 

[f/λ]

Thank you very much!

 

[paul_c5x4]

This link may be of interest: http://www.cloudynights.com/documents/large.pdf - Page 6 shows a modified Fidelity holder connected to a small vacuum pump. For larger formats, a similar arrangement would also work. I don't think the number and spacing of the holes is critical.

In answer to question #2 - Circuit cameras would have used large rolls of film, and Ilford certainly list them in their annual ULF programme (the uncut film is also stored as huge reels). Quite what the minimum bend radius would be for 7mil film stock, I don't know. Perhaps the Technical Support Dept. at Kodak or Ilford would be able to answer the question.

 

[DREW WILEY]

You need to differentiate the actual film based involved. For example, acetate films are far less dimensionally stable than those on polyester base; and therefore acetate is more affected in its physical properties by things like humidity changes. There is a lot of older literature out there on this subject, but easier to find in the old Kodak Graphics Arts Guides than in their photographic literature per se. Designing an effective vaccuum platen is an art is its own right, and again, you could study example from specialty companies in the printing industry like
Olec-Stoesser, for example. The idea is not just to get the film flat, but to extract air in a manner which pulls it consistent flat in the correct
direction and rate. That takes some serious prototyping regarding hole placement and size, vacuum draw and channeling, etc; and it will be
a different kind of problem for something feed on a roll rather than a presized sheet. I've made plenty of this stuff, and NO... it ain't all that
simple to do right. Not if you need something precise. Not like a piece of pegboard. But for use in the field, it's easier to build an adhesive
holder for sheet film than a vacuum back. In a lab or studio, vac is more useful. A common mistake is to use way to much vacuum draw.
You need some kind of bleeder valve to keep the pressure low. It doesn't take much to pull film or paper flat. Too much and you might even
up with compression marks on the film on paper, where the holes were.

 

[DREW WILEY]

Back to this. Something big and thin like an aerial film has to be tensioned and flattened very predictably, quickly, and reliably frame after frame, so a relevant vacuum platen requires some very serious engineering. If the holes aren't thought out well, the vac draw could actually
stretch or flatten the film in an uneven matter. Since aerial filming was often related to very precise mapping and needed rectilinear results,
keeping things exactly the same one image to another was crucial. It was hard enough just flying in a straight line. And there are still things
best done in this manner. If anyone has looked thru a good matching set of matching aerial photos thru a stereoscope, even if they were taken in the 1940's, it's downright amazing how much more detail, and how many more fine geological or archaeological features you can pick out than on something like Google Earth. That's why these things are still being done. Satellites do certain things well, but not so much other
things. Now there are all those little cheap drones out there too, which seem to have been custom designed for neighorhood burglars and
peeping toms.