film thickness variability

Has anyone noticed any major differences in the focus from shooting velvia loade d in Toyo holders vs., say, Astia? I noticed that velvia is a much thicker film while Astia, Neopan 400 presto, etc., are much thinner. As I'm about to embark o n a road trip, it makes me wonder if I should focus a tad farther when shooting the Astia/Neopan? I'm guessing that the differences in thickness are, most likel y, negligible compared to one's ability to focus accurately and the variability in holder depth. I was just wondering of anyone had run any tests for comparison s.

The greatest differences in film thickness are, according to my own tests, those between transparency and negative films. I have found these to vary by about 2 to 2.5 mils. As for transparency films there are differences also but on the films I have tested the difference is about 1 mil or less; These include velvia, Provia and Ektachrome. These differences are too small to worry about. Film holders themselves are subject to variations. On a limited sampling of Toyo the variations I found are about plus or minus 2 mils. The Lisco holders I tested vary by more than three times as much. Sinar guarantees theirs at plus or minus 1 mil. Additionally, have you tested your GG calibration? I think that you may find greater differences in holders and GG than you may in transparency films. It seems to me that focusing to compensate for variations can not be done with sufficient control. If you wanted to get closer to technical perfection you should have two backs for your camera, one for transparency film and another for negative film. Another source of variation is temperature and humidity, causing film to bow out. Sinar allows 0.5 mils for that in the design of their holders. Is all this worth the effort or the worry? I am not sure, but at the least, GG calibration seems to me the first priority. Several prior postings provide methods for so doing.

I have found that the quickload holder gives the best results for Fuji transparency film, partly, I assume, because the film is held to a uniform flatness by the pressure plate in the holder. Obviously one also avoids the problems of film being scratched, and dust, with Quickload, and I think it's worth the premium over loose sheet film, particularly if you are setting off on a road trip.

For black and white I also use Toyo holders, and they do a pretty good job overall - any errors are more likely to be due to my technique than the fault of the holder.

By the way, if you are using Neopan 400 in sheets (which I assume you obtained in Japan), how are you rating it? I have had good results with rating at 640, and developing in Microphen full strength. Has anyone tried PMK Pyro with this film?

A test run by a professional photo magazine a few years back came to the conclusion that quickload holders actually gave the worst film-plane registration, and that using quickloads in a Polaroid 545i gave better registration. Conventional Toyo or Fidelity holders were better, coming just behind the Sinar precision filmholder. The illustrations accompanying the article seemed pretty conclusive to me; but then I'm pretty much of the "stick to what you know works" school of thought, and I didn't even know that quickloads existed until I read the test. Needless to say, I've never bothered with them.

James, forgive me if I'm teaching you to suck eggs, but the thickness of the substrate has no direct effect on the focus position since the film is used with the emulsion towards the lens. The reference depth is to the front of the film sheet, not the back. Thicker film may be easier to keep flat in a conventional holder.

I think the Large Format Homepage refers to a test in which the old Grafmatic film holders came out best. The ultimate would be a vacuum back. You can check your holders with a stiff rod across the darkslide opening and a depth gauge or micrometer. As always in photography, it's consistency that matters, not accuracy.

Someone may have posted this page already, though, I have not seen it in any of the answers on the film flatness question. So I thought I'd toss it out. It may be of some use.

www.smu.edu/~rmonagha/mf/flat.html

Regards.

Jason

I've just briefly read the above article (flat.html) and I'm afraid I've got to question it's whole accuracy.

For a start it claims that contrast is affected by up to 48% by film plane inaccuracies. This is plainly nonsense, unless what is meant is MTF contrast, but then the specific spatial frequency should be cited as well. I'd also take issue with linking depth-of-focus directly to numerical aperture without any mention of what focal length lens is being referred to.

Lots of authoritative sounding sources are quoted, but simply on the basis of the two gaffs I've given here the whole thing seems a sloppy pastiche of half comprehended extracts.

As a follow-up, in the field, I performed a comparison of velvia and astia loaded in Toyo holders. The lenses used were the 90XL and 150XL, lens to subject distances of at least 60 ft and apertures ranging from f22-32 (subjects were Thor's Hammer in Bryce Canyon, White House ruin and Spider Rock at Canyon de Chelly). Only the aperture was adjusted by 1/3 stop (RVP rated at 40, RAP at 100) and shutter speed changed (focus was the same). For all three compositions, the velvia shots, when viewed under a Zeiss 5x loupe, are clearly sharper. I believe that at f22-32, the depth of focus should be ample, so the only explanation I can devise is that Velvia has superior sharpness as compared to Astia.