Specs for film to camera back distance.

[Don Cameron]

I posted a question about fresnel lenses on a crown graphic, and they were an option in the later years. Well I was dissapointed in the performance of the Kodak 127mm Ektar that came with it, so I got out my trusty depth micrometer, and started measuring to see if the fresnel was in the right place.It was, so I started measureing film holders, rollfilm holders, and even a "Polaroid 545 back: The results were scary! My plastic holders averaged .200" from flat into the film, the wood was.191" for same measurment, and I used the same piece of Tri-X film for all of the measuments. The Polaroid 545 was .207" with type 59 film installed. and last but not least was a graplic RH 10 holder that measured .175" to the film (FP-4) installed. The Crown Graphic Camera measued .209" to frosted glass with the fresnel in place, from the back of the camera. I would sure like to know what the specs are for that measurment? A difference of over .030 seems to eleminate shooting at bigger f stops. After almost thirty years making my living as a photographer I took this as agiven, and never bothered to check.

Don Cameron www.doncameron.com

[Eric Woodbury]

How much can it be off? Comes down to how much of the depth of focus you want to throw away. I read once that the spec (I think it was an ANSI spec) was +/- 0.007".

When I first got my 57 Deardorff (the back is old), it didn't seem to focus well and I did a measurement. I don't remember the numbers after all these years, but you might think that an old wooden back would wear down. Apparently this back had always been off, because in one corner it was at least 0.030" too high. And it wasn't even. I remachined the back to spec and it has worked fine since, but it has always made me cautious.

I've read about different holders, but mine are all of the new plastic type and I have not had a problem.

[Robert A. Zeichner]

There is no specification for "Film to Camera Back Distance". Every camera back is designed a little differently. The ANSI specification for 4x5 FILM HOLDER depth is 0.197" +/- 0.007". Measuring this is not as easy as you may think. First of all, the holder must be inserted into the back so that your measurements are made with the exact spring tension employed as would during an exposure. You must of course insert some film in the holder. The measurements need to be made with the back of the camera removed and the mating surface must be flat and free of damage. Reference measurements must be made using a non-compliant precision machined plate with holes at critical points across the entire surface of the film area. A depth gauge that resolves 0.0005 is recommended. If you have a Fresnel, you have to remove it and compensate for the focus shift. You would first make a series of measurements of ground glass depth. Next, you would insert a film holder and film and make an identical set of measurements. If you removed the Fresnel, you have to compensate for its removal. Lets say it was .060" thick. A Fresnel typically moves focus rearward by 1/3 of its thickness. So subtracting the Fresnel thickness and adding the shift back in, you should be about .040" shallower to the glass than to the film. Now as far as that 0.197" distance.... you'll never find it! Unless of course you measure just the holder, which I promise will give you erroneous results every time. You would have to build a fixture against which the ENTIRE bearing surface of the holder contacts, devise a pressure plate to apply the same tension your camera would impart to the holder and you would still have to subtract the thickness of the reference plate to get something close to that dimension. With the use of the back, you will get a more meaningful, integrated measurement. What you are really interested in is the difference between the distance from reference surface to gg vs. film. In an ideal world, your holders should be no deeper or shallower than 0.007". My advice is to test with film. See my article in the Nov. Dec 1996 issue of View Camera.