Film Holder T-depth and ANSI Standard

Quite often a reference is made to the T-depth of a film holder and if it is to the ANSI standard or not. My question (feel like should know this, but have been unable to find a good example) is how does one measure the T-depth of a film holder (my understanding is this where the film plane resides when the holder is inserted into the back so that the focus on the GG is in the same plane as the film - yes/no/close?)

And then there is the ANSI standard - where does one find this information as well?

Sorry for the basic sort of question, but would like a better understanding...and Thanks in advance as always.

Making these measurements using conventional depth gauges and similar measuring devices is not as easy as many have led on to believe. Some of the problems one might encounter relate to the condition of the bearing surfaces of the holder and the overall flatness of a particular holder. When the holder is inserted into the camera back, its depth may actually change as it is clamped in place by the spring mechanism. Then there is the issue of compliance of the septum. While it looks pretty solid, just touching it with the probe of a depth gauge will cause it to bend. After exhaustive tests, I have come to the conclusion that it is more important to check the condition of a holder with non-destructive film tests that take these and several other problems into account.

Z38.1.51-1951 is the American Standard Dimensions for photographic double film holders of the lock-rib type. "T" specs are as follows along with the tolerance for error. For 2-1/4 by 3-1/4, 3-1/4 by 4-1/4 and 4 by 5 holders the depth is 0.197" and the tolerance is +/- 0.007". For 5x7, depth is 0.228" and the tolerance is +/- 0.010". For 8x10, depth is 0.260" and the tolerance is +/- 0.016".

I agree. Its not as easy as people think to measure even if you have a very accurate micrometer. The most important thing is not the ansi standard but that your film holders match your ground glass depth and that depth is to the film surface and not the septum behind the film.

The best test is to photograph a distant horizon. If you can get that critically sharp then everything else should be OK.

Do you have suspect film holders or ground glass depth?

First, thanks for the response it is very good information. Do not have a suspect holder and looking for a better understanding of what is being discussed when I read T-depth. But, I use an old Eastman No. 2 in 7x11 format, so there are not a lot of holders around. Both S&S and AWB do make a holder for the 7x11 format, but seems that I have read that one or the other is not to made to the Eastman standard, that the T-depth was off. So, was trying to decide if I picked up additional holders how I would know if that were the issue (other than the focus would be off).

A flat metal bar of even thickness can be placed across the film holder and depth measurements can be made with a micrometer using the bar as a reference base. 3 across one end, 3 across the middle and 3 across the other end. If done with film in the holder, this will give you the depth that the GG should be. The measurements should be pretty close and can be averaged.
Care must be taken to measure only to where the micrometer touches the film and not to where the the micrometer torque click is heard because the micrometer will flex the film and septum before it clicks. The same applies when measuring the GG depth. And also it should be noted that the film, when vertical in the holder may fall foward or backward in the film guides or the holder so even this method of measuring may not be truly accurate.
Sinar used to make metal film holders which held 4x5 film under tension so this could not happen and they made 8x10 holders with a tacky septum which held the film very flat. But then again, most 4x5 users don't have these film holders and report no problems. But then again, many claim that fuji or kodak ready loads give greater accuracy because of the pressure plate they use to always put the film in the same plane. Everyones mileage differs. If its not a problem don't worry about it. Do an infinity focus check and if the results are critically sharp at optimum aperture, then there is no problem.

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Originally Posted by Rob Champagne

A flat metal bar of even thickness can be placed across the film holder and depth measurements can be made with a micrometer using the bar as a reference base. 3 across one end, 3 across the middle and 3 across the other end. If done with film in the holder, this will give you the depth that the GG should be. The measurements should be pretty close and can be averaged.
Care must be taken to measure only to where the micrometer touches the film and not to where the the micrometer torque click is heard because the micrometer will flex the film and septum before it clicks. The same applies when measuring the GG depth. And also it should be noted that the film, when vertical in the holder may fall foward or backward in the film guides or the holder so even this method of measuring may not be truly accurate.
Sinar used to make metal film holders which held 4x5 film under tension so this could not happen and they made 8x10 holders with a tacky septum which held the film very flat. But then again, most 4x5 users don't have these film holders and report no problems. But then again, many claim that fuji or kodak ready loads give greater accuracy because of the pressure plate they use to always put the film in the same plane. Everyones mileage differs. If its not a problem don't worry about it. Do an infinity focus check and if the results are critically sharp at optimum aperture, then there is no problem.

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There are several problems using a technique such as this. First of all, a metal bar that isn't a surface ground standard fixture is going to have a run out of several thousandths of an inch at very best and very likely much more. Assuming a holder for 7x11 is going to have a depth fairly close to 8x10 and a tolerance to match, you would want any fixtures you use to make meaningful measurements to have dimensional accuracy that is an order of magnitude better than the dimensional tolerance you are measuring. Such a fixture would also have to have sufficient thickness to resist warping or flexing.

The second problem relates to the contact area of the holder. A bar that doesn't contact the entire bearing surface of holder will telegraph the variations in the flatness and/or depth of the localized portions of the holder making any measurements inconclusive.

The third problem has to do with whatever measuring device contacts the surface of the film/septum. How much you press down will alter the measurement because the septum and film are actually flexible. When I used to do depth measurements on motion picture cameras, the dial depth gauges we used had a certain amount of spring tension against which the probe would retract. We needed to insert a surface ground plate into the film aperture to provide a stiff enough surface against which to make the measurement. Even hand pressure could cause this seemingly rigid assemblage to flex. The plate was 16mm wide, maybe 25mm long and about 4-5mm thick.

Assuming a dial depth gauge could be obtained with minimal enough spring tension to get around the last issue, a better way to make this type of measurement would to machine a flat plate the size of the holder and of sufficient thickness to resist flex and drilled with a number of holes through which the probe could be inserted. The measurement would actually need to be made from the inside of the camera back. A series of depth measurement would be made with the film holder in place and then removed, placing the gg back into the viewing position.

I'm not convinced it would be worth all the expense and trouble. You can make a definitive test with film using a step target and a normal lens at maximum aperture. Construction of this is fairly easy and inexpensive to do and described in my article.

Use a dial indicator and a long piece of material to span the outer rim of the holder. The dial indicator is zeroed on the edge and then allowed to drop down onto the film plane of the holder, then the depth is measured.

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Originally Posted by sabeluc

The distances are very small and it's hard to be confident, but I'm an amateur machinist and happy to take some measurements.

The film holder I ran the newspaper test with is a Lisco Regal II (modern plastic, not wood). I do notice a bit of a rise in the edge just under where the logo is, but it's flexible there and it gets pushed flat by the rear standard when in the camera (otherwise it would leak light). I measured T for the film holder at the 4 corners where it's easy to measure using my old browne and sharpe depth gauge and gut checked the measurements with my modern mitutoyo digital vernier calipers. The corners measure 0.2670" and 0.2675" at the bottom. At the top where the rim bows up I get 0.2750", 0.2740". I believe these are all a little high but within the spec for an 8x10 film holder. I don't have an appropriate flat bar to check the center, but a 150mm machinist rule placed across the span of the septum does not show any light through peeking underneath. It is flat enough for my purposes and I think the film holder is not the issue. Anecdotally, I saw the same focus issue across all 8 of my film holders, which are various brands of modern plastic holders.

Attachment 222294

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you're supposed to be measuring the depth to the front of the film, not to the septum.

In the ASA standard, the "T" depth is measured from the face of the holder to the face of the septum, not from the face of the holder to the front of the film. For film holders up to and including 4x5, this distance is .197 +/-.007 inches. For 5x7 holders it is .228, +/- .01 inches. For 8x10 holders it is .260 +/- .16 inches. The "H" distance, the slot in which the film rides, is .012 inches. This information is copied from The Camera Repairman's Handbook, revised edition 1966, and is the ASA Z38.1.51-1951 standard. I converted these figures to metric and compared them to ANSI/PIMA IT3.108-1998. Agreement was usually within 1%. Photographers obviously cannot rely on these standards where critical image quality is needed. The photograph is the ultimate test. However, blind adherence to standards and measurements can reduce the amount of film testing. One can shim the film against the front of the film slot and measure the distance from emulsion to the front of his own film holders for his own "T" standard. Of course this figure should coincide with the equivalent figure in the ground glass holder. What a bother! For a few, it may be essential. For many of us, good enough is good enough if our photographs are satisfactory. Many good photographs were made by the rule, "f/8 and be there!"