I was discussing the issue of film-flange register with a certain repair wizard who lives up on a mountain. The question was how you would check film-flange register on a given camera/lens combination. The conversation started with my idea of having my Hexar RF tested against a selection of lenses and concluded with my wondering if it was my M3 instead that needed to be checked.

First, besides telling me that as a real world test it is impossible to do because the film starts to bow inward after a minute (relative humidity changing); he also shared this interesting insight: Leica late LTM (IIIc and on) and M bodies contract over time, enough to cause the body focus to change. This is the same principle which makes boring out old engine blocks more attractive than using new ones; the cylinders keep their shape. If I recall, he called it "seasoning" of the alloy.

To be fair, this isn't just Leica, but anything with a diecast chassis (ever wonder why old SLRs sometimes focus a hair past infinity?). Leitz perceived this to be such a problem with the older screwmount cameras that it advertised that they were made of metal stampings to improve precision and stability. Then they started die-casting and the party line became that die- casting was better (in reality, die-casting allows smaller tolerances but apparently does nothing to promote stability).

Second, register problems do not manifest themselves with lenses like the Summilux 75 close-up, but rather with fast, wide lenses at infinity. Wide lenses have very little focus travel at the longer distances, and if there is a register problem (like body focus being too long), the lens will fall well short of focusing at infinity or focus well past it. This would tend to suggest that a lot of the people with troublesome Summilux 75s and Leica M6s close up are having rangefinder or lens problems, not register problems. Having used a 21/2.8 both on my M3 and my Hexar, both seem to be fine at f/2.8 at infinity at 50x. My interlocutor said that that fact suggested that it was unnecessary to test either camera.

My personal conclusion from this is that is that a lot of old M cameras probably have less than ideal body focus and that the modern Ms (of whatever brand) are heading that way. It also makes me think twice about all of this (probably manufactured) argument about the Hexar RF's register distance being slightly longer, (1) because most people who have complained about focusing problems have complained about long lenses not focusing (=rangefinder alignment); (2) because the Leica frame of reference on any camera before the M6 (1985) is a moving (contracting) target; and (3) given the nominal dimensions of the Hexar FFR (28.00 +/- 0.03mm) vs. the Leica M (27.95 +/- 0.01), it seems just as likely that after 10 years, a Hexar RF could have a FFR closer to Leica spec than a Leica does.

I suspect that the Hexar RF is now mfd to the same FFR as the Leica (27.95 to the inner rails). because it seems that everyone who has had real register problems has had a low-S/N Hexar. I surmise that they figured out the problem fairly early on.

The solution to all of this seems to be checking body focus every 10 years or so.

Strange.

-- Dante Stella (dante@dantestella.com), April 27, 2002

Answers

as you hint at one point, i really think film flatness issues will swamp register problems due to body problems in just about every case. as for probs at infinity, even at f2 the dof at infinity should solve any problem. as for the inability to check the problem, at least with m cameras that have a back flap, why would the traditional ground glass test not suffice? i checked my hexar rf in this way when i heard about the possibility of FFD troubles. in any event, a very interesting post!! probably something to think about with most barnack cameras. i must say, however, that if the problem were really severe enough to matter, would't there be plating probems? the effect is unlikely to be the same with respect to both brass and the chrome plating.

-- roger michel (michel@tcn.org), April 27, 2002.

Camera repairers are tradespeople who are *trained* rather than professionals who are *educated*. With time many of them pick up sound-bites of science or engineering but they still lack the background to sort and assimilate it, and the result is a lot of nonsense spoken in a very sincere and almost believable fashion.

Film flatness is an issue, but one that is independent of register tolerances. If the film register were not measurable, there would be no specs for it, nor would all camera manufacturers and (at least official) service facilities have custom-made jigs for just this purpose. The *measurement* is made without film in the camera. The *specifications* are computed to consider film flatness. If relative humidity were a sginificant factor in film flatness, the specs would need to be set to an average and then people in Arizona or Florida would not be able to get sharp images. Think about it.

It is also true that a metal casting can change dimensionally with time. However, this is a theoretic generality and varies with the type of alloy(s), the way the castings are treated, and the physical demensions. It is an extremely complex issue about which the person in question (while probably a top-notch craftsman) lacks the metallurgical knowledge to fully comprehend. If such a theory had practical ramifications in the camera industry, no one would be able to get sharp images from very old camera equipment, which is patently not the case.

-- Jay (infinitydt@aol.com), April 27, 2002.

You are right; the only reliable way to check focus is to use a first-surface mirror (better) or groundglass on the film rails. The problem is that a flat piece of glass (either) does not imitate the behavior of the film, which is flexible and coated on the image side with gelatin, a substance that loves to curl.

On the film flatness swamping all other issues, it is hard to say, because depth of focus varies from lens to lens (generally the inverse of depth of field).

I have noticed that motor-driven (2fps) sequences wide-open with a 21/2.8 at infinity with the Hexar do not work as well as one shot every one or two seconds (about the same speed as you would shoot an M3). If you shoot at a more leisurely pace, there is no difference between the K and the L. This confirmed what I read about a Zeiss test that concluded that 2-2.5fps is the practical limit for motor drive because the film has to settle. But this would not be a usual situation - wide-open and infinity would probably be a landscape, so no rush to shoot a million frames.

Based on my puttering around, I also think that pressure-plate geometry and spool rotation direction also play a role in flatness. Cameras designed with motor drives seem to have smaller pressure plates and deeper film channels. They also lack counter-rotating spools. This may affect film flatness, but I imagine the bigger determinant is time between shots. The repair expert told me that when you shoot wide and far with wide- angles, shoot two shots: one when the camera is wound and another one right after it winds (whether lever-wound or motor- wound),

-- Dante Stella (dante@dantestella.com), April 27, 2002.

Jay: the person in question got this from measuring M2s and M3s precisely on a specialized device. But you are right: the separate problems that affect ability to check register are (a) lack of similarity in film behavior to the test surface; (b) subject-side field curvature of the lens; and (c) the shape of the focused- plane projected on the film (paraboloid - meaning that you can optimize for the center, edges, or all).

As for the quantum of shrinkage, it can be a little or a lot. It may not even be as big a problem anymore. Just something to ruminate about.

-- Dante Stella (dante@dantestella.com), April 27, 2002.

Interesting stuff - I have to make one correction though - the flange to inner rail distance on an M is 27.75 - it's 27.95 to the outer rails. I have an early M6 which is short by 0.15mm from the flange to the film rails although I suspect this is by inaccurate machining of the back of the flange. It's not difficult to measure the actual distance BTW - a good vernier caliper will measure to 2/100 of a mm and a small bit of glass on the inner rails and a small straight edge across the flange will do the job. The other issue being is the shrinkage even and uniform? - fortunatley it's within the realms of very basic engineering ( a good Leica tech?) to machine the back of a lens flange on the body to the correct distance and parralel alignment. I have measured a selection of film emulsions and these vary by 0.02 mm - not withstanding th e bulge issue it's easy to get the engineering right but the actual placing of th efilm surface at the point of best lens focus is always going to be a compromise.

-- John Griffin (john@griffinphoto.u-net.com), April 27, 2002.

Dante, It is possible to use a piece of ground glass accuratley - all you have to do is mke sure it is at the correct distance from the inner rails ie M lenses are factory set to focus at infinity at 27.80mm from the lens flange ( back plate distance minus film thickness) as the iner rails are 27.75 the glass surface has to be .05mm away from the inner rails. I found a particular plastic film sleeve sheet which is just this thickness and have made a 'window' out of it to act as a spacer - as long as the glass is good (I cut mine from an old view camera) and you have a good powerfull loupe - it's very accurate!

-- John Griffin (john@griffinphoto.u-net.com), April 27, 2002.

I think everyone here is thinking way too much. As I just wrote on another list, Leica's had 80 years to come up with a set of numbers that do the job, regardless of theoretical issues to the contrary. When your camera goes in for repair, the repairman can check those numbers, and if they're not right, can shim them up to where they should be. If you get the four corners of the flange to the number, the middle will be there too, de facto. There are things you can't control (film being the primary one), and you therefore can't fixate and worry about them, though I'll bet that Leica has set the numbers to work with real-life film, anyway. It's not rocket science to set the lens flange where it should be, either.

The real question, as far as I'm concerned, is whether ANY camera that uses flexible film can be focused theoretically accurately (without a vacuum back or something similar), and, especially, whether many photographers can focus a long lens with a RF camera with enough assurance to blame the camera when something goes wrong. I'm absolutely insane about having my equipment adjusted and careful focus, and, still, getting my longer lenses in focus--especially at longer distances wide open, where it really matters--calls for judgements I'm well aware I can't make under a lot of real-life circumstances, resulting in slight misplacements of focus which I KNOW are my fault. The most famous photo taken with the 135/2.8, and used in Leica's own advertising (of a Vietnamese family swimming to safety) has the lens obviously focused on the water about 10 feet behind them. When I look at my own negs with a loupe, I often miss a bit, but it still works in a real print.

I've photographed critical things with 4x5, and it's hit or miss if you want the focus right on, with film slop, focus shift, and general equipment inaccuracies. Yesterday I heard that one of my colleagues has switched to a digital back focused in a magnified image on a computer screen to do the same job, and he's in much safer water.

If you want to cruise in these waters, please tell me why all of my Leica LTM-M adapters measure .91mm thick, but all my off-brand ones are 1.01mm? (Or do I have that exactly backwards--I don't have my caliper with me right now) Yet no one seems to be complaining about that obvious problem. . . .

-- Michael Darnton (mdarnton@hotmail.com), April 27, 2002.

A very, very, interesting thread and I think Michael has contributed the definitive "real world" perspective!! I doubt however that I shall lose any sleep over the tolerances issues - in my experience there are less esoteric factors that are almost always, universally, to blame in matters of sharpness (of which "focus" is only, admittedly, one consideration!)!! Even at 1/125 sec. the tripod makes the difference!!

-- Art Waldschmidt (afwaldschmidt@yahoo.com), April 27, 2002.

In 35mm format, the film makes a sort of lazy W shape across its width. IOW its middle is usually just slightly off the pressure plate. We tested this with Nikon and Canon bodies quite extensively back in the late 70's when one of my customers had the equipment to do so (hi precision comparitor, sort of like a microscope with long working distance and a precision rack calibrated to .01mm)

We also found that placing a ground glass on the inner rails of any of those cameras was NOT the place the camera focused! It was just a tad bit farther back, I think by something like .2mm (dont quote me) on Nikon.

What we concluded back then, was depending on rail, film and winding configuration, the "spec" distance and the actual measured distance would vary by model, to accomodate (what we thought was) the amount the film curved.

-- Charles (cbarcellona@telocity.com), April 27, 2002.

If I had a Noctilux, perhaps I'd worry.

At f/2 (my fastest lenses), poorly focused shots are my fault, not the camera/lens.

Best,

Jerry

-- Jerome R. Pfile, Jr. (JerryPfile@msn.com), April 27, 2002.