This is what I use to measure flange distance.
It's a depth gage with a gage block in the film plane. The two markers on the dial show the tolerance range for the K-Mount so the setup is precise enough to calibrate in this case. I think I got the gage for around $60 used and a small set of blocks for $30 or so (the blocks are also necessary to calibrate the depth gage with a 1" extension added).
But I'm not sure if this exact setup would work with a Leica iii, since you can't put a gage block in the film plane. You would have to put the depth gage against the pressure plate but also make sure that it isn't pushing the plate away. And you would need to correct for the difference in distance between the pressure plate surface and the film plane.
Thanks! Do you know what an accepted tolerance range is for such a flange distance?
Which gauge blocks can I use? I find sets on Ebay which are frankly of an unknown origin but advertised as Grade 1 precision for around 60€. All sets from known companies seem much more expensive.
Does anyone know of a good and budget friendly device to measure flange distance for adjustment (e.g. leica iii). What is commonly used. In the Leica repair manual they mention putting a mirror at the film plane and using a collimator but I imagine one could also use a depth gauge or similar?
Thanks again for your replies. And how do you actually to the adjustment on such a small scale on an old camera? you can get shims down to maybe 0.1mm or for that small screw size only 0.25mm. what are other options?
Thanks again for your replies. And how do you actually to the adjustment on such a small scale on an old camera? you can get shims down to maybe 0.1mm or for that small screw size only 0.25mm. what are other options?
Thanks again for your replies. And how do you actually to the adjustment on such a small scale on an old camera? you can get shims down to maybe 0.1mm or for that small screw size only 0.25mm. what are other options?
calipers will do the trick but most camera flange distances are known and can be looked up on the Internet or in books. For the Leica III I find a flange distance of 28.8 mm.
a sheet of paper has a thickness of 0.1 mm
For some cameras (e.g. Nikon F80) the mount flange distance is measured between the mount and the outer rails. Best to check the repair manual first.To give an update: measuring the distance with the above setup with a clock-like depth gauge seems to work quite well. Though I guess you should position the gauge plate with the polished side on the inner rails (the polished side is 9mm wide and 35mm tall I think as per ISO standards, mine was 20mm thick, Grade 1).
I see two problems with a caliper even if it‘s at 0.01mm precision (most are .1mm precision btw., which is 0.004inch or one sheet of office paper): 1) vertical placement (can you really place the depth rod perfectly vertical? The depth gauge has this wide and long flat feet on both sides that you can place on both sides of the bayonet ring. 2) precision of the surface of the depth rod tip. The precision depth gauge has a defined rounded tip of hardened steel to achieve the required precision, while most caliper I saw have a flat tip of the depth rod, making it difficult to perfectly align with the surface.
I think this hassle is less of a problem for SLRs since errors in flange distance do not affect focusing. However I think there is a reason for the small tolerances for rangefinders. Example: after a first rough adjustment I had only one (of 4) screws of the bayonet off by 0.10mm on the hexar RF (leica M mount). Using a split-prism focusing screen I adjusted the rangefinder for an object at 7meters (23feet) using a 90mm f2 lens (leica summicron), which is really close to infitiny on this lens. However the infinity was very visibly misaligned in the rangefinder. Correcting this one problem led to a good calibration of the rangefinder throught. So the effect of a single sheet of paper off (0.10mm), only in one corner!, was really visible in the center.
The required precision for setting the flange distance can be understood by considering the depth of focus at the film plane. A common value for acceptable circle-of-confusion for 35mm is 0.03mm, meaning a spot size of 0.03mm is considered "acceptable." Most depth-of-field marks on 35mm lenses correspond roughly to this choice. However, for critical focus/best results many people want to do a little better, and rate depth-of-field one or two stops more conservatively (a factor of 1.4-2x in spot size).
Let's say we are going to be about 1.5x more conservative, so our acceptable spot size is 0.02mm. Further let's say that we want to be able to achieve this with a f/2 lens. An f/2 beam emerging from the lens is a cone whose taper is: length is a factor of 2 times the base diameter. This means that the focus offset (length between the true plane of focus and the film) can be 2x the spot size, 2 x 0.02mm = 0.04mm. So the maximum error of the position of the film plane is +/- 0.04mm. That's really quite small - 40 microns, about half the diameter of a human hair. And, if one is 0.04mm off, one has used up all the conservative depth of focus in the film plane (depth of field in the subject plane) at f/2, which is not desirable. So when manufacturers specify the flange-film distance to 0.04mm or better, they aren't just being persnickety. It really does need to be that accurate.
As others have mentioned, you might get away with a small error with an SLR, as long as the focusing screen is off by the same amount (IOW, if you move the lens flange in or out, it should still focus ok, but the focusing scale may be off and you might not reach infinity). But for an interchangeable lens rangefinder it's got to be quite close.
I was negative early in the thread about improvising the measurement, because typical home tools like a caliper aren't precise or accurate enough. As others have said, a dial indicator, an accurate length standard (gauge block), and a very rigid fixture are needed. I'm glad to see that people have gotten it to work mechanically. If one isn't ready to buy that gear, I think optical alignment (autocollimation) is better than trying a vernier caliper.
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