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Canon Rangefinders - Peter Dechert and Peter Kitchingman Peter Dechert is best known for his Canon Rangefinder, Canon SLR, and Olympus Pen books, the latter two long out-of-print. He was a monthly columnist for many years for SHUTTERBUG magazine, and has contributed to many others. Most recently he has written about the pre-WW2 Zeiss 35mm cameras, but his interests in camera equipment and optics are many and varied. As a pro protographer and honorary life member of ASMP, Peter is also expert in using the gear! Peter Kitchingman - author of Canon Rangefinder Lens book Peter Kitchingman's 'Canon M39 Rangefinder Lenses 1939-71' book is the definitive source on these very interesting optics. His interests also go to the entire Canon Rangefinder system and beyond.

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Only Infinity
Old 10-07-2018   #1
mark_pw
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Is it true that the focusing thread of Canon M39 lens is slightly different from that of Leica M39?

I've bought a Canon 50mm f1.4 LTM from eBay and put it on by Leica CL, the developed photos showed the focus is only correct at infinity. Other closer targets were off.

Any comment?

Mine is the same as the one in the link below:

https://ebay.to/2Qxg4b0
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Old 10-07-2018   #2
leicapixie
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I purchased a Leica CL 90mm f4, which would NOT focus correctly on M3 and M2.
Wrong at infinity,,almost correct on 1 M, but totally out at close up (nearest).
Mid distance same bad!
Amount of out of correct focus varying on my M's.
I returned the lens.
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Old 10-07-2018   #3
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I’ve got many canon screw mount lenses and they have always worked well on both my leica screw mount cameras, leica screw mount copies and also my M5. If your other lenses work on your camera, then it’s likely your lens is not correct.
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Old 10-07-2018   #4
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No, that's not true.
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Old 10-07-2018   #5
Peter Jennings
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Any lens can have problems - especially an old one. Canon and Leica mounts are identical. Are you sure it’s the lens that’s off and not the camera? Do you have another camera to check it against? Tracking down the exact source and cause of a focus issue can be tricky.
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Old 10-07-2018   #6
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Well, the (original) Leica CL is a nice and good camera, but it's certainly not the first choice for f1.4...
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Old 10-07-2018   #7
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For decades I've used Canon 50/1.2s* on Leicas and had no problems, so I think you have a duff lens.

*Plural because of usual problem: bought one, sold it, regretted it, bought another...

Cheers,

R.
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Old 10-07-2018   #8
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First it is too broad a generalization to say Canon and Leica have the same lens mount. The original Canon thread mounts were 38.5mm x 1.1mm pitch and then true M39. Leica is NOT M39 because the thread pitch is not metric; it is 26 threads per inch. Leica should be called L39 because it is not M39. Canon switched to L39 in 1952. Leica thread mount was always L39.

So your 50mm f1.4 is L39 lens mount.

Now the question of the thread on the focusing mount in the lens. What has to be the same on all LTM and LBM lenses is the movement of the lens tab on the focusing cam. (I use the term "tab" even though on some Leica lenses it is actually a full circle tube.) Essentially the rangefinder doesn't know and doesn't care what focal length lens is mounted. It is just "solving" the triangle based on the two camera windows and the subject distance. The focusing pitch can be different for a given focal length as long as the focusing tab movement is the same for a given subject distance. On the 50mm Leica lenses, the back of the moving lens element (a full circle normally) bares directly on the focusing cam, so the pitch of the focusing mount has to be identical on all 50mm lenses. (There may be 50mm lenses that I don't know about for which this is not true. It is definitely true of the Canon 50mm 1.4) On longer focal length lenses the focusing mount has to have a steeper pitch because the glass has to move more to focus. On wide angle lenses the focusing mount has to have a shallower pitch because the glass has to move less to focus. It is hard to see on a wide angle lens, but the back focusing surface movement and the front of the lens do not move the same distance when focused. It can be easily examined with a caliper.

In control system terms the 50mm focusing mount has 0 gain. The telephoto focusing mount has negative gain. And the wide angle focusing mount has positive gain.

As an aside the Canon 0.95 is an example of a 50mm lens that has a tab instead of a full circle. But the movement is still 1 to 1.

Discussing further, the distance required to move a certain focal length to achieve focus is the same regardless of lens optical design. So the gain for a given focal length is the same regardless of the amount of rotation needed to focus the lens. You can have short throw and long through lenses based on focusing mount thread pitch, but the focusing tab movement has to be the same for a given subject distance.

Since these mechanical characteristics are machined into the lens, they are impossible to change. What is adjustable are: the focusing tab position by shimming the mount which is how a manufacturer accounts for small differences if as-built focal length; and the movement of the cam arms in the rangefinder. Also there is the possibility that the lens has been taken apart and reassembled incorrectly.

In my opinion, and it is just my opinion, the only way to check the lens shimming and the rangefinder adjustment with precision accuracy is with test equipment owned by a good camera repair shop. I don't feel that the rangefinder in the camera is accurate enough to check infinity focus. I know others in the forum regularly repair lenses and cameras without this equipment, but I feel that the best results are achieved with the equipment.

So my advice is to take or send the lens and camera reliable camera repair shop.
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Old 10-07-2018   #9
Roger Hicks
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Quote:
Originally Posted by View Range View Post
First it is too broad a generalization to say Canon and Leica have the same lens mount. The original Canon thread mounts were 38.5mm x 1.1mm pitch and then true M39. Leica is NOT M39 because the thread pitch is not metric; it is 26 threads per inch. Leica should be called L39 because it is not M39. Canon switched to L39 in 1952. Leica thread mount was always L39.

So your 50mm f1.4 is L39 lens mount.

Now the question of the thread on the focusing mount in the lens. What has to be the same on all LTM and LBM lenses is the movement of the lens tab on the focusing cam. (I use the term "tab" even though on some Leica lenses it is actually a full circle tube.) Essentially the rangefinder doesn't know and doesn't care what focal length lens is mounted. It is just "solving" the triangle based on the two camera windows and the subject distance. The focusing pitch can be different for a given focal length as long as the focusing tab movement is the same for a given subject distance. On the 50mm Leica lenses, the back of the moving lens element (a full circle normally) bares directly on the focusing cam, so the pitch of the focusing mount has to be identical on all 50mm lenses. (There may be 50mm lenses that I don't know about for which this is not true. It is definitely true of the Canon 50mm 1.4) On longer focal length lenses the focusing mount has to have a steeper pitch because the glass has to move more to focus. On wide angle lenses the focusing mount has to have a shallower pitch because the glass has to move less to focus. It is hard to see on a wide angle lens, but the back focusing surface movement and the front of the lens do not move the same distance when focused. It can be easily examined with a caliper.

In control system terms the 50mm focusing mount has 0 gain. The telephoto focusing mount has negative gain. And the wide angle focusing mount has positive gain.

As an aside the Canon 0.95 is an example of a 50mm lens that has a tab instead of a full circle. But the movement is still 1 to 1.

Discussing further, the distance required to move a certain focal length to achieve focus is the same regardless of lens optical design. So the gain for a given focal length is the same regardless of the amount of rotation needed to focus the lens. You can have short throw and long through lenses based on focusing mount thread pitch, but the focusing tab movement has to be the same for a given subject distance.

Since these mechanical characteristics are machined into the lens, they are impossible to change. What is adjustable are: the focusing tab position by shimming the mount which is how a manufacturer accounts for small differences if as-built focal length; and the movement of the cam arms in the rangefinder. Also there is the possibility that the lens has been taken apart and reassembled incorrectly.

In my opinion, and it is just my opinion, the only way to check the lens shimming and the rangefinder adjustment with precision accuracy is with test equipment owned by a good camera repair shop. I don't feel that the rangefinder in the camera is accurate enough to check infinity focus. I know others in the forum regularly repair lenses and cameras without this equipment, but I feel that the best results are achieved with the equipment.

So my advice is to take or send the lens and camera reliable camera repair shop.
An excellent explanation: thank you.

One further point, though, is that the coupling on a given rangefinder camera may not be designed or adjusted for a 50mm "standard". Sure, it is possible to machine a cam so that (say) a 50.8mm/ 2 inch lens "pretends" to be a 50mm for the purposes of triangulation, but equally, the camera may be designed to couple with a 50.8mm lens, and a true 50mm may have to "pretend" to be 2 inch.

See also the Leica habit of engraving precise focal lengths on lenses, as with SOOZI the 90mm Summicron on my .eu site, about 80-90% through.

Cheers,

R.
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Old 10-07-2018   #10
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Thanks for the addition, Roger. The exact design focal length for the "0 gain" lens is extremely important. (This design basis has never been totally resolved for Nikon RF vs Contax RF cameras. The issue is that the lens element movements do not produce the same rangefinder movements in the two cameras resulting in the lenses not being interchangeably usable on the two systems. Some say the reason is that the Nikon is designed for a 50mm standard lens while the Contax is designed for a 58mm standard lens.)

I should also have added that the arm leverage lengths (rangefinder gain) inside the rangefinder have to be appropriate to solve the triangle for the design cam movement. So while Barnack Leicas (~ 39mm), the early Canon with integrated viewfinder / rangefinder (~ 39mm), later Canons (~ 44mm), Canon 7 (~ 62mm) and M Leicas (~ 69mm) have different rangefinder baselines, the two rangefinder images must merge for the same subject distance and cam movement. (Don't quote me on the rangefinder baselines. I have not measured them with any rigor.)
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