Canon lens cams & compatibility question

[Charles D. Orze]

Hi all,

I'm sure there must be a thread somewhere on this but I can't find it. I've noticed that some of my Canon screw lenses (my 50 f1.8, for instance) have a cam that protrudes just a smidge less than other similar screw mount 50s. On the R-D1 it is just shy of perfect match in the rangefinder at infinity and close up wide open the focus is slightly forward of where it should be. But on the Canon P focus is spot on. Now on some lenses it is easy to modify the amount the cam protrudes, but on the f1.8 Serenar there is no provision to make the cam longer! Is there a systematic difference in the mount to film plane distance in old Canons as opposed to old Leicas?

Thanks

Charlie

 

[John Shriver]

If the cam doesn't protrude exactly 7.5mm from the mounting plane when the lens is focused at infinity, something is wrong. There is no incompatibility between Canon and Leica LTM cameras.

It's possible that the cam is on the wrong start of the threads. However, I don't remember exactly how that helical is assembled.

Or, the real problem may be that the lens isn't poperly collimated for infinity, has the wrong shim, or no shim, in it.

Any of the Leica-specialized repair people should be able to put the lens in order. You need quite a few special jigs to do it right.

 

[Charles D. Orze]

Canon cam

Canon cam

Hi John,

Thanks for the reply. The cam is indeed short by about .25mm. The Serenar with focus lock has a single cam, multiple entry thread but no provision for adjustment--the lock is part of the single-piece machined cam itself and there is only one position which brings it flat home in the right spot with no additional clearance. Certainly I can shim the lens (I've been working on all sorts of optics, large and small for forty years) to make it focus correctly on my R-D1, but this does not address the fundamental problem. What I find surprising is that it seems to work perfectly on my Canon P. Therefore my question--I do remember seeing somewhere that some Canons were set up to a slightly different distance. But now I can't find it.

 

[Charles D. Orze]

Dantestella's wisdom on it

Dantestella's wisdom on it

Hi,

Dantestella's site confirms my analysis of the construction of the 50mm f1.8 Serenar:

"the infinity stop is set for where the optical helix is when the optics are focused at infinity, and the back of the helix is ground until the test rangefinder reads at infinity. This is the type of mechanism used on Canon rangefinder lenses."

In short, this cam appears to have been ground down too far. But this still leaves the good performance on my Canon P--perhaps just a coincidence of it being "off" in the correct way?

Best,

Charlie

 

[Charles D. Orze]

Cam mystery deepens

Cam mystery deepens

Hi folks,

I took out all the Canon lenses I have with me (50mm f1.2, 35mm f1.5, 50mm f1.8, 100mm f3.5) and my CV lenses (28mm f1.9, 50mm f1.5, 75mm f2.5) and compared the cams back to back fashion at infinity. All the Canon cams are exactly the same length (consistency leads one to surmise this is intended). But the Canon cams are shorter than the cams of the CV 28 and 75 by a tiny amount (about a quarter of a millimeter). The Canon cams are also a lot shorter than that of the CV 50mm which is a good millimeter longer. Does anyone know why the CVs are longer (assuming the 50mm is out of spec)?

Thanks,

Charlie

 

[Xmas]

Charlie

If they all focus at infinity ok
They should all have the same depth between the lens flange flat and the cam flat, they should be the same better than 0.001 of an inch.

The RD1 is more likely to have a problem sorry.

Noel

 

[Charles D. Orze]

Cams

Cams

Ah yes-there is the rub--they are not all the same length as I noted above, and while they all focus to infinity on the Canon P the Canon 50 f1.8 does not quite reach infinity on either the R-D1 or on the Leica M5 (actually, the CV 50 nokton doesn't either!). Maybe all three of my adapter rings are out of spec (I swapped out all three to see if one of them got things closer on the R-D1 or the M5--one did, but not enough to reach full infinity. Hmmmm

Does anyone remember what the thickness of the screw to M adapter rings should be or where that thread is? I'm trying to eliminate some of the variables.

Best,

Charlie

 

[John Shriver]

The M adapter ring should be exactly 1mm thick.

Digital sensors are fussier about exact focusing, the light sensitive portion of the sensor is thinner than film.