Infinity focus

[alex7075]

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That tiny distance is all it takes, we are talking fractions of a mm here - the focussing roller moves about 4mm as you focus from 0.7m to infinity and two thirds of that 4mm is used up by the time you get to 2m, meaning that the roller only moves about 1.5mm as you focus from 2m all the way to infinity.

 

I agree with HC's diagnosis; a lens like the 35/2 has a double helicoid; as you rotate the focussing ring, the focussing cam moves at one rate (X mm/degree rotation) and the lens head moves at another (Y mm/degree rotation), X and Y being determined by the machined pitch of the helicoids. For a wide-angle lens, Y < X, for a longer lens, Y > X, typically. X and Y are chosen to give as accurate focussing as possible which is then made exact by profiling the lens cam.

 

The infinity adjustment is done by rotating the helicoids with respect to the lens mount and each other; with the focussing ring at infinity, the helicoids are rotated in the lens mount (and secured in position by set screws) so that the focussing cam and lens head are both in the correct position, using the bayonet flange as a reference.

 

I think these two lenses need to go back to Leica for calibration.

 

On my Elmarit-M 28 ASPH the focusing cam is at the right distance (the rangefinder reaches infinity) but the lens head is 40 micron closer to the bayonet than it should be. Result: noticeable back-focus.

 

The distance of the group focusing cam + lens head from the bayonet can be changed operating on a ring screwed on the helicoid of the focusing cam, just under the bayonet, which determines the distance of the group from the bayonet and is kept in place by three screws. Turning it counterclockwise by a fraction of a millimeter is enough but it requires many tries.

 

I did that and now focus is spot-on at any distance; even if, obviously, the rangefinder cannot reach infinity anymore. Pictures at infinity are sharp anyway because of the depth of field of the lens even at maximum aperture.

 

I don't dare to go further and try to disassemble the lens head from the focusing cam...