Backfocus and Focus Shift: The Plot Thickens

[Guest guy_mancuso]

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When you send a lens in for coding , it is indeed true they completely replace the mount with a new coded one and than the lens is calibrated to spec. using whatever method they use in the manufacturing of the lens.

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[hankg]

Except we'd have another self-selecting survey again.

 

Look--I don't doubt for a moment that people who are having focus problems have actual focus problems.

 

But they should then get the lenses and camera checked.

 

Period.

 

There is no amount of conjecture or theory that will explain all those perfectly working 35 luxes, noctis, 28 crons out there in M8-land (mine included) without--for the love of pete (whoever that is)--taking into account the lens / camera connection first. No matter how many samples you've tried out of the box.

 

Case in point: my first M8 focussed just fine; the second one needed an adjustment.

 

But that does not mean camera or lens is flawed by design.

 

 

YMMV.

 

I agree.

 

I had my M8 RF checked (by DAG) and it was dead on. Every used lens I bought neeeded adjustment (4 lenses), some where off a mile. DAG couldn't adjust a new Leica 90/4 so that was returned. My Leica 28/2 was good out of the box.

 

There is nothing different about the M8 focus wise other then the fact that now you can check focus at 1000% instantly. So a lot that went unnoticed is or only noticed after lots of shooting and printing is being seen as soon as the lens is out of the box.

 

So first -make sure your RF is calibrated then if you have problems have someone competent adjust your lenses.

[Chuck Hatcher]

To clarify part of your statement; the focusing cam that moves the rangefinder roller inside the M8 body is not associated with the lens mount on most M lenses...

 

Oops, you are right, of course. I was looking at the lens mount and mistakenly identified the machined cutout as the part the rangefinder roller contacts. Now that you mention it, since the bayonet never moves (once the lens is attached), it could not possibly be used for focusing. That makes me feel better about having older lenses coded since there is less that could go wrong. Still, I would guess they must have to shim the new mounts to position the optics the correct distance from the mount face.

 

Several have mentioned having lenses shimmed or adjusted for correct focus. Is that a routine part of CLA at reputable repair shops, or something you have to ask for specifically?

[nzav]

Still, I would guess they must have to shim the new mounts to position the optics the correct distance from the mount face.

 

Several have mentioned having lenses shimmed or adjusted for correct focus. Is that a routine part of CLA at reputable repair shops, or something you have to ask for specifically?

 

Any good shop would check the lens on an optical bench after working on it and before sending it back to the customer. If the mount is the same and the technician reassembled it using the original shims (if any), then there should not be any issues. Of course a new mount may not be exactly the same as the replaced one, so some adjustment may be needed.

 

The lens to focal plane is a precise fixed distance at infinity and this is what needs to be adjusted if the lens is disassembled. The focus cam is ground to match each lens' exact focal length (i.e. all 50mm lenses are not always 50mm!) and once it is focused accurately at infinity, the rest of the distances fall in line.