Link to post on M lens calibration

[jim_buchanan]

Hi, folks.

Here is a link to a post I wrote on my experiences with the Leica rangefinder and M lenses:
http://www.l-camera-forum.com/leica-forum/leica-m8-forum/108069-study-m-lens-focus-accuracy.html

 

[Chriscrawfordphoto]

If you shim the lens like that you screw up close focus accuracy. What you're essentially doing is making the lens slightly front focus. At infinity, things closer to you will be sharp since you've actually shimmed the lens to focus at a closer distance when set at infinity, and depth of field ensures that far off things are sharp too. Problem is, at close distances, your front focusing leaves the actual focused distance too far behind the point the lens is really focusing on due to your shim.

Why didn't you just adjust the rangefinder of your camera for proper infinity focus? If the shop you went to wouldn't do it right, find another, but dont mess up your lenses!

 

[Dante_Stella]

The OP is on the right track. Some lenses need optical recalibration (adjusting the physical position of the optical cell) to be usable due to issues with focus shift (e.g. 75mm Summilux) or miscollimation (Leica's historic tolerances were apparently much greater than a flat sensor will tolerate). Changing the RF calibration will not help (and will hurt performance with lenses that don't suffer from these problems).

Dante

If you shim the lens like that you screw up close focus accuracy. What you're essentially doing is making the lens slightly front focus. At infinity, things closer to you will be sharp since you've actually shimmed the lens to focus at a closer distance when set at infinity, and depth of field ensures that far off things are sharp too. Problem is, at close distances, your front focusing leaves the actual focused distance too far behind the point the lens is really focusing on due to your shim.

Why didn't you just adjust the rangefinder of your camera for proper infinity focus? If the shop you went to wouldn't do it right, find another, but dont mess up your lenses!

 

[woodleica]

The OP is on the right track. Some lenses need optical recalibration (adjusting the physical position of the optical cell) to be usable due to issues with focus shift (e.g. 75mm Summilux) or miscollimation (Leica's historic tolerances were apparently much greater than a flat sensor will tolerate). Changing the RF calibration will not help (and will hurt performance with lenses that don't suffer from these problems).

Dante

Does that imply that purchasing some of the newer lenses ( such as the summarits) may produce better results at less price overall than a really expensive older lens?

 

[Dante_Stella]

More modern lenses (ASPH, APO) seem to suffer this less. I would imagine that anything made after the M8 should be fine. I think with the older ones, it is a combinatin of focus shift, looser tolerances for collimation, and collimation for a different type of surface. Summarits should be fine. They're actually impressive little lenses...

Dante

Does that imply that purchasing some of the newer lenses ( such as the summarits) may produce better results at less price overall than a really expensive older lens?

 

[Olsen]

I bought this Leica 35 mm 2,0 2.hand a few years back, now. I sent it to Leica for calibration. Which they did. Free of charge. I did not even pay the transport cost via the Norwegian dealer Interfoto.

 

[jim_buchanan]

If you shim the lens like that you screw up close focus accuracy. What you're essentially doing is making the lens slightly front focus. At infinity, things closer to you will be sharp since you've actually shimmed the lens to focus at a closer distance when set at infinity, and depth of field ensures that far off things are sharp too. Problem is, at close distances, your front focusing leaves the actual focused distance too far behind the point the lens is really focusing on due to your shim.

Why didn't you just adjust the rangefinder of your camera for proper infinity focus? If the shop you went to wouldn't do it right, find another, but dont mess up your lenses!

I'm thinking you didn't read the post critically. Your reply really doesn't make any sense. Sorry. In case I wasn't clear, I have perfect focus with all 3 lenses from 1 meter to infinity.

 

[jaapv]

The OP is on the right track. Some lenses need optical recalibration (adjusting the physical position of the optical cell) to be usable due to issues with focus shift (e.g. 75mm Summilux) or miscollimation (Leica's historic tolerances were apparently much greater than a flat sensor will tolerate). Changing the RF calibration will not help (and will hurt performance with lenses that don't suffer from these problems).

Dante

Chrtis is 100% right. The optical cell and the RF coupling are two separate adjustments. If you move the whole lens at the mount to "adjust" the one you screw up the other. If you want to adjust the optical centre-sensor distance you change the shims inside the lens that control that tolerance. If you want to adjust the RF mechanism at lens level - which is the most likely cause of non-aligning RF patches in this case you adjust the helicoid.

 

[jim_buchanan]

Chrtis is 100% right. The optical cell and the RF coupling are two separate adjustments. If you move the whole lens at the mount to "adjust" the one you screw up the other. If you want to adjust the optical centre-sensor distance you change the shims inside the lens that control that tolerance. If you want to adjust the RF mechanism at lens level - which is the most likely cause of non-aligning RF patches in this case you adjust the helicoid.

This is a good point to bring up, but I'm not completely sure it pertains to my scenario. Most of my responses are in the original thread. If my camera and lenses were all over the map in its adjustments, then I would not be hit or miss on this. After accurate infinity adjustment, all 3 lenses were back focusing about 10mm on 1 meter. Not much in the first place. By tuning the roller follower a very small amount to bring the 2 images together, the shift in focus at the short end eliminated the 10mm back focus. To restate, after I've adjusted and confirmed infinity focus, the backfocusing went away on all 3 lenses. Lucky Me!

 

[jaapv]

Well done. 🙂 I will not follow your example however 😉

 

[aldobonnard]

actually a very good point. I've had to send my 2nd hand lux and cron to Leica for lens recalibration (decentring on 1 and optical centre move on 1 lens, possibly due to shocks, and some internal rings to be changed). The result is brilliant now.
2nd hand lenses should be bought with having in mind that they will need a full calibration service in the first place.

 

[jim_buchanan]

Well done. 🙂 I will not follow your example however 😉

That's OK with me. If I had paid 3 times as much for a M9 like I see in your photo, I would make them correct any imprecision.

 

[jim_buchanan]

Was this a problem with just these three lenses? I was thinking that a single shim on the camera bayonet might have brought all the lenses within acceptable range.

Well, the 3 lenses mentioned are the only ones I have at the moment, but each focused differently at the infinity stop. The 90 over focused by a good amount, and a .0015 shim brought it in. the 50 was right on and needed nothing. The 28 focused at infinity but was over focused, and a .001 shim brought it in. (See the original post) Even if they were all even, I'm not sure I would want to remove the camera lens mount and fashion a shim for under it. Its too easy to shim the bayonet. Be aware of the difference between shimming the optical cell and the mount. Shimming the optical cell would require major disassembly of the lens and I'm not even certain thats possible, even though jaapv says it is, and I have no reason to doubt it.

 

[regular]

I have a question : when a lens is recalibrated for a M8/M9 camera, can it be used optimally on any other M8/M9 ? Or do you need to recalibrate it again and again?