Lens RF coupling

[edwardkaraa]

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Hello. I have a slight problem with one of my lenses. My M240 RF is perfectly adjusted with all of my lenses but one. The lens focuses fine at infinity but the RF doesn't fully coincide, basically causing back focusing at all distances. The error is really very slight so I'm a bit reluctant to send the lens away for repair. Is there an easy fix that I can do myself? Thanks!

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[adan]

Short version - No.

 

Longer version - your camera RF works with all your other lenses, so it is not a camera adjustment problem. Which is unfortunate, since it is relatively easy to user-adjust the camera's rangefinder.

 

Your problem lens focuses at infinity, set via the focus scale, so the glass itself is likely in the proper location, relative to the flange on the lens and the lens mount ring on the camera.

 

Which leaves the lens's RF cam - the moving part inside the back of the lens, the rim of which touches the camera's RF roller and produces movement of the RF images when you turn the focus ring - as the likely problem part.

 

It is either out of position, or actually a bad part, with an error to its shape that doesn't correctly move the RF the right amount as the lens moves.

 

Diagnosing and working on that really requires major disassembly of the lens - there is not a simple screw or lever that can be moved to adjust the lens's RF cam position.

 

In the cut-away view linked below, the focus cam is the threaded brass tube immediately surrounding the rear lens element. In a 50mm lens, it simple moves with the glass. In any other focal length, it moves differentially from the glass via yet another thread. In either case, a fairly complex bit of interlocked engineering, not to be interfered with casually.

 

http://petapixel.com/assets/uploads/2011/05/cutaway4.jpg

[Lenshacker]

What lens is it, what type of RF cam is on the lens?

 

If the actual focus of the lens is good at infinity, but the RF is not being driven to coincide at infinity- the RF cam can be built up. I use copper tape, which is ~0.04mm thick.

 

nokton_coppercam by fiftyonepointsix, on Flickr

[adan]

A-Ha! We discover the source of your forum name.

[edwardkaraa]

Thank you very much Adan and Lenshacker. That's a great info you gave me, greatly appreciated. It's my new ZM 35/1.4. The back focusing is so tiny I wouldn't want to mess it up myself. So I will leave it as is.

[duckrider]

Just for curiosity: does Your 85/2 focus correctly?

In Your case I would send the 35/1.4 lens to Zeiss for overwork the problem.

Thomas

[edwardkaraa]

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Yes the sonnar is quite accurate. I would say the distagon is within tolerance so I'm reluctant to send it away for such a tiny issue.

[sklocke]

 I use copper tape, which is ~0.04mm thick.

oppercam by fiftyonepointsix, on Flickr

 

Interesting discussion. I observe backfocus with the new ZM35 as well. In my case a single layer of household adhesive cures the backfocus, my estimation is 0.038 mm thickness (10 strips stacked and then measured). My only reservation is the weakness of the adhesive. This works so well that I am reluctant to send it back. 

 

I would like to ask Lenshacker about the make of the copper tape ? I checked the 3M offerings and they indicate a thickness of the copper layer of 0.036 mm but combined with the adhesive it is 0.066 mm. 

 

Stefan 

[edwardkaraa]

Please pardon my ignorance but where do you add the scotch tape? On the thin edge?

[sklocke]

Please pardon my ignorance but where do you add the scotch tape? On the thin edge?

 

This is my way of attaching the transparent adhesive tape. Cut a thin strip of a rolling dispenser and then pushed in place with a finger nail. Finally trimmed the end of. 

 

 

[edwardkaraa]

This is my way of attaching the transparent adhesive tape. Cut a thin strip of a rolling dispenser and then pushed in place with a finger nail. Finally trimmed the end of.

 

 

Thank you very much! That looks quite simple!

[rsolomon]

why and how does this solve the problem?  if i bring my camera and lenses to leica NJ can they do this procedure ?

 

thanks ! 

[pico]

I don't understand why adding thickness to the focusing cam will fix front-focus, but there are many thing I do not understand.

 

I will add that my 50mm Summicron had front-focus issues because it had become slightly unscrewed from the mount. A simple twist to seat it solved the problem.

[edwardkaraa]

The principle is quite simple actually. If the lens is further away from the ideal flange distance it will front focus/not reach infinity. If it's too close, it will back focus/exceed infinity. In this case, the lens is correctly collimated so the physical settings on the focusing ring are accurate. However the RF cam is slightly too short, making the camera think the lens is further away, front focusing. So the rangefinder will tell you to focus further back, causing back focus.

 

Now the difference is just the thickness of a thin scotch tape, as measured above to be around 0.03mm. It is very difficult to adjust such a tiny amount by a service center as it is normally within the margin of error, that is why if a simple fix like adding a layer of tape works, it would be preferable to sending the lens away.

[pico]

[...]If it's too close, it will back focus/exceed infinity.[...]

 

The very idea of seeing beyond infinity sends chills up my spine. Something from the Twilight Zone.

[edwardkaraa]

LOL

[Lenshacker]

 

 

I would like to ask Lenshacker about the make of the copper tape ? I checked the 3M offerings and they indicate a thickness of the copper layer of 0.036 mm but combined with the adhesive it is 0.066 mm. 

 

Stefan 

I put the 3M copper tape on the lens- check the focus, then run the side of a jeweler's screwdriver (Wiha) over the copper tape to thin it down. The adhesive has some give to it. 

 

Lens manufacturers make assumptions for their cameras, especially true in film cameras- about flatness. In this day and age of digital, the same may be true of microlenses and Dye in Mosaic filters. My M8 and M9 "disagree" about focus on some lenses but not others. I've calibrated a lot of lenses for other photographer's M9 using my own, get good agreement. Same with my M Monochrom and other M Monochrom's- great agreement. Use the M9 to calibrate for the M Monochrom, "ever so slight" disagreement. Usually the disagreement is within a 0.02mm shim. Sometimes can be used to an advantage; The C-Sonnar is perfect at F1.5 on the M Monochrom and is perfect at F2 on the M9. 

 

- Brian

[rsolomon]

does anyone know if i sent my lenses to leica can they make this modification adjustment ?

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[sklocke]

I put the 3M copper tape on the lens- check the focus, then run the side of a jeweler's screwdriver (Wiha) over the copper tape to thin it down. The adhesive has some give to it. 

 

Use the M9 to calibrate for the M Monochrom, "ever so slight" disagreement. Usually the disagreement is within a 0.02mm shim. Sometimes can be used to an advantage; The C-Sonnar is perfect at F1.5 on the M Monochrom and is perfect at F2 on the M9. 

 

Thank you Brian for the reply, do you recall what kind of 3M copper tape you use? I will try to get 1181. 

 

Regarding the differences between M9 and MM9 (and M240 in my case). I have spend a lot of time adjusting the rangefinder of all three. I never managed to achieve agreement so that the Zeiss 85 F2 could be used with equal satisfaction on all three. When giving the same task to Leica ( with more/other lenses in the equation) they did not achieve this either. I now think it is not possible, especially for lenses with an inclined brass "Cam" if that is the word (like 50 Summilux). This category has to react differently when the rangefinder arm is adjusted longer or shorter since the contact point of the roller is shifted. Since then I have split lenses into those that work on the MM9 and others that work on the 240. For some of these, I employ strategies like focussing from infinity or from near (clockwise or counter-clockwise turning only, good for offsets less than 0.02mm). 

 

With admiration if have followed the adventures of lindolfi, never dared to be this bold however: 

http://www.l-camera-forum.com/topic/107823-nokton-50mm-f11-backfocus/page-2

 

Question did you ever tackle front focusing issues? Are you using a diamond-file or take lenses apart? 

 

Stefan 

[sklocke]

does anyone know if i sent my lenses to leica can they make this modification adjustment ?

I think they can (especially if not too many other body or lens dependencies exist). Leica will work against a calibration standard and this means that they need your camera body most likely as well. I understand that with the M240 body more of the body site calibration can be automated, the lens work seems to me more of a human judgement call.