What exactly causes a lens to come out of alignment?

[Takkun]

Today I got two of my favorite lenses back from repair-- an Elcan 90 Tele-Elmarit, and a Canon 50 1.4 LTM. Both fairly suddenly started to severely front-focus both on film and digital, and both DAG and a local tech were able to fix them. What I can't figure out is what would cause this. Neither took any trauma that I know of.

In researching the matter I found a number of articles about shimming lenses, both professionally and hack jobs, but I can't help but wonder how a lens that presumably was working for a past owner no longer did. Any ideas here?

 

[colyn]

Possibly retaining rings holding lens elements in place started to become loose or de-cementing of the glued elements. Beyond that I'd have to say impact damage but you say that did not happen..so I go with the first possibility...

 

[peterm1]

It is true that an impact can affect alignment - I have had it happen to me when a lens took a tumble from a couple of feet onto a carpeted floor. No physical damage in the way of dents etc but a defintie loss of focus.

But I have to say I think its more common with digital Leica that its actually the rangefinder in the camera that drifts more so than the lens. At least this seems to be a more frequent occurrence in my experience as I am constantly struggling with it. I have seen reports by others (maybe it was Dante Stella) saying that M8s etc are prone and that frequent mounting/dismounting of lenses affects it. I dont know the reason for this as my rangefinder "guy" assures me that the rangfinder in a digital M is identical to those in the film Ms. But clearly judging from my own experience and the number of posts on the topic on the Web this is an issue for many, many people.

As to lenses I dont know whay it happens other than the reasons given in the other psot. Some lenses are able to be shimmed (for backfocus) but apparently not all allow this. BTW to test for the amount of correction required and as a temporary fix I have used finger nail polish to build up the tab on the back of the lens that engages the Ms rangefinder arm / wheel mechanism. When dried it becomes quite hard, hard enough for the rangfinder arm to engage against. I would not trust it for a permanent fix but for a quick fix it does work well enough so long as its alloowed to tdry thoroughly.

 

[NazgulKing]

Today I got two of my favorite lenses back from repair-- an Elcan 90 Tele-Elmarit, and a Canon 50 1.4 LTM. Both fairly suddenly started to severely front-focus both on film and digital, and both DAG and a local tech were able to fix them. What I can't figure out is what would cause this. Neither took any trauma that I know of.

In researching the matter I found a number of articles about shimming lenses, both professionally and hack jobs, but I can't help but wonder how a lens that presumably was working for a past owner no longer did. Any ideas here?

Alignments of this kind is often down to the millimeter. Miss by a little and the difference becomes very obvious.

If by anything, mechanical parts tend to run after a while, meaning they can get lose, misalign etc. after long enough use. This is why these days, camera parts have a combination of mechanical and electronic alignment corrections. Partly it is cheaper, but with electronic alignment tools, you can correct for mechanical misalignment.

 

[Phil_F_NM]

Even contraction or contraction of the chassis of the lens can affect focus. The RF in the Leica is calibrated to 51.6mm nominal focal length and this is the true focal length of every Leica 50mm except the DR Summicron which is 51.9mm. Any cam variation greater than about .3mm will yield you a gross focusing error, especially with a fast lens and a tele.
Two lenses won't come out of adjustment at the same time though. This is statistically possible but so unlikely that the cause would be camera RF alignment.
Do note that the 90 TE is very sensitive to focus on that digital Leica and I never got mine quite perfect. It seemed to hunt for focus and on a tripod it had a bit of front focus and a shift with regard to the aperture being used. I took some lovely portraits with it but I had to bracket the focus by breathing a bit to change my position while keeping focus the same and then shooting. When it was on it was ON. Stellar image maker, I just had very little luck on a Leica digital.

Phil Forrest

 

[Dan Daniel]

Certain types of vibration can loosen screws, retaining rings, etc.

I was once given a Minolta Autocord to overhaul by someone who couldn't understand why everything was loose and out of alignment. In discussing how she used the camera, it turns out that she did a lot of shooting at the front row of music concerts, so standing near the speaker stacks had basically vibrated the camera apart. I reassembled everything with medium loctite (which I never use in general), made sure every screw was cinched down, and four months later it all loosened up again.

Anyway, were the lenses exposed to vibrations before they went out? Airplane, motorcycle, speaker stacks....?

 

[Monochrom]

Hi, lenses don´t come off, what does is the rangefinder of your camera.

 

[kzphoto]

If it's a modern lens like a canon or nikon, the lenses have inside them adjustment collars that allow very small adjustments to the planarity of the optics. These adjustments ensure that the elements stay parallel to one another.

In older lenses, most of the time there is a small retaining ring or clip which keeps the glass pressed flat against it's intended mount. Occasionally things move, shift, or adhesives on the rings / clips may come lose causing the optics to move very slightly.

Alpa claims that a tolerance of a hundredth or thousandth of a millimeter can cause focusing issues with digital gear. It's pretty amazing to think that manufacturer can produce glass with such tolerances to keep photos sharp across the frame. It amazes me at least.

 

[mfunnell]

But I have to say I think its more common with digital Leica that its actually the rangefinder in the camera that drifts more so than the lens. At least this seems to be a more frequent occurrence in my experience as I am constantly struggling with it. I have seen reports by others (maybe it was Dante Stella) saying that M8s etc are prone and that frequent mounting/dismounting of lenses affects it. I dont know the reason for this as my rangefinder "guy" assures me that the rangfinder in a digital M is identical to those in the film Ms. But clearly judging from my own experience and the number of posts on the topic on the Web this is an issue for many, many people.

I think this is because any mis-adjustment at all is far more obvious - screamingly so - with digital than it is with film. I've just recently acquired an M type 240 which focuses all my lenses just about perfectly except my favourite, my Summilux 75. The Summilux focus with my film cameras (Hexar RF, M3) is just fine at all distances, but it is way off on my digital M. It is spot on at distances to about 2.5 meters but front-focuses more and more drastically with increasing subject distance. I'm pretty sure this is because the thickness of film allows more latitude, versus the all-or-nothing plane of the digital sensor. So my lovely old lens is off with Camera Clinic for a re-working of it's focus for digital. (They wanted both the camera and lens, but I'm taking a punt that since all my other lenses focus correctly, adjusting the 75 for digital will make it work just fine too.)

I understand, from the Leica bumf, that the RF in the M type 240 is engineered to be more resistant to going out of alignment than other Ms (film and digital) because of this very issue. I guess I'll see, over time, just how well they have or haven't achieved that objective.

...Mike

 

[Takkun]

Thanks for the responses, but I would like to clarify a few things:
1- not looking for advice; the lenses were in fact repaired and working nicely. More just curious about the nature of these problems.
2- it was not an RF alignment issue; they happened at different times and the RFs were calibrated professionally.
3- yes, the lenses do in fact go out of alignment, that's why they were in the shop!

Interesting about the Minolta story. I have a feeling that something similar might have caused my problems.

 

[semi-ambivalent]

The RF in the Leica is calibrated to 51.6mm nominal focal length and this is the true focal length of every Leica 50mm except the DR Summicron which is 51.9mm.

I have a 50 Summicron V4 that is marked '22' right on the focusing ring. All the Leica literature I have been able to find says this means the focal length of my lens is 52.2mm. What are we to make of that?

Thanks,
s-a

 

[NazgulKing]

I have a 50 Summicron V4 that is marked '22' right on the focusing ring. All the Leica literature I have been able to find says this means the focal length of my lens is 52.2mm. What are we to make of that?

Thanks,
s-a

Lens surface curvature... spacing between elements... The focal length changes with every bit of difference in these two parameters.

 

[John Shriver]

If the lens was disassembled to the point that an element was removed, and it was not re-seated properly, it would cause the focal length of the lens to change. The focusing cam on the lens has to match the focal length of the lens. So the re-repair may have been re-seating the element properly. The Leica elements can be very tight fits, some lenses you have to heat the barrel to expand it enough to remove and seat an element.

Both Leica and Canon would make the optical blocks, measure their focal length, and then match them to a focusing mount whose thread pitch was in the right focal length range.

In the Canon 50/1.4, another possibility is that infinity collimation was off the first time it was returned. There is a selected shim between the removable lens barrel and the focusing mount. It may have been left out, or some dirt caught in that area.

I don't know how the Tele-Elmarit is collimated. But it probably also has a adjustment for the length of the focusing cam, which would ride on a cam way up front in the focusing mount.

There's a LOT of things that have to be right for a Leica camera and lens to focus accurately.

There's a really practical cost reason that SLRs were dominant, all that has to be right is that the distance from the lens mount to focusing screen (via mirror) and lens mount to film (no mirror) is the same. That's done by shimming the screen and adjusting mirror tilt.

 

[John Shriver]

I'm afraid Phil's not right. There are multiple focal length ranges for Leica 50mm lenses. For instance, for Elmar 50/3.5 lenses, the focusing mounts were made for lenses of 50.1, 50.4, 50.7, 51.0, 51.3, 51.6, and 51.9mm focal length. This is indicated by the single digit stamped on the bottom of the infinity lock. (See the Leica Lens Compendium.)

So an Elmar of 50.2mm focal length would go in a 50.1mm mount, and one of 50.3mm would go in a 50.4mm mount.

Different mounts have different thread pitches, and subtly different camming on the rangefinder coupling. The coupling has to be one particular distance from the lens mount when the lens is focused at (say) 3 meters. No matter what the focal length is.

The Dual-Range Summicron was made in only one focal length, because the focusing mount was so complicated. It was not practical to make the mount in different focal lengths. (They might have also had to make different goggles for each focal length group.) Optical blocks of the "right" focal length were used in the Dual-Range, the others were used to make "Rigid" Summicrons.

As for why is it hard to control the focal length of lenses, the reason is that it is easy to control the radius of curvature of a lens element, but much harder to control the final thickness. Different thickness does affect the focal length (diopter) of the element. They pick a group of elements that are compatible, assemble, and measure the focal length.