Why focus calibration with rangefinder lenses is so difficult

Almost everyone who’s shot with a variety of rangefinder cameras and lenses will have had an experience like the following. You buy a lens with high confidence it will work properly, because it’s either brand new, or a pre-owned lens that’s been tested by the previous owner. The seller might even specify something like “this lens works perfectly on my M11 with no issues; focus is dead-on.” Then, upon receiving the lens and trying it on your own camera, a dreadful realization occurs: the lens consistently backfocuses or frontfocuses.

It’s a frustrating problem that makes the lens basically unusable without being sent to a professional for adjustment. It’s possible to compensate for backfocusing or frontfocusing by adjusting the camera’s rangefinder, but this will causes every other lens to be out of calibration.

Until recently, I assumed that this happened because some rangefinder lenses are simply defective from the factory or become out of spec after being used for a while. And I accepted a sad reality that buying a lens meant a roughly 25% chance it would be defective. Indeed, I’ve compiled a list of almost every rangefinder lens I’ve owned or rented since 2017, and the ones with focusing issues (listed in bold) make up about 1 in 4:

Leica 28/2 - backfocused
Leica 35/2 ASPH
Leica 35/1.4 pre-FLE - backfocused
Leica 35/1.4 FLE
Leica 50/2 rigid
Leica 50/2 v5
Leica 50/1.4 ASPH - 1 of 3 copies frontfocused
Leica 90/2 ASPH
Voigtlander 15/4.5 III
Voigtlander 21/1.8
Voigtlander 21/1.4
Voigtlander 21/3.5
Voigtlander 28/2
Voigtlander 28/1.5 - 1 of 3 copies backfocused
Voigtlander 35/2.5
Voigtlander 35/2 APO
Voigtlander 35/1.5
Voigtlander 35/1.2 III - frontfocused
Voigtlander 50/2 APO - 1 of 2 copies backfocused
Voigtlander 50/2.2
Voigtlander 50/1.5 II
Voigtlander 90/2.8 - frontfocused
Zeiss 35/2
Zeiss 35/1.4
Zeiss 50/2 - 1 of 2 copies frontfocused
Zeiss 85/4

But a recent experience with a 50 Summilux ASPH helped me discover that there’s more to the story than just defective lenses: flange-to-sensor distance variation makes the problem even more complicated.

I realized this after buying a practically unused, very recently manufactured 50 Summilux ASPH and testing it on my Leica M10. This is what I found when I focused the lens at infinity:

The lens was frontfocusing, to the point where at the infinity hard stop, distant objects were obviously out of focus. The fact that achieving infinity focus was impossible means that something was definitely wrong with the distance between the lens and sensor, rather than an issue with rangefinder calibration. I previously would have attributed this to yet another faulty lens, but I found it unusual that a Leica lens manufactured so recently would be this far out of spec. Furthermore, the previous owner noted that it focused perfectly on his camera, which was newer than my M10. I began to suspect the issue was not with the lens but the flange-to-sensor distance on my M10.

At this point it would help to take a step back and describe how rangefinder cameras focus.

Turning the focus ring on the lens moves the glass elements forwards and backwards relative to the lens mount and sensor. This changes the distance from the glass elements to the sensor, which changes the focus distance (lens closer to sensor = focuses on farther objects, lens farther from sensor = focuses on closer objects).

To allow for rangefinder focusing, the lens also has to have a mechanism that has no commonly agreed-upon name, but which I will call the “rangefinder helicoid.” This ring is designed to contact the arm of the camera’s rangefinder mechanism, and as the lens focus is changed, the rangefinder helicoid moves back and forth in a very specific way that is consistent among all rangefinder lenses. It is this precise movement of the rangefinder helicoid that allows the camera’s rangefinder to know if the lens is focused at infinity, 0.7m, or anywhere in between.

The red arrows in the photo below demonstrate this consistent design feature, regardless of where the glass elements sit in the lens.

Because both lenses are set to infinity in this picture, the rangefinder helicoid is in an identical position relative to the lens mount.

What this unfortunately means is that for a rangefinder camera and lens to focus correctly, at least four different variables must be correct:

The movement of the lens’s glass elements relative to the lens mount.
The distance from the lens mount to the sensor (also called flange-to-sensor distance).
The movement of the lens’s rangefinder helicoid.
The calibration of the rangefinder cam to the rangefinder mechanism.

If a single one of these variables is off by a significant amount, the lens will frontfocus or backfocus. And what’s perhaps more unfortunate is that the variance in the lens and camera can compound with each other. For example, a lens that is just slightly too far forward relative to the lens mount, used with a camera with a flange-to-sensor distance that is just slightly too long, can combine to create significant frontfocusing. And individually, that lens and camera may both appear to be functioning properly when used with other cameras/lenses.

Of these four variables, only #4 is user-adjustable, by applying a 2mm Allen wrench to the rangefinder cam (see below). Variables #1, 2, and 3 can only be adjusted by a technician.

Going back to my 50 Summilux ASPH, I decided to send both my M10 and the lens together to Leica for repair. This way the technicians could guarantee the lens focused properly on my camera, eliminating any guesswork with camera-to-camera variance. I also hoped that if there was anything out-of-spec with my camera, it would be fixed.

When I got the camera and lens back from Leica servicing, I was pleased to find that it focused perfectly, including at the infinity hard stop. But shortly after, I found an irresistible deal on a used Leica M10-R and bought it to replace my M10. To my dismay, my 50 Summilux ASPH backfocused on the M10-R. I considered that this could simply be due to the rangefinder calibration on the M10-R being off, but the situation seemed more complicated than that because my 35 Summilux FLE didn’t display the same behavior. It focused correctly on the M10 and M10-R, and it wasn’t possible to adjust the M10-R’s rangefinder so that it focused accurately with both lenses. I theorized that the Leica repair department addressed my frontfocusing complaint by shimming the 50 Summilux closer to the lens mount without inspecting the flange-to-sensor distance on my M10.

To investigate this theory I set up an experiment: I tested the 50 Summilux ASPH back-to-back on the M10 and M10-R. Both cameras were mounted to a tripod with an Arca Swiss plate, ensuring that they were in identical positions. I used tape on the Summilux’s barrel to lock the focus ring in the same exact position for the two cameras, and photographed an oblique surface to precisely show the focus distance.

It’s not the most obvious difference in the world, but the lens focused closer on the M10 than the M10-R. The only possible explanation for this is that the cameras have different flange-to-sensor distances, as the other variables have been eliminated. And the difference is large enough to make photos noticeably out of focus at f/1.4. I finally had proof that flange-to-sensor distance can cause rangefinder focusing problems, and sadly, it also showed that Leica “corrected” my frontfocusing complaint by compensating with the lens rather than addressing the root cause (the camera).

I’m not the only person to have thought about this. I highly recommend these two articles from the Lens Rentals founder, Roger Cicala:

The Great Flange-to-Sensor Distance Article: Part 1 — Cine Cameras

The Great Flange-to-Sensor Distance Article. Part II: Photo Cameras

The TL;DR is that they tested a huge sample of cameras with a specialty device for precisely measuring flange-to-sensor distance. They found a significant amount of variance, but this didn’t cause significant issues for the mirrorless and DSLR cameras because (1) mirrorless autofocus systems don’t care if the flange-to-sensor distance is off, (2) DSLR autofocus systems have a microadjustment setting that can fix most calibration issues, and (3) mirrorless and DSLR lenses focus past infinity by design, so a frontfocusing lens/camera combination can still focus to infinity.

Unfortunately for rangefinder cameras, flange-to-sensor distance variation is far more damaging, for multiple reasons. First, rangefinder lenses always have a hard infinity stop, so if the flange-to-sensor distance is too long, it’s impossible to focus at infinity. Second, it’s not possible to fully correct for flange-to-sensor variance by adjusting the camera’s rangefinder. A DSLR autofocus system can add specific microadjustment values for each paired lens, and focus perfectly with all of them despite flange-to-sensor variance. But with a rangefinder camera, any adjustment to the rangefinder cam will change the focus calibration of all lenses. Making it further complicated is that wide angle lenses are affected more by flange-to-sensor distance variation than longer focal lengths. This is why I was unable to adjust my M10-R to focus correctly with both my 35 Summilux and 50 Summilux, even though both lenses focused correctly on my M10.

Why am I taking the time to write about this? Because I hope that it clears up a lot of the uncertainty and frustration faced by people buying rangefinder cameras and lenses. The incorrect assumption that focus issues are only caused by defective lenses can create a lot of confusion. Take for example this nasty exchange of feedback on the Fred Miranda Buy & Sell forum:

Both the buyer and seller in this transaction are completely convinced of their contradicting realities regarding whether this lens is defective or not. As a result of this confusion, they both hate each other and money was lost in the process. The actual reality is that rangefinder focus calibration is so complicated that it’s near impossible to know for sure whether the lens was out of spec, one of the cameras was out of spec, neither, or both.

Other than the comfort of gaining understanding of the issue, what can be done? I think there are several lessons to take away from this:

Leica needs to reduce the amount of flange-to-sensor distance variation in their cameras. It’s possible they have already done this. I have noticed a trend that recently-manufactured Leica cameras paired with recently-manufactured Leica lenses have a high success rate in terms of focusing accuracy.
When Leica cameras are sent in for servicing, the technicians need to test for flange-to-sensor distance and bring it within spec of a newly manufactured camera. It’s disappointing they didn’t do this with my Leica M10.
If you own a rangefinder camera+lens combination that focuses perfectly, think carefully before selling either the camera or lens. There’s no guarantee that the next camera or lens will also be calibrated perfectly.
When buying a rangefinder lens, try to buy it in a way that allows you to test it on your camera before committing to the purchase. For new lenses, purchase from a store that allows free returns. When buying used lenses, prioritize local transactions that allow you to test the lens in-person (and save on shipping and fees, too).
When selling used rangefinder lenses online, strongly consider offering a return policy in case the lens isn’t calibrated to the buyer’s camera. I personally allow returns within 7 days of delivery if the lens is in the same condition and the buyer is willing to pay for the return shipping. Again, this does not mean the lens is “defective”; all that can be determined is that this specific camera+lens combination is not a good match.
If you do end up owning a camera+lens combination that is not focusing correctly, strongly consider sending both the camera and lens to Leica to ensure they are calibrated to each other. Unfortunately, there’s no way to do this for Voigtlander or other non-Leica rangefinder lenses.
Everyone with a Leica rangefinder should own a 2mm Allen wrench. Despite all the drama of this article, sometimes the problem is as simple as the rangefinder cam being out of calibration, and fixing this at home in 2 minutes is far better than sending it to a technician.

If you made it all the way through this article, congratulations and thank you for reading. Please use the knowledge to make the rangefinder community more courteous when buying and selling gear, and direct people to this article when focus calibration issues arise.