How can two lenses couple differently at infinity?

[mdemeyer]

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I have a number of lenses - Leica, Zeiss ZM and Voigtländer - and a 1 year old M10. Two of the lenses, a Summilux 50 ASPH and an Elmarit 90 (last version) don’t bring the RF quite into alignment when at the infinity stop. The others all do. The Summilux was back focusing a bit so I sent them both to DAG - the 50 to be adjusted and the 90 to be 6-bit coded and checked.

 

Subject to more thorough testing, they both seem to be focusing accurately close in with the RF and sharp at infinity when dialed to the infinity stop. But the RF patch is still noticeably mis-aligned at infinity.

 

Since my other 7 lenses all are correct regarding the RF patch at infinity, I checked with Don and he said that this seems like a body issue. But my (simplistic) mental picture of the RF coupling mechanism when at infinity says that the part of the lens that touches the cam in the body needs to be at a certain place and that place should be the same for all lenses. So, if the body were adjusted to correct the infinity point on one of these lenses, it would be off on the 7 that are correct now.

 

What am I missing?

Edited June 14, 2018 by mdemeyer

[jdlaing]

Perhaps the two that don’t align at infinity will align if you go to the stop and back up just a little?

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[mdemeyer]

They don’t go past alignment. The RF patches never reach alignment. So it’s not the lens going past infinity.

Edited June 14, 2018 by mdemeyer

[Exodies]

I had a Summicron 90 Apo where the patches didn’t align by the time infinity was reached. It had previously been ok. I sent it to Leica and it came back perfect.

[Mark II]

My Summilux 50mm ASPH does not quite reach infinity. However, the calibration is otherwise spot on and there does not seem to be any issue even with shots made wide-open at any distance.

[adan]

Allow me to "unpack" the mechanics of Leica's rangefinder focusing a bit.

 

Let's start with the "fixed" - or at least unchanging from lens to lens - RF mechanism in the camera. Through levers and fulcrums it takes an in-and-out movement of the lens or camera roller, and transforms it into an angular movement of the prism that transmits the secondary rangefinder-patch-only image in the viewfinder. We see that angular movement as a "panning" side-to-side movement, such that we can focus the lens until the two RF images "pan" into alignment. The geometry of the fulcrums and levers does not change, and therefore there is a fixed relationship between the amount of lens movement in and out, and the amount of side-to-side movement indicating a given focusing distance.

 

Thus (made-up numbers for illustrative purposes) a 1mm movement of the lens forward from infinity will always produce, perhaps, a 2-degree rotation of the prism and movement of the RF image, thus indicating, perhaps, that focus is now at 50 feet/15.15 meters.

 

HOWEVER - that only applies to a single focal length - longer lenses will move more to get from infinity to 50 feet, and shorter lenses will move less to change focus from infinity to 50 feet. For reasons of historical contingency, the geometry of the Leica rangefinders assume one is always using a focal length of 50mm exactly (the original 50mm Elmar - except not even then).

 

Therefore, any lens that is not exactly 50mm in true focal length will misfocus, if the roller simply touches the back of the moving optical cell (barrel holding the actual glass). Instead, every lens Leica makes has a cam: a separate brass inner barrel that moves in and out to simulate the movement of an ideal 50mm lens, while the actual lens moves in and out a different amount. Deep in the lens, the cam that touches the RF in the camera is threaded inside the focus ring that moves the glass, with a differential threading that moves the cam 1% or 45% or 270% LESS than the actual glass movement (for 52mm or 90mm or 135mm lenses) and 70% or 42% MORE than the actual glass movement (for a 35mm or 21mm lens).

 

OK - we are good so far. The actual movement of the cam and rangefinder is now the same regardless of how much the actual focal length being used is moving. "50 feet" looks the same to the RF with a 21 or a 50 or a 90, even though the lenses are actually moving very different amounts.

 

But the next flaw in the ointment is that a "50mm" lens is not always (or even usually) actually 50mm in focal length. Most Leica "50s" are really something other than 50mm - they are 51mm or 52.2mm or 51.5mm in true focal length. "90s" may be 90mm, or 89.5mm, or 90.5mm, etc. etc. Leica builts a purported 90mm lens - and then tests it to see what it really is - 89.5 or 91mm, or whatever. And then tweaks the cam to reflect the real focal length and real movement needed for correct focusing - and also engraves a little 2-digit number on the focus ring beside the "m"-for-meters engraving, to show the actual measured focal length, for future reference by the repair techs if/when they need to know (and the enjoyment of Leica collectors ). Check your own 50s or 75s or 90s or 135s for the little numbers  - "16" on a 50 means it is really a 51.6mm lens; "22" means it is a 52.2mm lens; for 90s, "00" means it really is a true 90mm, and "10" means it is actually a 91.0m lens. Even the Barnack Elmars come in different "flavors" - from 50.1mm to 51.9mm.

 

BTW - this extra precision is usually only needed for normals and teles - wide-angles have enough depth fo field even at f/1.4 (or other factors such as focus shift that produce much bigger errors anyway) that the focusing geometry works fine regardless of whether a given "35mm" is actually 34.6mm or 35.5mm. So they do not have the "true focal-length" numbers engraved.

 

HOWEVER (again) - even that correction for true focal length is not always enough to get a high-performance lens into focusing spec at longer focal lengths and larger apertures, and therefore the back surface of the brass focus cam in a lens (which will contact the camera's RF roller) may be machined delicately, one lens at a time, to adjust the in-and-out movement (subtle hills and valleys) as the cam rotates with focusing, nailing focus at various points (with smooth transitions between points) for that specific unit.

 

But note that that means that if your camera RF roller that contacts the lens is off, to one side or the other, by even 0.1mm - it may be "feeling" a different part of the machined cam surface than Leica's factory-standard test camera did. Thus one may get generally good focusing, and correct focus at infinity using the stop/lens scale, but the RF images may not align perfectly at infinity (which is, after all, not possible to measure exactly anyway - the Leica RF geometry and focusing image will look the same whether you focus on a building 2 miles away, or the moon 250,000 miles away - either one counts as "infinity" to the rangefinder.

 

Note - the 90 APO-Summicron and the 135 APO-Telyt (and some other older lenses, e.g. early versions of the 75 f/1.4 and 90 f/2 (1980) or 135mm Tele-Elmar) actually have their cam surface way up inside the lens, with a narrow (blackened) spring-loaded brass bar or rail that transfers the in-and-out movement of the cam to the camera - via a tip like this -  \___/  - that is all you see sticking out the back. That system saves some weight, and also eliminates the problem of where your roller actually touches the lens feeler (it doesn't rotate side to side, it just moves in and out.) But is extra complexity not used in shorter focal lengths.

 

At any rate, the short version is there are lots of complexities working behind the scenes, and each complexity can introduce a tiny error here or there at times. It may or may not be enough to produce a noticable focus problem, for any given lens sample or photographer.

Edited June 14, 2018 by adan

[250swb]

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What am I missing?

 

If the range of focusing is otherwise accurate, and 'infinity' is achieved just by bumping up against the stop, why bother doing anything?

[mdemeyer]

It seems to me that, if the RF patch is off at infinity, it’s also off at longer distances. So, yes, I can turn to infinity when that’s what I want. But I also need accurate focusing at longer, but not infinity, distances.

 

Also, if 7 lenses work properly and two do not, I’m trying to see if Don’s suggestion (he is not being adamant) that the camera is at fault makes sense. The long version explaination above (thanks!) provides something to check - lateral positioning error in the roller - but that doesn’t seem likely to me since even my M-Rokkor 40mm with the sloping contact surface focuses precisely. If the roller were displaced laterally that one should have major issues.

 

If the range of focusing is otherwise accurate, and 'infinity' is achieved just by bumping up against the stop, why bother doing anything?

[wizard]

It seems to me that, if the RF patch is off at infinity, it’s also off at longer distances. So, yes, I can turn to infinity when that’s what I want. But I also need accurate focusing at longer, but not infinity, distances.

 

...

 

If 7 lenses work properly and 2 do not, I am pretty certain that the camera is NOT the culprit. Adan provided a superb explanation above, and it looks like two of your lenses may have a small problem. However, if the results you are getting with those lenses are good. then I would not change anything. In other words, if you get accureate focusing at whatever the distance is, why change anything?

[Fgcm]

Since my other 7 lenses all are correct regarding the RF patch at infinity, I checked with Don and he said that this seems like a body issue. But my (simplistic) mental picture of the RF coupling mechanism when at infinity says that the part of the lens that touches the cam in the body needs to be at a certain place and that place should be the same for all lenses. So, if the body were adjusted to correct the infinity point on one of these lenses, it would be off on the 7 that are correct now.

 

What am I missing?

 

Don is right. Adjusting properly the body, the patch will be perfectly aligned with ALL your lenses, including those that now seem to be different.

 

When the body is misadjusted, different lenses gives different alignment at infinity because the profile of the focusing cam is not flat / parallel to the flange, thus, if the lever is too long or too short, the roller touches the cam in the wrong position. 

So, what you are missing is that adjusting the arm and the roller in the body you change both the slope of the rangefinder and the exact position where the roller touches the focusing cam on the lens barrel.

[helged]

Allow me to "unpack" the mechanics of Leica's rangefinder focusing a bit.

...

 

Thanks Adan, this was quite a bit more than a bit... Much appreciated! Edited June 15, 2018 by helged

[jaapv]

It seems to me that, if the RF patch is off at infinity, it’s also off at longer distances. So, yes, I can turn to infinity when that’s what I want. But I also need accurate focusing at longer, but not infinity, distances.

That is not necessarily true. The geometry is a bit more elaborate than that.

Infinity is adjusted by a different control than close range; infinity is  by the roller wheel, close focus by the screw on the other side of the arm deeper inside the body. Theoretically mid-range could be adjusted by bending the arm but that is very rarely done/needed and certainly not DIY.

[mdemeyer]

Yes, close in can be fine (and seems to be) but my concern relates to focus at distance. There the RF is operating in the same range as infinity so, if infinity is improperly coupled, I believe the part of the focusing range near infinity will also be incorrectly adjusted. Right?

 

That is not necessarily true. The geometry is a bit more elaborate than that.

Infinity is adjusted by a different control than close range; infinity is by the roller wheel, close focus by the screw on the other side of the arm deeper inside the body. Theoretically mid-range could be adjusted by bending the arm but that is very rarely done/needed and certainly not DIY.

[jaapv]

That is correct. It is a gradual transition. What is at a distance? If you look at the focussing scale you'll see that it is logarithmic.

[mdemeyer]

Seems the best way to get an answer is visit the Leica store and try these lenses on another camera. Will advise what I learn...

[adan]

The thing with focusing long-but-not-infinite distances using an RF is that there is a "black hole" between where the triangle connecting the two RF windows and the subject gets too narrow and skinny to accurately resolve differences in distance, and the hard-wired "infinity" stop, which so long as it is set correctly, guarantees focus there. If you are trying to accurately focus on, say, 50 yards/meters vs. 55 yards/meters, the Leica RF baseline of 68mm/2.7 inches (x 0.7x viewfinder magnification) is just too short to show an angular difference, even with an eyepiece magnifier.

 

There's a reason the RFs used by naval gunners (pre-radar) to accurately distinguish ranges up to thousands of m/yds had base lengths between their windows of 2-6 meters (6.6-20 feet).

 

http://enacademic.com/dic.nsf/enwiki/7678408

 

Depth of field gets larger at long distances, (as mentioned before, a building at 2 miles and the moon behind it at 250,000 miles are usually both in focus with the lens set to infinity - that's DoF of at least 249,998 miles!) - but doesn't track exactly with the degradation of RF effectiveness.

 

My 135mm f/4 aligns the RF images perfectly when set to infinity (and is focused correctly on things several miles or more away) - but still sometimes has problems nailing exact focus at 100-200 feet. That is its "black hole" region.

Edited June 15, 2018 by adan