Whats the number on the focusing ring mean?

[awesom3]

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I just got myself a 50 summilux and noticed the number 15 on the focusing scale next to the "m" marker, its the only number thats at 90 degrees to the focusing scale numbers. Any idea what it means?

 

thanks!

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[matlep]

It means that the actual length of the lens is 51.5mm and not 50mm.

[adan]

Like matlep says. The numbers are the two digits either side of the decimal point of the actual focal length. 51.5 = 5(1.5) = 15.

 

Leica has quit doing this recently (at least on some lenses), but for many years they put these numbers on 50mm or longer lenses for precise calibration of the focusing cam (or recalibration years later, if needed). Some 50mm lenses are 51.5mm in true focal length, some 52.2 (engraved "22"). Some 135s are actually 136mm (engraved "60") etc. The focal lengths are occasionally slightly less than the nominal length - e.g. a 90mm lens engraved "99" is 89.9mm in reality.

 

I think there have been 6 variations on "50mm" and maybe 2-4 each for 75s, 90s, and 135s.

[lars_bergquist]

It is a fact that ever since Berek (meaning 1925 or so) the actual design focal length of Leitz/Leica "5cm" or "50mm" lenses has been close to 52mm.

 

The old man from the Berek Age

[bybrett]

Yes, 14 seems to be the prevalent on the 50mm Summilux ASPH

[jaapv]

It is a fact that ever since Berek (meaning 1925 or so) the actual design focal length of Leitz/Leica "5cm" or "50mm" lenses has been close to 52mm.

 

The old man from the Berek Age

That is correct, and actually the number on the barrel does not denote the difference from the nominal focal length, but the tolerance from the focal length of the factory reference lens.

[adan]

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Jaap. maybe it's just a language thing, but I don't think the number represents EITHER a "tolerance" or a "difference" (which sound the same to me) - but the ACTUAL focal length pure and simple.

 

I.E. a 75 lens marked "60" means a 76mm lens. The difference or tolerance in that case would be 1.0mm (75mm + 1.0mm) - but the lens is not marked "01" or "10."

[tobey bilek]

Many years back I tested a bunch of 50 2.8 for use on a stereo camera. No nunber on the lens as a guide. I did photos and measured the projected image and could not find two that matched.

[jaapv]

Andy, sorry for being unclear, but that was not what I meant. The point is that the design of for instance a Summicron ( as Lars mentions) makes for a 53 mm lens. That is the actual focal length. The nominal focal length is 50 mm, and we can assume that the reference lens is indeed exactly 53 mm. So a lens marked for instance 15 would make that lens a nominal 51.5 mm lens, but if you would measure it you would arrive at an actual 54.5. Not that that is of any practical interest, of course, for taking photographs, but it can be very practical for a technician who has to adjust the lens I imagine. And, as mentioned above, if one needs two exactly matched lenses.

[pico]

[...]Leica has quit doing this recently (at least on some lenses), but for many years they put these numbers on 50mm or longer lenses for precise calibration of the focusing cam [...]

 

Is there any correspondence between unmarked lenses and the occasional focusing problems with the M9?

 

I worked with a lot of large format aerial photo lenses, every one of which was measured and marked individually with true focal length, usually with a hand etching tool. It was typical to find the actually focal length to vary far more than could be accommodated with one number per lot as stamped in the Leica lenses. While that might be due to the greater length of the aerial lenses which ranged upward from 76mm, it is also true that greater focal length gives greater depth-of-focus (not depth-of-field). However, aerial lenses have far more strident standards than most general use lenses (for example, to be certified for mapping, they are tested at maximum aperture.)

 

For calibrating, would it not be better to shim each lens to a given focal length? It is easier to machine the focusing cam to exact tolerances than it is to grind and assemble lenses to specific lengths. This all leads me to question the accuracy of the added FC engraving. 75/76 is a huge difference, an unacceptable tolerance range, IMHO.

[adan]

"So a lens marked for instance 15 would make that lens a nominal 51.5 mm lens, but if you would measure it you would arrive at an actual 54.5."

 

That is where I believe you are mistaken. If you measure a "50mm" lens marked "15", you will find it to be a 51.5mm lens, not a 54.5mm lens. I agree that the true focal length Leica aims for is actually 52mm or thereabouts (maybe 52.2mm), but lenses that hit that target exactly are not marked "00", they are marked "20." (or "22").

 

A 135 lens marked "60" is most definitely NOT a 141mm lens.

[jaapv]

That is the point, Andy. A 50 mm lens marked 00 will be those 53 mm. Probably marketing. It is hard to sell a Summicron 53 to Joe Public;) I've never seen a lens marked "60" btw

[jaapv]

Is there any correspondence between unmarked lenses and the occasional focusing problems with the M9?

 

I worked with a lot of large format aerial photo lenses, every one of which was measured and marked individually with true focal length, usually with a hand etching tool. It was typical to find the actually focal length to vary far more than could be accommodated with one number per lot as stamped in the Leica lenses. While that might be due to the greater length of the aerial lenses which ranged upward from 76mm, it is also true that greater focal length gives greater depth-of-focus (not depth-of-field). However, aerial lenses have far more strident standards than most general use lenses (for example, to be certified for mapping, they are tested at maximum aperture.)

 

For calibrating, would it not be better to shim each lens to a given focal length? It is easier to machine the focusing cam to exact tolerances than it is to grind and assemble lenses to specific lengths. This all leads me to question the accuracy of the added FC engraving. 75/76 is a huge difference, an unacceptable tolerance range, IMHO.

The focal length is an optical value at infinity - shimming the lens and adjusting the focussing cam to correct the shim would not make one whit of difference as the optical cell would be in the same position again to throw a sharp image.

[adan]

"For calibrating, would it not be better to shim each lens to a given focal length?"

 

How would you go about shimming a lens to change its focal length? Shims for individual elements would just foul up the optical formula.

 

Shims for the whole lens would just move it forward or backward, but not change the focal length. Any more than moving it forward or backward with the focus ring (excepting the modern floating-element designs).

 

"It is easier to machine the focusing cam to exact tolerances than it is to grind and assemble lenses to specific lengths"

 

Exactly. Which is what Leica does (or did).

 

They measure the actual focal length of each individual lens on an optical bench, and mark that on the barrel in small numbers for future reference. Both the immediate future (the next step in the assembly line) and the long-term future (in case it ever needs rebuilding).

 

Then they pick, from a selection of cams* pre-machined for the different "flavors" of focal length (and labelled, and stored in separate parts bins), one that matches that real focal length.

 

*I use "cams" loosely, since depending on focal length, sometimes they are cammed, and sometimes not. Better, but longer, would perhaps be "surface to mate with the camera rangefinder wheel, directly or via a push-rod."

[adan]

Jaap, again, I think the reality is that a 53mm lens is one marked "30."

 

(EDIT: Actually, I'm not sure there any such thing as a "53mm" lens marked 30 or otherwise - the design target seems to be 51.6mm, with variations around that target up to 52.2 maximum)

 

That's the information I have from multiple sources including Puts' Lens Compendium and Laney's Collector's Guide.

 

Also: Leica FAQ — Lens white numbers

 

Also: http://greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=005cTt

 

I've owned a 135 Tele-Elmar marked "60" (i.e.136.0 mm).

 

My CURRENT 135 Tele-Elmar is marked "55" - apply your theory and tell me what you believe its actual focal length to be. My theory says 13(5.5)mm

[jaapv]

I know it is explained like that, Andy. I just happen to think they are wrong to disregard the actual focal length of the basic design. I gues the only one who knows exactly is Leica.

[Nicoleica]

I'm willing to bet that the OP had no idea of the 'debate' that would arise from a harmless sounding question.

 

For what it's worth, and after reading the thread on 35mm lenses, my personal theory is that this number signifies the decibel level of the rattling sound if you shake the lens.

[ho_co]

Jaap, Andy is correct.

 

Design focal length of DR Summicron was 51.9 mm as I recall, and DR Summicrons were marked as 51.9.

 

The actual focal length of a lens unit is important when mating it to a focusing mount.

 

(Imagine what would happen if the engraving showed 'the deviation from design' instead of 'the deviation from nominal': Once no one remembered exactly what the design focal length of a particular Xenitar was, for example, the number would have no meaning. And we'd be having discussions like this one. )

 

The actual focal length was typically engraved only on lenses of 50mm and longer, because only for them were different focusing mounts produced. Wider lenses could live with marginal discrepancies in those days.

 

The reason so many lenses today show no discrepancy from the nominal focal length is simply that current design and production standards force the lens to zero tolerance. (See Puts on "stressed design.")

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[pico]

"For calibrating, would it not be better to shim each lens to a given focal length?"

 

How would you go about shimming a lens to change its focal length? Shims for individual elements would just foul up the optical formula.

 

I was not speaking of shimming the elements, but shimming the back where the lens meets the lens mount. That will not mess up the optical design. It can place the lens exactly where it should be when the focal length is longer than the nominal length.

 

They measure the actual focal length of each individual lens on an optical bench, and mark that on the barrel in small numbers for future reference. Both the immediate future (the next step in the assembly line) and the long-term future (in case it ever needs rebuilding).

 

Then they pick, from a selection of cams* pre-machined for the different "flavors" of focal length (and labelled, and stored in separate parts bins), one that matches that real focal length.

 

OK, thanks for that. I see the confusion regarding the cam. I guess eventually I will understand why they cannot shim in front of the rear lens mount. One would think that if they do hand-match a cam for a focal length, then they should be able to cam for focus shift, too. N'est pas?

 

*I use "cams" loosely, since depending on focal length, sometimes they are cammed, and sometimes not. Better, but longer, would perhaps be "surface to mate with the camera rangefinder wheel, directly or via a push-rod."

 

Understood.

[jaapv]

Jaap, Andy is correct.

 

Design focal length of DR Summicron was 51.9 mm as I recall, and DR Summicrons were marked as 51.9.

 

The actual focal length of a lens unit is important when mating it to a focusing mount.

 

(Imagine what would happen if the engraving showed 'the deviation from design' instead of 'the deviation from nominal': Once no one remembered exactly what the design focal length of a particular Xenitar was, for example, the number would have no meaning. And we'd be having discussions like this one. )

 

The actual focal length was typically engraved only on lenses of 50mm and longer, because only for them were different focusing mounts produced. Wider lenses could live with marginal discrepancies in those days.

 

The reason so many lenses today show no discrepancy from the nominal focal length is simply that current design and production standards force the lens to zero tolerance. (See Puts on "stressed design.")

Ok,Ok, I relent