Will we ever see a Noctilux 35mm?

[IkarusJohn]

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Olaf,

 

This sort of reply is not really instructive, except to provide a graphic example of why it is that sometimes people get annoyed with your contributions to the forum.

 

While this may all be terribly tedious for you, for the mere mortals among us, this sort of discussion is very helpful; whereas, your post isn't.

 

A bit of explanation without making us feel completely stupid in the process would be really good. Conversely, we know you're knowledgeable and clever, so we don't need reminding in this way.

 

Cheers

John

[01af]

A bit of explanation without making us feel completely stupid in the process would be really good.

Sorry. Peter's analysis of the ratio between focal length and front-vertex-to-entry-pupil distance doesn't lead anywhere because there is no clear connection between that and the lenses' focal length. The size of the front element does not only depend on entry pupil distance and angle-of-view (as in the window analogy) but also on the design of the lens. Two lenses with the same focal length, same maximum aperture, and same coverage can have significantly different front element diameters. In particular, modern wide-angle lenses tend to have much smaller front elements than older ones with the same focal length and speed—even when their back-focus distances are about equal.

 

Back-focus distance is the distance between the rear element's rear vertex and the image.

 

A lens is retro-focus when the back-focus distance (at infinitiy focus) is longer than the focal length. A lens is telecentric (on the image side) when the exit pupil is at infinity. Note how these two terms have nothing to do with each other—being telecentric has nothing to do with back-focus distance or focal length, and being retro-focus has nothing to do with exit pupil position.

 

By the way, virtually all wide-angle lenses for SLR camera are retro-focus while wide-angle lenses for rangefinder cameras usually are not; even the modern ones aren't. Some of them may have longer back-focus distances than some of their predecessors had (most obviously in the 21 mm focal length), but they still are not retro-focus. In fact, the only retro-focus Leica M wide-angle lens I am aware of is the Tri-Elmar-M 16-18-21 mm Asph.

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[pgk]

I wonder if the Wiki should have an explanatory section which explains technical terms and then links them specifically to Leica lenses? I for one would find it very useful to have a diagrammatic lens with appropriate labeling - and this labeling could then be referred to from each lens - as an example. CRC (close range correction or whatever technical terms is used), could be diagrammatically shown and those lenses with it could link to this - in this case it would differentiate the 35mm Summiluxes. Just a thought;).

[01af]

... the only retro-focus Leica M wide-angle lens I am aware of is the Tri-Elmar-M 16-18-21 mm Asph.

Umm, wait—the Tri-Elmar-M 28-35-50 mm Asph also is retro-focus, at least at 28 mm and 35 mm. At the 50 mm setting, focal length and back-focus distance are almost equal, so not sure if it's retro-focus here.

[ynp]

I wonder if the Wiki should have an explanatory section which explains technical terms and then links them specifically to Leica lenses? I for one would find it very useful to have a diagrammatic lens with appropriate labeling - and this labeling could then be referred to from each lens - as an example. CRC (close range correction or whatever technical terms is used), could be diagrammatically shown and those lenses with it could link to this - in this case it would differentiate the 35mm Summiluxes. Just a thought;).

 

Yes, I am learning the new tech words and it's meaning now, not very easy...when English is your third language....

[CheshireCat]

Yeah. I'd say that [$15000] is in the ballpark.

 

Although production cost and retail price are not necessarily correlated for this kind of products.

[Peter Branch]

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The question has been raised as to the definitions of some of the key terms being used in this thread. The problem, as anyone who has done the usual searches on the web soon discovers, is that so called definitions are not in agreement. This is perhaps understandable because they relate, with one exception, to characteristics.

 

The term retro-focus has a wide range of uses however the fundamental concept is straight forward. In simple optical theory all lens elements are treated as though they have no thickness, so called thin lens theory. All lenses have two key parameters, normally called the Entry and the Exit pupils. In a very simple thin lens they coincide. As lens designs become more complex by combining many thick elements these parameters tend not to be coincident. The term retro-focus is concerned with the position of the exit pupil. There are at least three descriptive definitions. The first describes any lens where the exit pupil is displaced from the “centre” in the direction of the image plane. The second describes a lens where the exit pupil is outside the rearmost glass surface of the physical lens again in the direction of the image. The third is concerned with having enough physical space between the rear of the lens and the image plane for items such as SLR mirrors or beam-splitters to be inserted in the optical path. Hence all SLR lenses with a focal length less than the space taken up by the moving mirror are, of necessity, retro-focus. Leica seem, based on a review of their literature over the last 20/30 years, to have applied the term retro-focus at different times to different lenses having any of these essentially descriptive properties. The company has not, in general, published data relating to the position of the exit pupil, unlike the very precise data available for the position of the entry pupil. Essentially the position of the exit pupil is of little use to practical photography but the entry pupil is important for a number of applications such as close range panoramic sequences.

 

The other term is telecentric which does have a precise definition. It means a lens which has its exit pupil at infinity measured in the direction away from the image plane. The key property of such lenses is that the cone of light falling on the sensor for each and every image point has an axis which is normal to the surface of the sensor.

 

With the advent of digital sensors it is generally desirable that the angle of incidence of the light falling on them is as near normal as practicable. Micro lenses, and even more significant off-set micro lenses, are a technical response to the fact that practical photographic lenses are not telecentric. However Leica and many other lens manufacturers have in recent years designed lenses which tend towards having the properties of a true telecentric lens. Leica now use the term but in the context of tending to have telecentric properties by which they mean that the exit pupil has been deliberately designed to be as far from the image plane as practicable to optimise performance with digital sensors.

 

Such lenses may, or may not, have any of the characteristics of one of the practical definitions of a retro-focus lens.

[01af]

The question has been raised as to the definitions of some of the key terms being used in this thread. The problem, as anyone who has done the usual searches on the web soon discovers, is that so-called definitions are not in agreement.

There is no problem whatsoever because 'so-called definitions' aren't. The terms in question—namely retrofocus and telecentric—both have definite and crystal-clear definitions. If there's any disagreements then it's in the understanding thereof, not in the definitions.

 

 

The term retro-focus has a wide range of uses however the fundamental concept is straight forward. In simple optical theory all lens elements are treated as though they have no thickness, so called thin lens theory. All lenses have two key parameters, normally called the entry and the exit pupils. [...] The term retro-focus is concerned with the position of the exit pupil.

Please note that the term retro-focus is in no way concerned with entry or exit pupils—as I already pointed out in an earlier post above.

 

 

There are at least three descriptive definitions.

No, there are not. There's exactly one definition, period.

 

 

The first describes any lens where the exit pupil is displaced from the “centre” in the direction of the image plane.

As a lens does not have a 'centre,' it cannot be part of a description—much less a definition—of any lens parameter.

 

 

The second describes a lens where the exit pupil is outside the rearmost glass surface of the physical lens again in the direction of the image.

Such a lens does not, and cannot, exist in the first place because it would require bending of rays of light in mid-air. So again, this can hardly be part of a description or definition of any lens parameter.

 

It seems you're confusing the exit pupil with the rear principal plane. If you replace the former with the latter then your second 'description' will coincide with your third.

 

 

Leica seem, based on a review of their literature over the last 20 - 30 years, to have applied the term retro-focus at different times to different lenses having any of these essentially descriptive properties.

Well—I haven't checked all the Leica literature of the last 30 years ... but I did check all the PDF documents you can download for each current lens from their website. In none of these, a lens was described as being retro-focus, not even the only current lens that actually is (the WATE). Two lenses—Elmar-M 24 mm Asph and Super-Elmar-M 21 mm Asph—are described as 'retrofocus-like,' i. e. not actually retrofocus. This means the back-focus distance is by design somewhat longer than it naturally would be but still not long enough to be retrofocus. This is not wrong, yet it does not make much sense to describe them this way because these two lenses are no more and no less 'retrofocus-like' than all the other Leica M wide-angle lenses (except the WATE).

 

 

However Leica and many other lens manufacturers have in recent years designed lenses which tend towards having the properties of a true telecentric lens.

No, they haven't.

 

Clueless marketing jerks sometimes are mis-using this term ... I guess it's because it sounds so cool. It's just a buzzword. It has no place in actual photographic lens descriptions. Even the word 'telecentric-like' is mostly just an outrageous exaggeration—unlike the more moderate term 'retrofocus-like' which does make some sense.

 

 

Leica now use the term ...

Right now I am not aware of any Leica publication where the term is used, and I sincerely hope there isn't any. Or is there?

 

But then, Leica's technical documentation, in particular product brochures and manuals, occasionally contain hard-to-believe nonsense, so I wouldn't be too surprised ... for example, in the description of the Apo-Summicron-M 75 mm Asph, Leica's website states that using it on an M8 'turns the compact lens into a powerful telephoto lens with a focal length of 100 mm'. More nonsense like this can be found in the little black lens manuals coming with new Leica M lenses. In other words, don't take everything Leica says as gospel.

 

 

... the exit pupil has been deliberately designed to be as far from the image plane as practicable to optimise performance with digital sensors.

I am not aware of one single Leica M lens where the exit pupil has been deliberately designed to be as far from the image plane as practicable to optimise performance with digital sensors. Are you?

[jaapv]

Umm, wait—the Tri-Elmar-M 28-35-50 mm Asph also is retro-focus, at least at 28 mm and 35 mm. At the 50 mm setting, focal length and back-focus distance are almost equal, so not sure if it's retro-focus here.

According to Erwin Puts (quoted from the Leica Compendium) the Elmarit 21/2.8 was Leica's first retro-focus design in the 21 class, the 21/2.8 asph is listed as retrofocus as well. I doubt the EP does not understand the definitions..

Edited December 14, 2012 by jaapv

[01af]

According to Erwin Puts (quoted from the Leica Compendium) the Elmarit 21/2.8 was Leica's first retro-focus design in the 21 class, the 21/2.8 Asph is listed as retrofocus as well.

Yeah ... except they aren't

 

The Elmarit-M 21 mm Asph has a back-focus distance of approximately 16.5 mm—which most definitely is less than the focal length. So it's not retrofocus. In fact, no Leica M lens ever was, or is, retrofocus, except the Tri-Elmars.

 

Instead, most modern (and many older) Leica M wide-angle lenses share some design principles that are typical for retrofocus lenses. The purpose is to make both the back-focus distance as well as the exit-pupil distance longer than they would be naturally. This has several advantages—most notably, more room for TTL metering and less natural vignetting. Still, those back-focus distances are no longer than the respective focal lengths. So one can say they are retrofocus-like. The designs are similar to retrofocus designs. Some are almost retrofocus ... but not quite.

 

 

I doubt the EP does not understand the definitions.

What is the EP?

[jaapv]

The Erwin Puts As an authority on optical design certainly not the least. If he goes out of his way to mention it, I have a strong tendency to dismiss other opinions.

[01af]

As an authority on optical design certainly not the least.

Not the least for sure ... but neither the first. He knows an awful lot—still, not everything he writes in his books and articles about optics in general and Leica in particular is correct. In the recent years, my respect for his expertise has sprung a few cracks. Sorry.

[lct]

"Retrofocus-like" would not mean anything if the retrofocus definition were reduced to the aim or consequence of the inverted tele design i.e. by the length of backfocus compared to focal length. The former is not "longer-like", it is longer than the latter or it is not.

[IkarusJohn]

Sorry to ask, but why is "retro-focus" a good thing?

 

Seems just like another label to me.

[lct]

Retrofocus is just a name given to the inverted telephoto design which consists of a negative front lens group and a rear positive lens group. Il allows to increase the backfocus length of wide lenses, which is specially useful for (d)SLRs. It is useful for rangefinders as well in that it has the light more perpendicular to the film or sensor. Also it tends to reduce vignetting but it would produce more distortion and makes for larger lenses, hence its lesser success at Leica's i would guess.

[jaapv]

According to the EP it increases the scope of the designer to correct aberrations as well.

[lct]

The physical size argument is relative though. The Zuiko 28/2 (right) is retrofocus but is not much larger than the Summicron 28/2 (left). Now both of them look like dwarfs compared to many other retrofocus wides like the Zeiss Distagon 28/2.

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[01af]

Sorry to ask, but why is "retro-focus" a good thing?

Huh? Did anyone say it was a good thing?

[jaapv]

Good nor bad I would say. Simply a design choice. That is what people like Peter Karbe get paid for.

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Hallo,

Look here: Leica M

[IkarusJohn]

Huh? Did anyone say it was a good thing?

 

I just wondered why so much has been posted about it? and if I should care?

 

Sometimes (mostly) I find this forum incredibly useful and other times really quite confusing. On the one hand, we have people saying the latest ASPH designs are the bees knees, then others seem to be saying that the only thing better than a Mandler design is something made before WW2, and now hair splitting over definitions which don't seem that important.

 

To be honest, I would like to know more about real optical terms (as opposed to a manufacturer's marketing words - planar, distagon, biogon being examples). But I've come to the conclusion that understanding the real meaning of "telephoto", "retro-focus", "Double Gauss", "ASPH", "Aspherical", "FLE", "APO" and much of the rest doesn't help me with lens selection on the day or make me a better photographer.

 

Sorry, I don't mean to be dismissive. I learn a lot from these discussions sometimes, but too much of it seems to be a pissing competition. Maybe I'm just too stupid (cue bill), but I've found that my time is better sent taking pictures, and working out why the ones I like work, and what went wrong with the rest (or rather, how I can make them better).

 

Back on topic, I love the way the Noctillux renders (when I get it right), and a 35 Noctilux would be very interesting, though incredibly expensive. Being a simple soul, if the 35 Summilux is smaller than the 50 Summilux, and obscures the viewfinder less (mine does on both counts), I don't really understand why a 35 Noct would be bigger, heavier or more obscuring than the 50 version.

 

Cheers

John

Edited December 15, 2012 by IkarusJohn