kram
Well-known
I have read that a 50mm f2 lens does not have to be a retrofocus design on a SLR as there is plenty of room between the film plane and rear element of the lens for the mirror box. With a 50mm f1.4 it does need to be a retro focus design (just, depends on camera make there is some variation in this distance) and with f1.2 definitely because the rear element is closer to the film plane than the mirror box will allow.
With a rangefinder - no mirror box so a 50mm f1.4, f1.2 etc does not have to be of a retrofocus design, therefore can be made simpler and smaller (and more easy to higher optical corrections).
However, with the light meter in modern 35mm RFs, there needs to be room to take the light measurement, for example, the Zeiss ZM needs 15mm between the film plane and rear element.
My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...?
Also, the size of the rear element of the lens, for say a leica m mount camera, increases in size with aperature. When would this cause problems in the m mount? 25 f1.4? Any equations out there to calculate this stuff?
With a rangefinder - no mirror box so a 50mm f1.4, f1.2 etc does not have to be of a retrofocus design, therefore can be made simpler and smaller (and more easy to higher optical corrections).
However, with the light meter in modern 35mm RFs, there needs to be room to take the light measurement, for example, the Zeiss ZM needs 15mm between the film plane and rear element.
My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...?
Also, the size of the rear element of the lens, for say a leica m mount camera, increases in size with aperature. When would this cause problems in the m mount? 25 f1.4? Any equations out there to calculate this stuff?
kram
Well-known
Surely someone has an answer
ClaremontPhoto
Jon Claremont
We don't need to work out these figures.
If Zeiss, or Vogtlander, or Leica make a lens it will work with any M rangefinder.
They have already done the optical calculation and lens design.
If Zeiss, or Vogtlander, or Leica make a lens it will work with any M rangefinder.
They have already done the optical calculation and lens design.
ampguy
Veteran
just a comment - with RF glass, the back element doesn't necessarily go deeper with faster lenses. The Noctilux (latest version) is relatively shallow going back from the flange, while the 35/2 asph, and ZM 28/2.8 Biogon go much deepler back from the flange. The latter two go so far back that I never used them with my back to back DAG M adapters (which give more space and a dust free divider compared to the Heavystar grey ones).
Lenses I recall that are relatively shallow are the Nokton 1.5, Rokkor 40/2s, Hexanon 50/2, J8, I61 (with M adapters).
Deep lenses are the J12, many collapsibles if not adjusted for limited collapsing, and as mentioned the 35/2 asph, and 28/2.8 ZM Biogon.
Lenses I recall that are relatively shallow are the Nokton 1.5, Rokkor 40/2s, Hexanon 50/2, J8, I61 (with M adapters).
Deep lenses are the J12, many collapsibles if not adjusted for limited collapsing, and as mentioned the 35/2 asph, and 28/2.8 ZM Biogon.
Spyderman
Well-known
The aperture numbers mean something like the ratio between diameter of the lens opening and the focal length. So you needn't put lens closer to film to get a fast lens, just make the opening larger.
Most SLR 50mm lenses are so called double-gauss or Planar design, be it 50/2 or 50/1.4, even the 50/1.2 of some manufacturers.
You're right about wideangle lenses - on SLRs they need to be retrofocal designs. On RF they can be symmetrical, thus having lower distortion but more light fall-off in the corners.
rxmd
May contain traces of nut
A lot of this depends on the construction of the lens. Here's a couple of general ideas:
The aperture number is more or less as the focal length divided by the lens' entrance pupil (the entrance pupil of the lens is the apparent diameter of the aperture). How the apparent diameter is related to the physical diameter depends on the lens design. A 50/f1, for example, needs an apparent aperture as big as the focal length. A 200/f1.8 for a SLR needs an apparent aperture of more than 10cm in diameter, which is big. While the apparent diameter can be quite different from the physical diameter, this gives you a very general idea how big a lens has to be diameter-wise for a given speed. (That's one of the reasons why a 21/f4 can be built much smaller than a 500/f4.) It also creates all sorts of interesting construction problems when building things like a constant-aperture zoom, because, say, in a 24-105/f4 lens, the apparent diameter has to change from 6mm to 26mm over the zoom range. That's why there are no constant-aperture superzooms - try the numbers on a hypothetical 18-200/f2.8.
If you have a more or less symmetrical lens, like the typical 50, the exit pupil of the lens needs to be about as big as the entrance pupil. With a more or less symmetrical 50/f1, for example, the exit pupil should be about 50mm in size. Now the M mount only has 41mm or so in diameter, which leads to all sorts of constraints because you can't obviously build the exit pupil of the lens bigger than the throat of the bayonet it's supposed to go through. This is one of the reasons why the Noctilux vignettes wide open. To avoid vignetting you have to modify the lens construction to make it less symmetrical, which makes it more difficult to correct aberrations.
Another constraint is the size of the image circle; a 25/f1.4 lens is relatively easy to build symmetrically (after all you only need an aperture of 18mm or so, which gives you an idea of the size of the lens elements). But because it sits so close to the film plane, it would only cast a relatively small image circle, so the construction would have to become more extreme with bigger lenses. Then you run into a size problem with the bayonet throat relatively quickly. A classic example is the Biogon 35/f2.8 for the Contax, which has a huge rear element that sits close to the film plane to make a relatively fast non-retrofocus lens which covers 35mm film, but which couldn't be built any faster because the rear element would no longer fit through the relatively narrow throat. The only way out is a retrofocus, non-symmetrical construction, which invalidates many of the assumptions underlying these simply calculations, and which also leads to asymmetrical relations between the front and rear elements. That's why lenses such as the Canon 24/f1.4 L have huge front elements in relation to the relatively short focal length of the lens. I have a Flektogon 50/f4 medium format wideangle that covers 6x6 and needs a lot of clearance for the Pentacon 6 mirror box - focal lens is 50mm, lens register is 74mm, and it really needs those 74mm clearance. To accomodate this, the construction is completely asymmetrical with a small rear element of less than 20mm (so no problems whatsoever with the bayonet size), but with a huge front element that needs a 86mm filter ring.
Symmetrical lenses were interesting because they were easy to correct for distortion and aberrations, but in effect nowadays they are largely overrated, because computer-aided lens construction has made it so easy to calculate distortions that there is really not much of an advantage, while there are a lot of drawbacks when building fast lenses. The idea that rangefinders were superior because you can build better symmetrical wideangles was correct up until the 1970s or so, but is now largely obsolete IMHO.
When you start building asymmetrical, retrofocus lenses which use a tele construction (or for wideangles an inverted tele construction), much of this becomes much more difficult to calculate because the construction affects things like the apparent aperture size. However, calculating difficulty is no longer a problem now that everything can be simulated. In principle then you can build lenses arbitrarily fast without caring for the physical parameters of the bayonet. The tradeoff for this is that the construction needs extra lenses and tends to become huge very quickly.
Philipp
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pvdhaar
Peter
There have been non-retrofocal wide angle lenses for SLRs. You just had to lock up the mirror and mount an accessory finder. Maybe not very practical, but then again, a visoflex isn't either..
From these questions, one might draw the conclusion that non-retrofocal designs are some kind of holy grail. They're not.. ask anyone who needs a center filter on their ultra wides to compensate for light fall-off in the corners..
kram
Well-known
Interesting responses.
kram
Well-known
Sorry about the typos -should had preveiwed response:-(
rxmd
May contain traces of nut
You can always edit your post and move the brackets around
As I said, the rangefinder "advantage" in wideangles is somewhat overrated as soon as you want higher lens speeds. As soon as you go beyond f/2 or beyond 28mm this is really noticeable! For example the Voigtländer Ultron 28/f1.9 is 47mm long, 53mm thick and weighs 280g, the Canon FD 28/f2 is 47mm long, 63mm thick because of the bayonet, weighs 265g, is only marginally slower, but has a lot more mechanics inside. The Zeiss Distagon ZM 15/f2.8 is 92mm long, 78mm in diameter and weighs 550g, the new Canon 14/f2.8 L II is 90mm long, 80mm in diameter and weighs 645g, which includes the AF motor, aperture actuators and all sorts of extra electronics and mechanics. The Distagon is probably a bit better, then again the Canon is a bit wider. You can't just assume that rangefinder lenses are always smaller and lighter.
The reason is that the constructive advantage of the rangefinder (the short lens register, and with the M mount the relatively wide bayonet throat in relation to the rangefinder) are increasingly difficult to exploit as soon as the focal length becomes shorter than the lens register. A Summilux 35/1.4 can be built much smaller than a SLR 35/1.4. But if your lens register is 27.8 mm, 28mm is something of a constructive boundary for fast lenses. There is simply no way to build a small non-retrofocus 25mm lens, to take your example, that is both fast and covers a large enough image circle. So it's all inverted tele constructions again, you need lots of lens elements, and suddenly your lenses aren't much smaller than SLR lenses anymore if you want them to be fast.
As pvdhaar has pointed out, non-retrofocus ultrawideangles suck anyway, because the cos^4 law will mercilessly give you lots of vignetting in the corners as your angle of view increases, leaving you in centerfilter land.
In addition, rangefinder bodies are smaller. The Canon 28/f2 feels quite compact on a T-90 body; the Voigtländer Ultron 28/f1.9 feels like a big lens on an M-mount body. People are less likely to put up with the size constraints on fast lenses, because in relation to the body it will seem bigger. As a result, I doubt we will ever see any 28/f1.4 or 25/f2 or faster lenses on rangefinders, because they would be big and feel even bigger. No matter how rangefinder geeks beg Zeiss and Leica for fast wideangles, they're just not going to happen, because the same rangefinder geeks don't realize the constraints that would make them not buy those lenses in the end.
Philipp
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DrLeoB
Shoot a IIIc "K" !!!
In answer to the original question, there is optical design software for calculating many of the questions posed, but it is quite expensive. However, Lambda Research (I have no association) has a free "educational version" for downloading (( http://www.lambdares.com/education/oslo_edu/ )). The program is named Oslo and is, by no means, the only one. It is popular with the amateur astronomer community because telescopes rarely go to ten survaces, which is the limit to this version.
kram
Well-known
rxmd
May contain traces of nut
Hi,
when you quote things, it helps if you put the quoted text between the square ][ brackets of the block.
I wouldn't expect a rangefinder 28/f1.4 to be a small lens. But this is irrelevant, as it is somewhat unlikely that such a lens will ever be produced. At this point it's a completely hypothetical discussion with no grounding in reality. Hoping that such a lens will be small seems even less rewarding than hoping it will be built. If/when something like this does appear, then it's the time for size comparisons, not earlier.
Also, the ZM lens is some twenty-odd years younger. A more sensible comparison would be between the ZM 25/f2.8 and ZF 25/f2.8 Distagons, both are new and by the same maker.
At this point discussions of "optical quality" become somewhat pointless. Try comparing lenses on photographic merit instead and don't bother too much about theory and hypothetical fast wideangles that will never see the light of day.
Philipp
when you quote things, it helps if you put the quoted text between the square ][ brackets of the block.
Yes.
There isn't really a clear-cut "limit", just the general notion that wideangle lenses lose their size advantage as they get wider and faster with the observation that at 28/f2 you already have lenses that are pretty similar in size.
I wouldn't expect a rangefinder 28/f1.4 to be a small lens. But this is irrelevant, as it is somewhat unlikely that such a lens will ever be produced. At this point it's a completely hypothetical discussion with no grounding in reality. Hoping that such a lens will be small seems even less rewarding than hoping it will be built. If/when something like this does appear, then it's the time for size comparisons, not earlier.
I don't know which Nikkor 24/f2.8 you have, it's an oldish design that went through quite a number of revisions if I read this page correctly. But it appears to be quite a good lens in itself, with a number of significant advantages over the ZM (such as focusing down to 1', which can be quite useful with a wideangle).
Also, the ZM lens is some twenty-odd years younger. A more sensible comparison would be between the ZM 25/f2.8 and ZF 25/f2.8 Distagons, both are new and by the same maker.
At this point discussions of "optical quality" become somewhat pointless. Try comparing lenses on photographic merit instead and don't bother too much about theory and hypothetical fast wideangles that will never see the light of day.
Philipp
Rob-F
Likes Leicas
Great question, and great answers! Great article, RxMD! Some observations to add to the info already given:
Increasing the speed of a lens may increase its bulk, because faster lenses usually have more elements, which take up more room. But note that the 35mm Pre-ASPH Summilux is no bigger than the 35mm Pre-ASPH Summicrons. Why? Because the additional element in the Pre-ASPH Lux is very thin and small; and the Version IV has the same schematic layout, while the version I actually has two such thin elements. Yet these lenses are all about the same size. Moral: there is no overarching rule that can cover all cases, here.
I don't think that increased speed necessarily calls for going to a retrofocus design. But I think that today, the retrofocus lens offers some performance advantages, especially--and perhaps mainly--in acheiving more uniform corner-to-corner illumination. Also, designers in recent times have been increasing the diameter of front and rear elements, to reduce artificial vignetting. This, of course, makes the lens bulkier.
In general, making a lens larger gives more freedom to make it better. As films and sensors improve, there is competition to make better lenses that can take advantage of these improvements. This, too, may partly explain why lenses are getting bigger. Example: the 35mm Zeiss lens for the Leica M mount is bigger than the 35/2 offerings from Leica. I'm sure that made it easier to design. But again, there seems not to be one overarching rule. The little 50/2 pancake lens for the Nikon SLR seems just as good as the larger version--if you can get your fingers around its tiny controls.
So the size of some short focal length lens may be due only in part to their needing to be retrofocus. The large size may be also the result of the designers wanting to maximize the performance in respect to illumination, abberation correction, etc.
Edit: when I say that the retrofocus design can reduce corner falloff, I'm referring to the fourth-power cosine law, which essentially states that the light falls off very fast as the angle at which the light ray strikes the film increases from center to edge. By moving the rear element farther away from the image plane, the angle becomes less extreme; therefore there is less falloff.
Increasing the speed of a lens may increase its bulk, because faster lenses usually have more elements, which take up more room. But note that the 35mm Pre-ASPH Summilux is no bigger than the 35mm Pre-ASPH Summicrons. Why? Because the additional element in the Pre-ASPH Lux is very thin and small; and the Version IV has the same schematic layout, while the version I actually has two such thin elements. Yet these lenses are all about the same size. Moral: there is no overarching rule that can cover all cases, here.
I don't think that increased speed necessarily calls for going to a retrofocus design. But I think that today, the retrofocus lens offers some performance advantages, especially--and perhaps mainly--in acheiving more uniform corner-to-corner illumination. Also, designers in recent times have been increasing the diameter of front and rear elements, to reduce artificial vignetting. This, of course, makes the lens bulkier.
In general, making a lens larger gives more freedom to make it better. As films and sensors improve, there is competition to make better lenses that can take advantage of these improvements. This, too, may partly explain why lenses are getting bigger. Example: the 35mm Zeiss lens for the Leica M mount is bigger than the 35/2 offerings from Leica. I'm sure that made it easier to design. But again, there seems not to be one overarching rule. The little 50/2 pancake lens for the Nikon SLR seems just as good as the larger version--if you can get your fingers around its tiny controls.
So the size of some short focal length lens may be due only in part to their needing to be retrofocus. The large size may be also the result of the designers wanting to maximize the performance in respect to illumination, abberation correction, etc.
Edit: when I say that the retrofocus design can reduce corner falloff, I'm referring to the fourth-power cosine law, which essentially states that the light falls off very fast as the angle at which the light ray strikes the film increases from center to edge. By moving the rear element farther away from the image plane, the angle becomes less extreme; therefore there is less falloff.
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