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#31
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Olympus OM enthusiasts' digital prayers have been answered ...
In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A
writes On 2012-02-08 16:30:15 +0000, Kennedy McEwen said: In article , Rol_Lei Nut writes On 2/7/2012 14:12, Kennedy McEwen wrote: OM lenses work just as well on digital as they did on film, which doesn't mean telecentric lenses can't work better, but the argument was false to begin with and was merely an attempt by Olympus to justify their investment in cheap chips. I have a Panasonic M43 and use *lots* of made for film lenses on it (M42, Zeiss, Leica both M & R, M39 & others). Some lenses, especially fast one, often don't work well at their faster apertures (by "not well" I mean visibly less well than on a film camera). Most lenses will exhibit vignetting when wide open. That has nothing whatsoever to do with the sensor, whether digital or film, but due to the exit pupil being truncated. You can see this happening just by looking through the lens while tilting it - that circular aperture wide open closes down to a "cat's eye" when significantly off axis. The "cat's eye" is the main cause of the corner vignette. Stop the lens down a little and tilting it has no effect on the aperture. This was just the same with film as it is on a digital sensor - whether you noticed it or not. Furthermore, stopping the lens down doesn't change its telecentricity - so the very fact that you get a different level of vignette fully open demonstrates that telecentricity isn't the issue that Olympus claim(ed) it to be. If a lens is rear telecentric then its rays are parallel i.e. the angle of incidence is zero. How can an image form with parallel rays? It is the *principle* rays which are parallel in a telecentric lens, not all the rays and certainly not the peripheral rays which determine the f/#. If what you claimed was true then any pinhole lens would be telecentric - which is certainly not the case. Stopping it down cannot reduce the angle of incidence any further. For a non-telecentric lens, of course stopping it down reduces the angle of incidence thereby making it _nearer to_ rear telecentric than when it is wide-open. Stopping a non-telecentric lens down does not change the angle of incidence of the principle rays to any point on the focal plane. Thus, by definition, stopping a lens down does *NOT* make a lens "nearer to" telecentric in any way. Lots of compact, allegedly useless on digital because they aren't telecentric, OM lenses are used on Canon and other FF digital cameras without problem, hence their used prices are holding up quite well. Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the least telecentric lenses on the OM lineup due to the tiny rear pupil, Hang on a minute - a tiny rear exit pupil ensures that rays will not strike the sensor/film at an oblique angle Only close to the optic axis, ie. in the centre of the frame. A tiny exit pupil ensures that the principle rays at the corner of the frame strike the focal plane at a *much* more oblique angle than those at the centre of the frame. To be telecentric, the principle rays for each image point in the frame are parallel - a small rear element, only slightly larger than the on-axis pupil, ensures that they are not. , therefore how can it be one of the least telecentric lenses in the OM lineup? That paradox in your theory is something for you to ponder. There is no paradox in mine. One of the benefits of telecentricity is that geometric distortion is minimised and objects remain the same size as the focus travels through them. The design of the OM series Zuiko 18mm maximises both of these effects, the object field magnifies at close focus and shrinks as focus moves to infinity. Exaggerated geometry, not to be confused with the exaggerated perspective due to the short focal length, was a selling feature of that lens - achieved by its extremely non-telecentric design. -- Kennedy Yes, Socrates himself is particularly missed; A lovely little thinker, but a bugger when he's ****ed. Python Philosophers (replace 'nospam' with 'kennedym' when replying) |
#32
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Olympus OM enthusiasts' digital prayers have been answered ...
"Bruce" wrote in message news I only suggested it didn't match my experience and you have now explained why. Thank you! You're welcome. But I didn't add anything that wasn't already in the post you decided not to read. ;-) So show me where you previously mentioned the use of Kodak sensors which my Canon doesn't have, and possibly explains why I don't have the problems you are sure exist with the Olympus cameras. Trevor. |
#33
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Olympus OM enthusiasts' digital prayers have been answered ...
In article , Bruce
writes The suggestion that the least telecentric lenses are always those with tiny rear elements is completely risible. Nobody said that is *always* the case. I gave an example, which is certainly the most common situation, where it *IS* the case. In some cases, they are among the *most* telecentric! That is a completely ridiculous statement - we can all restrict our comparisons to the "some cases" which *can* be worse. In most cases that is NOT the case. I referred to the entire OM range. There are many aspects of the OM 18mm which demonstrate it's poor telecentricity. However poor rear telecentricity is guaranteed *because* it has a small rear element. That, together with the proximity of the rear element to the focal plane (it projects into the lens mount and just clears the mirror!), restricts the angle of incidence of the principle rays at the corners of the focal plane to very oblique angles: the very problem that Olympus claim makes such a non-telecentric lens unsuitable for digital. Yet it works extremely well and is one of the most sought after OM Zuikos for FF dSLRs! The implied converse, that a larger rear element means the lens is closer to telecentric is *complete nonsense*. That is *not* the implied converse. Only an idiot thinks that "all cats are furry animals" implies "all furry animals are cats"! The converse is that telecentricity, more specifically rear-telecentricity which is the version under discussion here, *requires* a large rear element! Should Bruce Almighty dispute that, I am sure he can point us to a ray diagram which shows otherwise. (That doesn't mean a diagram showing that telecentricity *can* be worse on a large rear element design, that is obvious with the standard textbook design of a front-telecentric lens being such an example.) -- Kennedy Yes, Socrates himself is particularly missed; A lovely little thinker, but a bugger when he's ****ed. Python Philosophers (replace 'nospam' with 'kennedym' when replying) |
#34
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Olympus OM enthusiasts' digital prayers have been answered ...
On 2012-02-08 23:44:00 +0000, Kennedy McEwen said:
In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A writes On 2012-02-08 16:30:15 +0000, Kennedy McEwen said: In article , Rol_Lei Nut writes On 2/7/2012 14:12, Kennedy McEwen wrote: OM lenses work just as well on digital as they did on film, which doesn't mean telecentric lenses can't work better, but the argument was false to begin with and was merely an attempt by Olympus to justify their investment in cheap chips. I have a Panasonic M43 and use *lots* of made for film lenses on it (M42, Zeiss, Leica both M & R, M39 & others). Some lenses, especially fast one, often don't work well at their faster apertures (by "not well" I mean visibly less well than on a film camera). Most lenses will exhibit vignetting when wide open. That has nothing whatsoever to do with the sensor, whether digital or film, but due to the exit pupil being truncated. You can see this happening just by looking through the lens while tilting it - that circular aperture wide open closes down to a "cat's eye" when significantly off axis. The "cat's eye" is the main cause of the corner vignette. Stop the lens down a little and tilting it has no effect on the aperture. This was just the same with film as it is on a digital sensor - whether you noticed it or not. Furthermore, stopping the lens down doesn't change its telecentricity - so the very fact that you get a different level of vignette fully open demonstrates that telecentricity isn't the issue that Olympus claim(ed) it to be. If a lens is rear telecentric then its rays are parallel i.e. the angle of incidence is zero. How can an image form with parallel rays? It is the *principle* rays which are parallel in a telecentric lens, not all the rays and certainly not the peripheral rays which determine the f/#. Indeed, the principle ray contains no light whatsoever because it's infinitely thin. Yes, the aperture affects the cone of peripheral rays. If what you claimed was true then any pinhole lens would be telecentric - which is certainly not the case. A pinhole camera exhibits cos^4 light falloff at the film. Having a very small aperture results in a correspondingly small cone therefore it is primarily the angle of the principle ray that causes the light falloff in such a camera. At 33 degrees the falloff is 1 f-stop. With an image-side (rear) telecentric lens, it is only the angle of the cone that causes light falloff. The cone does not vary across the image plane therefore the image doesn't suffer from corner vignetting. If the lens is badly designed then mechanical vignetting may cause light falloff in the image corners. Very wide-angle lenses suffer from light falloff on the object side, but this is a separate issue. In any mirror-less system, the lens designer is free to position the exit pupil very close to the image plane, which places an unrealistic demand on the design of image sensors; especially Bayer CFA sensors. Specifying "near-telecentric" simply means placing the exit pupil _reasonably_ far away from the sensor. So, it is a big issue and perfectly warranted in a mirror-less system specification. Stopping it down cannot reduce the angle of incidence any further. For a non-telecentric lens, of course stopping it down reduces the angle of incidence thereby making it _nearer to_ rear telecentric than when it is wide-open. Stopping a non-telecentric lens down does not change the angle of incidence of the principle rays to any point on the focal plane. Thus, by definition, stopping a lens down does *NOT* make a lens "nearer to" telecentric in any way. Literally, you are correct. In essence, stopping down reduces the rays that are problematic to the sensor. Lots of compact, allegedly useless on digital because they aren't telecentric, OM lenses are used on Canon and other FF digital cameras without problem, hence their used prices are holding up quite well. Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the least telecentric lenses on the OM lineup due to the tiny rear pupil, Hang on a minute - a tiny rear exit pupil ensures that rays will not strike the sensor/film at an oblique angle Only close to the optic axis, ie. in the centre of the frame. A tiny exit pupil ensures that the principle rays at the corner of the frame strike the focal plane at a *much* more oblique angle than those at the centre of the frame. To be telecentric, the principle rays for each image point in the frame are parallel - a small rear element, only slightly larger than the on-axis pupil, ensures that they are not. The tiny exit pupil you mentioned is not the problem. The distance of the exit pupil from the image plane is the problem. , therefore how can it be one of the least telecentric lenses in the OM lineup? That paradox in your theory is something for you to ponder. There is no paradox in mine. There is no paradox, you misstated cause and effect. That isn't a criticism - I frequently write things that make sense only to me. One of the benefits of telecentricity is that geometric distortion is minimised and objects remain the same size as the focus travels through them. The design of the OM series Zuiko 18mm maximises both of these effects, the object field magnifies at close focus and shrinks as focus moves to infinity. Exaggerated geometry, not to be confused with the exaggerated perspective due to the short focal length, was a selling feature of that lens - achieved by its extremely non-telecentric design. That sounds like a very interesting lens. |
#35
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Olympus OM enthusiasts' digital prayers have been answered ...
Why would a user of the 24x36mm OM-1/2/3/4 be anxious for a 1/4 frame
(17.3 x 13 mm) look-a-like? -- Mike |
#36
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Olympus OM enthusiasts' digital prayers have been answered ...
In article 2012020918152437271-pete3attkins@nospamntlworldcom, Pete A
writes On 2012-02-08 23:44:00 +0000, Kennedy McEwen said: In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A writes If a lens is rear telecentric then its rays are parallel i.e. the angle of incidence is zero. How can an image form with parallel rays? It is the *principle* rays which are parallel in a telecentric lens, not all the rays and certainly not the peripheral rays which determine the f/#. Indeed, the principle ray contains no light whatsoever because it's infinitely thin. Yes, the aperture affects the cone of peripheral rays. Of course it does, but those peripheral rays don't affect the telecentricity. It is the angle of the principle rays which determine that. If what you claimed was true then any pinhole lens would be telecentric - which is certainly not the case. A pinhole camera exhibits cos^4 light falloff at the film. Having a very small aperture results in a correspondingly small cone therefore it is primarily the angle of the principle ray that causes the light falloff in such a camera. At 33 degrees the falloff is 1 f-stop. With an image-side (rear) telecentric lens, it is only the angle of the cone that causes light falloff. The cone does not vary across the image plane therefore the image doesn't suffer from corner vignetting. Exactly what I said previously! A pinhole lens is NOT telecentric - the narrow cone angle of the high f/# does not affect the telecentricity of a lens, which is determined by angle of the principle rays to each point on the image plane. In any mirror-less system, the lens designer is free to position the exit pupil very close to the image plane, which places an unrealistic demand on the design of image sensors; especially Bayer CFA sensors. Specifying "near-telecentric" simply means placing the exit pupil _reasonably_ far away from the sensor. So, it is a big issue and perfectly warranted in a mirror-less system specification. We are discussing the use of OM series lenses in particular, which were not mirrorless cameras, and lenses from other film SLR cameras in general. So the issue of "placing an exit pupil close to the image plane" is irrelevant. Nevertheless, the same constraints on lens designs existed for mirrorless film cameras, such as Leica rangefinders. Furthermore, the lens specification for the 4-turds standard required compatibility with dSLRs as well, so that standard has the same restriction on rear element position as well. Olympus claimed they had to use small sensors because digital sensors in dSLRs required telecentric optics to work properly and the lenses designed for film would not cope with full size digital sensors. That was a lie and remains demonstrably so by anyone with an OM adapter and a FF dSLR. Stopping a non-telecentric lens down does not change the angle of incidence of the principle rays to any point on the focal plane. Thus, by definition, stopping a lens down does *NOT* make a lens "nearer to" telecentric in any way. Literally, you are correct. In essence, stopping down reduces the rays that are problematic to the sensor. It reduces just as many rays that are less oblique, and thus *less problematic*, as it does rays that are more oblique than the principle ray - it has no effect on the telecentricity or the response of the sensor. The change in light fall-off when stopping down is almost entirely a consequence of the exit pupil being partially obscured when fully open, not the angle of incidence of the principle rays - they remain fixed. The tiny exit pupil you mentioned is not the problem. The distance of the exit pupil from the image plane is the problem. You can't place a larger rear element any closer to the focal plane on an SLR without obstructing the mirror, consequently the 18/3.5 is "one of the least telecentric lenses in the OM series" due to its small rear element. If the lens design utilised a larger convergent rear element it could be placed at exactly the same distance with better telecentricity - so it *is* the size that makes this lens "one of the least telecentric lenses in the OM lineup". , therefore how can it be one of the least telecentric lenses in the OM lineup? That paradox in your theory is something for you to ponder. There is no paradox in mine. There is no paradox, you misstated cause and effect. I have re-read exactly what I wrote and I certainly did NOT mis-state cause and effect. On the contrary, you claimed that "a tiny rear exit pupil *ensures* that rays will not strike the sensor/film at an oblique angle" which is not only confusing cause and effect but also wrong! However, this shouldn't degrade to a "he said, she said" issue, the fact remains that telecentricity or the lack of it in lenses designed for film has no more effect on the solid state sensors in modern digital cameras than it did on film. One of the benefits of telecentricity is that geometric distortion is minimised and objects remain the same size as the focus travels through them. The design of the OM series Zuiko 18mm maximises both of these effects, the object field magnifies at close focus and shrinks as focus moves to infinity. That sounds like a very interesting lens. All non-telecentric lenses exhibit this effect to some degree. In fact it is one of the simplest tests of rear telecentricity - fill the frame with a subject and pull focus from closest point through to infinity. If the object remains the same size independent of the focus position (the distance of the lens from the focal plane) then the lens is rear telecentric. The more that image size changes, and hence geometry is distorted, with focus the less telecentric the lens is. -- Kennedy Yes, Socrates himself is particularly missed; A lovely little thinker, but a bugger when he's ****ed. Python Philosophers (replace 'nospam' with 'kennedym' when replying) |
#37
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Olympus OM enthusiasts' digital prayers have been answered ...
On 2/9/2012 19:50, Mike wrote:
Why would a user of the 24x36mm OM-1/2/3/4 be anxious for a 1/4 frame (17.3 x 13 mm) look-a-like? Not anxious, but there are things to like: "Free film" (well, once you've more or less heavily invested in equipment and any necessary peripherals), "instant developing" (though if you shoot raw that also takes some time & most images can use some adjusting before printing or viewing), some quite decent lenses (some of the very few WA zooms I've found to be good - I normally use Zeiss & Leica with film) and very compact and light (even compared to the original OM series). What I like less it that the "form follows function" idea got lost in translation and the useless false pentraprism adds quite a bit to the effective size of the camera. Also the hand grips seem to be a lame attempt to label the camera as a "pro" model (marketing predominating over content). Perhaps Olympus should introduce some oversized white telephoto lenses which can be parked at the side of sport fields, then they'd have succeeded in emulating the ultimate marketing brand.... That said, if the image quality is as good as it should be and the camera seems tough enough, I'll probably end up buying one when they get affordable. A water resistant, tough (hopefully) camera with decent (or hopefully more than decent) quality and tiny but good lenses is something to like. But I'll still use my film Leicas and Rolleiflexes when I want to do something special... |
#38
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Olympus OM enthusiasts' digital prayers have been answered ...
On Thu, 09 Feb 2012 14:55:37 +0000, Bruce
wrote: RichA wrote: On Feb 8, 11:30*am, Kennedy McEwen wrote: In article , Rol_Lei Nut writes On 2/7/2012 14:12, Kennedy McEwen wrote: OM lenses work just as well on digital as they did on film, which doesn't mean telecentric lenses can't work better, but the argument was false to begin with and was merely an attempt by Olympus to justify their investment in cheap chips. I have a Panasonic M43 and use *lots* of made for film lenses on it (M42, Zeiss, Leica both M & R, M39 & others). Some lenses, especially fast one, often don't work well at their faster apertures (by "not well" I mean visibly less well than on a film camera). Most lenses will exhibit vignetting when wide open. *That has nothing whatsoever to do with the sensor, whether digital or film, but due to the exit pupil being truncated. *You can see this happening just by looking through the lens while tilting it - that circular aperture wide open closes down to a "cat's eye" when significantly off axis. *The "cat's eye" is the main cause of the corner vignette. *Stop the lens down a little and tilting it has no effect on the aperture. This was just the same with film as it is on a digital sensor - whether you noticed it or not. *Furthermore, stopping the lens down doesn't change its telecentricity - so the very fact that you get a different level of vignette fully open demonstrates that telecentricity isn't the issue that Olympus claim(ed) it to be. Lots of compact, allegedly useless on digital because they aren't telecentric, OM lenses are used on Canon and other FF digital cameras without problem, hence their used prices are holding up quite well. Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the least telecentric lenses on the OM lineup due to the tiny rear pupil, and it is extremely flat to the corners when stopped down to f/5.6 - less than quarter a stop. I think there is more to this than just telecentricity. I tested an 85mm f2.0 OM lens on a m4/3rds camera and it vignetted, right down to f5.6. If an 18mm OM doesn't vignette horribly on a FF, then something else is at work. I find this discussion highly entertaining because it is the blind leading the blind. http://www.opto-engineering.com/tele...-tutorial.html may be of some help. The suggestion that the least telecentric lenses are always those with tiny rear elements is completely risible. In some cases, they are among the *most* telecentric! The implied converse, that a larger rear element means the lens is closer to telecentric is *complete nonsense*. It is extremely naive to assume that because the size of the rear element is closer to the size of the sensor, that the rays must be emerging almost parallel to each other and therefore perpendicular to the sensor. Once again, the truth can be the exact opposite, because a larger rear element can allow light rays to emerge at *more* oblique angles! That might appear counter-intuitive to some, but that merely demonstrates that intuition is highly subjective, and often wrong. Regards, Eric Stevens |
#39
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Olympus OM enthusiasts' digital prayers have been answered ...
On 2012-02-09 19:27:46 +0000, Kennedy McEwen said:
In article 2012020918152437271-pete3attkins@nospamntlworldcom, Pete A writes On 2012-02-08 23:44:00 +0000, Kennedy McEwen said: In article 2012020819270899904-pete3attkins@nospamntlworldcom, Pete A writes If a lens is rear telecentric then its rays are parallel i.e. the angle of incidence is zero. How can an image form with parallel rays? It is the *principle* rays which are parallel in a telecentric lens, not all the rays and certainly not the peripheral rays which determine the f/#. Indeed, the principle ray contains no light whatsoever because it's infinitely thin. Yes, the aperture affects the cone of peripheral rays. Of course it does, but those peripheral rays don't affect the telecentricity. It is the angle of the principle rays which determine that. If what you claimed was true then any pinhole lens would be telecentric - which is certainly not the case. A pinhole camera exhibits cos^4 light falloff at the film. Having a very small aperture results in a correspondingly small cone therefore it is primarily the angle of the principle ray that causes the light falloff in such a camera. At 33 degrees the falloff is 1 f-stop. With an image-side (rear) telecentric lens, it is only the angle of the cone that causes light falloff. The cone does not vary across the image plane therefore the image doesn't suffer from corner vignetting. Exactly what I said previously! A pinhole lens is NOT telecentric - the narrow cone angle of the high f/# does not affect the telecentricity of a lens, which is determined by angle of the principle rays to each point on the image plane. In any mirror-less system, the lens designer is free to position the exit pupil very close to the image plane, which places an unrealistic demand on the design of image sensors; especially Bayer CFA sensors. Specifying "near-telecentric" simply means placing the exit pupil _reasonably_ far away from the sensor. So, it is a big issue and perfectly warranted in a mirror-less system specification. We are discussing the use of OM series lenses in particular, which were not mirrorless cameras, and lenses from other film SLR cameras in general. So the issue of "placing an exit pupil close to the image plane" is irrelevant. Nevertheless, the same constraints on lens designs existed for mirrorless film cameras, such as Leica rangefinders. Furthermore, the lens specification for the 4-turds standard required compatibility with dSLRs as well, so that standard has the same restriction on rear element position as well. Olympus claimed they had to use small sensors because digital sensors in dSLRs required telecentric optics to work properly and the lenses designed for film would not cope with full size digital sensors. That was a lie and remains demonstrably so by anyone with an OM adapter and a FF dSLR. Stopping a non-telecentric lens down does not change the angle of incidence of the principle rays to any point on the focal plane. Thus, by definition, stopping a lens down does *NOT* make a lens "nearer to" telecentric in any way. Literally, you are correct. In essence, stopping down reduces the rays that are problematic to the sensor. It reduces just as many rays that are less oblique, and thus *less problematic*, as it does rays that are more oblique than the principle ray - it has no effect on the telecentricity or the response of the sensor. The change in light fall-off when stopping down is almost entirely a consequence of the exit pupil being partially obscured when fully open, not the angle of incidence of the principle rays - they remain fixed. The tiny exit pupil you mentioned is not the problem. The distance of the exit pupil from the image plane is the problem. You can't place a larger rear element any closer to the focal plane on an SLR without obstructing the mirror, consequently the 18/3.5 is "one of the least telecentric lenses in the OM series" due to its small rear element. If the lens design utilised a larger convergent rear element it could be placed at exactly the same distance with better telecentricity - so it *is* the size that makes this lens "one of the least telecentric lenses in the OM lineup". , therefore how can it be one of the least telecentric lenses in the OM lineup? That paradox in your theory is something for you to ponder. There is no paradox in mine. There is no paradox, you misstated cause and effect. I have re-read exactly what I wrote and I certainly did NOT mis-state cause and effect. On the contrary, you claimed that "a tiny rear exit pupil *ensures* that rays will not strike the sensor/film at an oblique angle" which is not only confusing cause and effect but also wrong! However, this shouldn't degrade to a "he said, she said" issue, the fact remains that telecentricity or the lack of it in lenses designed for film has no more effect on the solid state sensors in modern digital cameras than it did on film. One of the benefits of telecentricity is that geometric distortion is minimised and objects remain the same size as the focus travels through them. The design of the OM series Zuiko 18mm maximises both of these effects, the object field magnifies at close focus and shrinks as focus moves to infinity. That sounds like a very interesting lens. All non-telecentric lenses exhibit this effect to some degree. In fact it is one of the simplest tests of rear telecentricity - fill the frame with a subject and pull focus from closest point through to infinity. If the object remains the same size independent of the focus position (the distance of the lens from the focal plane) then the lens is rear telecentric. The more that image size changes, and hence geometry is distorted, with focus the less telecentric the lens is. The axe that you are grinding is with Olympus. My condoning of Olympus recommendations for practical reasons obviously irritates you - this is obvious not by your replies, it is obvious from what you snip from my replies. Enjoy your photography. |
#40
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Olympus OM enthusiasts' digital prayers have been answered ...
On 2012-02-09 20:12:27 +0000, Eric Stevens said:
On Thu, 09 Feb 2012 14:55:37 +0000, Bruce wrote: RichA wrote: On Feb 8, 11:30*am, Kennedy McEwen wrote: In article , Rol_Lei Nut writes On 2/7/2012 14:12, Kennedy McEwen wrote: OM lenses work just as well on digital as they did on film, which doesn't mean telecentric lenses can't work better, but the argument was false to begin with and was merely an attempt by Olympus to justify their investment in cheap chips. I have a Panasonic M43 and use *lots* of made for film lenses on it (M42, Zeiss, Leica both M & R, M39 & others). Some lenses, especially fast one, often don't work well at their faster apertures (by "not well" I mean visibly less well than on a film camera). Most lenses will exhibit vignetting when wide open. *That has nothing whatsoever to do with the sensor, whether digital or film, but due to the exit pupil being truncated. *You can see this happening just by looking through the lens while tilting it - that circular aperture wide open closes down to a "cat's eye" when significantly off axis. *The "cat's eye" is the main cause of the corner vignette. *Stop the lens down a little and tilting it has no effect on the aperture. This was just the same with film as it is on a digital sensor - whether you noticed it or not. *Furthermore, stopping the lens down doesn't change its telecentricity - so the very fact that you get a different level of vignette fully open demonstrates that telecentricity isn't the issue that Olympus claim(ed) it to be. Lots of compact, allegedly useless on digital because they aren't telecentric, OM lenses are used on Canon and other FF digital cameras without problem, hence their used prices are holding up quite well. Only this morning I was shooting with the Zuiko 18mm f/3.5, one of the least telecentric lenses on the OM lineup due to the tiny rear pupil, and it is extremely flat to the corners when stopped down to f/5.6 - less than quarter a stop. I think there is more to this than just telecentricity. I tested an 85mm f2.0 OM lens on a m4/3rds camera and it vignetted, right down to f5.6. If an 18mm OM doesn't vignette horribly on a FF, then something else is at work. I find this discussion highly entertaining because it is the blind leading the blind. http://www.opto-engineering.com/tele...-tutorial.html may be of some help. Thanks Eric. Some may find these interesting: http://www.pco.de/fileadmin/user_upload/db/download/pco_cooKe_kb_shading_0603_s.pdf http://en.wikipedia.org/wiki/Vignetting http://toothwalker.org/optics/vignetting.html The suggestion that the least telecentric lenses are always those with tiny rear elements is completely risible. In some cases, they are among the *most* telecentric! The implied converse, that a larger rear element means the lens is closer to telecentric is *complete nonsense*. It is extremely naive to assume that because the size of the rear element is closer to the size of the sensor, that the rays must be emerging almost parallel to each other and therefore perpendicular to the sensor. Once again, the truth can be the exact opposite, because a larger rear element can allow light rays to emerge at *more* oblique angles! That might appear counter-intuitive to some, but that merely demonstrates that intuition is highly subjective, and often wrong. I totally agree with Bruce. A true bi-telecentric (afocal) lens demands a front element somewhat larger than the object and a rear element somewhat larger than the sensor; a near-telecentric image-space lens has no such demands. Very few people can reverse-engineer a lens just by looking at it and jumping to intuitive conclusions :-) |
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Olympus OM enthusiasts' digital prayers have been answered ... | Chloe | 35mm Photo Equipment | 15 | February 13th 12 04:11 PM |