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Photo Forum / Digital Photography / DSLR Cameras / April 2006How would lens optics/engineering drive toward an optimal sensor/film size?
Hi all, I have learned a lot as the practitioners and engineers and such (you know who you are :-) have debated sensor size, physics, angles, and limits. The related question I have is, if we started with some goals for the glass and worked toward a sensor what kind of system would we end up with? Is a 50mm 1.8 35mm lens a great lens optically, and for moderate cost, because its in the sweet spot for the 'physics of glass' or is just the most engineered / largest selling lens and market size reduced the cost? For wide angle for a given field of view, is a 24m easier to get 'good' (distortion, flair, etc) vs an 18mm lens? So body and sensor wars aside, if we could start over and pick a format driven by optics what would it be? If, optically, bigger is always better (e.g. 200 inch in some fields is nice) then is this just a size/quality/cost/preferences trade-off debate? > Hi all, > I have learned a lot as the practitioners and engineers and such (you [quoted text clipped - 11 lines] > If, optically, bigger is always better (e.g. 200 inch in some fields is > nice) then is this just a size/quality/cost/preferences trade-off debate? I'll take a stab at it. No guarantees though <g>. It is a matter of knowing the trade-offs and which to chose for your need. Small sensor format has more depth of field and is easier to get long focal lengths and a large sensor format has very limited depth of field, and is impractical to make long focal lengths for but has room for more pixels and larger pixels with less noise but again it's impractical to make large sensors because the drive behind technology these days is miniaturization. Small sensor formats perform better in low light allowing faster shutter speeds but the noise associated with ISO boost seems to counteract this advantage so that the high ISO performance of a larger sensor holds the advantage up to a point, when you get to big view cameras it's completely impractical to improve the already too expensive sensor's ISO rating because it's too big and probably near quantum limits anyways. We'll see if I screwed up anything (probably). >It is a matter of knowing the trade-offs and which to chose for your need. You have to be careful not to change too many parameters at the same time. >Small sensor format has more depth of field and is easier to get long >focal lengths and a large sensor format has very limited depth of field, This is not true. Getting DOF from a larger format requires using a higher numerical aperture. But that is not magic. Even diffraction doesn't get in the way. However, for this to be true, the larger format has to be as sensitive (per pixel) as the smaller format, and this require a much higher ISO sensitivity. Usually, larger formats are used to get more resolution, and more resolution comes at the expense of DOF (assuming a constant amount of light). >and is impractical to make long focal lengths True. >has room for more >pixels and larger pixels with less noise True >again it's impractical to >make large sensors because the drive behind technology these days is >miniaturization. It is impractical because it is hard to make large sensors. Miniaturization doesn't have much to do with it. Miniaturization does make certain things cheaper over time. >Small sensor formats perform better in low light >allowing faster shutter speeds but the noise associated with ISO boost >seems to counteract this advantage so that the high ISO performance of a >larger sensor holds the advantage up to a point, I don't see any reason why small sensor would perform better in low light. (well, there are dark current and read-out noise, but I am not sure how they scale, and photon shot noise tends to be the dominant source of noise in many low light applications). >when you get to big >view cameras it's completely impractical to improve the already too >expensive sensor's ISO rating because it's too big and probably near >quantum limits anyways. The quantum limits are the same for everyone. It is much more likely that in many larger formats, read-out electronics is not as optimized as in smaller formats.  SignatureThat was it. Done. The faulty Monk was turned out into the desert where it could believe what it liked, including the idea that it had been hard done by. It was allowed to keep its horse, since horses were so cheap to make. -- Douglas Adams in Dirk Gently's Holistic Detective Agency >>It is a matter of knowing the trade-offs and which to chose for your need. > [quoted text clipped - 11 lines] > (per pixel) as the smaller format, and this require a much higher > ISO sensitivity. This is why smaller formats perform better in low light. Below a certain size, quantum mechanics & diffraction start to interfere though. So there ought to be a chart that shows where these factors collide & an optimal sensor size can be chosen (for a given use). > Usually, larger formats are used to get more resolution, and more resolution > comes at the expense of DOF (assuming a constant amount of light). [quoted text clipped - 25 lines] > scale, and photon shot noise tends to be the dominant source of noise in > many low light applications). Assuming a fixed budget of $3000 I'm thinking it might be possible to make a super fast P&S better than 35mm? I guess not though if you look at a 4/3 sensor with a honking big expensive lens in that budget I don't think that's any faster than APS so that suggests APS is pretty close to optimal. Full frame 35mm is faster but you need longer lenses to match & that could blow the budget. >>when you get to big >>view cameras it's completely impractical to improve the already too [quoted text clipped - 4 lines] > in many larger formats, read-out electronics is not as optimized as in > smaller formats. >> However, for this to be true, the larger format has to be as sensitive >> (per pixel) as the smaller format, and this require a much higher >> ISO sensitivity. > >This is why smaller formats perform better in low light. I don't see why this has to be true. Nobody makes 6 Mpixel medium format backs in the same process technology as is used for 6 Mpixel P&S sensors. >> I don't see any reason why small sensor would perform better in low light. >> (well, there are dark current and read-out noise, but I am not sure how they [quoted text clipped - 7 lines] >optimal. Full frame 35mm is faster but you need longer lenses to match & >that could blow the budget. Yes, and for $1000 the P&S is likely to win from the APS. The original question was about optics. When you spend $3000 on a full frame 35mm system, you don't have any money left to buy lenses. If you remove the sensor from the equation, then $3000 buys you much better optics on full frame 35mm than on a tiny P&S format. SignatureThat was it. Done. The faulty Monk was turned out into the desert where it could believe what it liked, including the idea that it had been hard done by. It was allowed to keep its horse, since horses were so cheap to make. -- Douglas Adams in Dirk Gently's Holistic Detective Agency >>>However, for this to be true, the larger format has to be as sensitive >>>(per pixel) as the smaller format, and this require a much higher [quoted text clipped - 4 lines] > I don't see why this has to be true. Nobody makes 6 Mpixel medium format backs > in the same process technology as is used for 6 Mpixel P&S sensors. The quantum efficiency of sensors doesn't change with sensor and pixel size, so sensitivity does not change. But larger pixels (in larger sensors) collect more photons per pixel. See: The Signal-to-Noise of Digital Camera images and Comparison to Film http://www.clarkvision.com/imagedetail/digital.signal.to.noise The f/ratio Myth and Digital Cameras http://www.clarkvision.com/photoinfo/f-ratio_myth Another factor is that as sensor size shrinks, a wall between sensors must be maintained between pixels. That wall is not photon sensitive, so the fractional photo sensitive area goes down. In the data I've seen on read sensor noise, some of which is presented in tables on the first web page above, the larger sensors with more expensive electronics have lower read noise than cheap P&S cameras. Camera performance is about lenses and sensors. Collecting adequate photons for good signal-to-noise ratio images was something not appreciated before digital cameras came along. So having pixel sizes large enough to collect lots of photons is important. That means larger pixels, but too large and you can't pack as many in the camera or it becomes too large. The sweet spot is currently 6 to 9 micron pixels in my opinion. This also happens to be a good size for lens aberrations, including diffraction. Roger >>>> However, for this to be true, the larger format has to be as sensitive >>>> (per pixel) as the smaller format, and this require a much higher [quoted text clipped - 16 lines] > The f/ratio Myth and Digital Cameras > http://www.clarkvision.com/photoinfo/f-ratio_myth from the last paragraph on that page: "If you tried to make a smaller camera that collects the same number of photons as a larger camera, you must keep the aperture constant. Given a camera, for example, with a 50 mm f/1.4 lens and then shrink the camera 2x, you would need a 25 mm f/0.7 lens that had double the resolution if you wanted to keep the same detail in the image. That means the smaller camera would not be much smaller, and might be more expensive due to the lens specifications." Just to be clear though about this f/ratio Myth, the exposure time remains the same for f/stops on different size sensors, it's just the signal to noise ratio or degree of detail that's compromised by not using a larger aperture. And the DOF change. And to get the miniature camera with high performance you describe would have to be some magical high-res film? If you were willing to make small-format fast lenses, at what point does the pixel size limit things? 6-9 microns? > Another factor is that as sensor size shrinks, a wall > between sensors must be maintained between pixels. That wall [quoted text clipped - 15 lines] > in my opinion. This also happens to be a good size > for lens aberrations, including diffraction. > And to get the miniature camera with high performance you describe would > have to be some magical high-res film? If you were willing to make > small-format fast lenses, at what point does the pixel size limit > things? 6-9 microns? It depends on what you mean by limiting. If you don't mind not using higher numbered f/ratios, and with a fast lens it delivers the optical resolution, then you could do quite well in the 3 micron range, except for the number of photoelectrons you can store in the small well. Performance would drop more rapidly as you went below 3 microns and approached the wavelength of light (diffraction on the optical side, and small electron well capacity on the sensor side). Roger >> And to get the miniature camera with high performance you describe >> would have to be some magical high-res film? If you were willing to [quoted text clipped - 3 lines] > ...except for the number of photoelectrons > you can store in the small well... That's what I'm understanding is the main limitation even if you have a super fast mini-lens pumping the same number of photons into a smaller sensor, can the sensor hold them? >>> And to get the miniature camera with high performance you describe >>> would have to be some magical high-res film? If you were willing to [quoted text clipped - 7 lines] > super fast mini-lens pumping the same number of photons into a smaller > sensor, can the sensor hold them? No, at least not with present technology. Roger >> And to get the miniature camera with high performance you describe would >> have to be some magical high-res film? If you were willing to make [quoted text clipped - 11 lines] > >Roger But just think of it; A FF with 3um pixels. About 35 megapixel resolution? Limit it to moderate contrast image at 100 ISO or less (landscapes?) and provided you had lenses that could make use of it, it would be terrific. -Rich >>>And to get the miniature camera with high performance you describe would >>>have to be some magical high-res film? If you were willing to make [quoted text clipped - 17 lines] > make use of it, it would be terrific. > -Rich Yeah, but 3-micron pixels will not hold many photons. But use pixels in the 20D size, 6.4 microns and you get 35.8/0.0064 * 23.9/0.0064 = 20.9 megapixels. A lot of high quality pixels, each holding about 50,0000 photoelectrons. The 1Ds Mark II is almost there. Roger [] > If, optically, bigger is always better (e.g. 200 inch in some fields > is nice) then is this just a size/quality/cost/preferences trade-off > debate? Thanks for your comments, Tom. It triggers a couple of questions. I see it as more a sensor noise and camera function trade-off. Taking two of the current popular camera styles, so-called P&S (or small sensor) and DSLR: small sensor: - shorter focal lengths, so mechanical tolerances are tighter - larger depth of field - diffraction more important - no need for long back focus - less glass so cheaper material costs - no need for interchangeable mount DSLR - longer focal lengths, easier mechanical tolerances - smaller depth of field - diffraction less important - longer back-focus required to clear mirror - more glass, so bigger and more expensive material costs - interchangeable mount required Now extend those comparisons to larger cameras. Of course, the actual sensor plays a major part in the resulting performance, not just the optics. The 35mm lenses have been developed intensively over the last 50 years, so you would expect them to have developed to a peak of performance. Question 1: You see some 12:1 zoom lenses for small-sensor cameras that were unmatched in the 35mm world until the recent Nikon 18 - 200mm VR zoom. Why is that? Perhaps because the smaller sensor lenses are "designed for digital" in the sense that they don't need a very high ultimate resolution - they need an MTF curve with a sharper cut-off than a long tail. Question 2: Why are lenses for 4/3 sensors (about half the linear size of 35mm film), not half the linear size and an eighth of the weight? It seems to me that the sensor size is more important, and that lens engineering adapts to the sensor size requirements. David Why do the small sensor cameras not offer really wide angle? Is that just a matter of marketing appeal or a physical limitation? I know you can get screw-on adapters but I assume that's a big hit on image quality. Large format is usually fairly wide angle though I'm not aware of extreme wide lenses for those either, maybe just a matter of so many different lenses developed for 35mm because there has been a big market with lots of room for innovation. > Why do the small sensor cameras not offer really wide angle? Is that > just a matter of marketing appeal or a physical limitation? I know you [quoted text clipped - 3 lines] > so many different lenses developed for 35mm because there has been a > big market with lots of room for innovation. You can get 23mm and 24mm equivalent, and the Nikon add-on for the 8400 gives (IIRC) and 18mm equivalent FOV at high quality. No "big hit on image quality". I suspect that the real reason is that it ywould not be cost-effective for the relatively small number of sales. Who is going to buy a $600 lens for a $200 camera? David >I suspect that the real reason is that it ywould not be cost-effective for >the relatively small number of sales. Who is going to buy a $600 lens for >a $200 camera? A smart photographer...? Seriously, my lenses equal or exceed the value of my camera body because I know that's where the image is formed. >> I suspect that the real reason is that it ywould not be >> cost-effective for the relatively small number of sales. Who is [quoted text clipped - 4 lines] > Seriously, my lenses equal or exceed the value of my camera body > because I know that's where the image is formed. Whilst this may be true for the SLR enthusiast (and it was true for my own SLR outfit), I don't believe it /as/ true for the $200 digital camera owner (which is why I set the camera price rather low). Once DSLRs are offered at US $200 perhaps it will be true. David >>> I suspect that the real reason is that it ywould not be >>> cost-effective for the relatively small number of sales. Who is [quoted text clipped - 9 lines] >owner (which is why I set the camera price rather low). Once DSLRs are >offered at US $200 perhaps it will be true. Yes, for P&S, I agree completely. Hence the joke...did I forget the smiley? >>>> I suspect that the real reason is that it ywould not be >>>> cost-effective for the relatively small number of sales. Who is [quoted text clipped - 13 lines] > > Hence the joke...did I forget the smiley? Probably not - it's still (relatively) early in the day over here! David >>Why do the small sensor cameras not offer really wide angle? Is that >>just a matter of marketing appeal or a physical limitation? I know you [quoted text clipped - 5 lines] > > You can get 23mm and 24mm equivalent, Alright, that's not bad (15mm on APS DSLR). That's only on the fancier models though, right? Last time I looked the Oly models seemed the only reasonable option and they don't go very long. > and the Nikon add-on for the 8400 > gives (IIRC) and 18mm equivalent FOV at high quality. No "big hit on [quoted text clipped - 3 lines] > the relatively small number of sales. Who is going to buy a $600 lens for > a $200 camera? [] >> You can get 23mm and 24mm equivalent, > > Alright, that's not bad (15mm on APS DSLR). That's only on the fancier > models though, right? Last time I looked the Oly models seemed the > only reasonable option and they don't go very long. The Nikon 8400 (24mm) was towards the top end of the Nikon range, yes, but I would not describe the Kodak V570 as a "fancier model". http://www.dpreview.com/reviews/nikoncp8400/ http://www.dpreview.com/reviews/specs/Kodak/kodak_v570.asp I believe there are more cameras now offering 24mm, and doubtless someone will give the URL for listing cameras by widest field of view on DP Review or wherever! David > [] > [quoted text clipped - 8 lines] > > http://www.dpreview.com/reviews/nikoncp8400/ 24 - 85 mm F2.6 - F4.9 > http://www.dpreview.com/reviews/specs/Kodak/kodak_v570.asp 39 - 117 mm F3.9 - F4.4 Oly C8080: http://www.steves-digicams.com/2004_reviews/c8080.html 28 - 140mm f2.4 > I believe there are more cameras now offering 24mm, and doubtless someone > will give the URL for listing cameras by widest field of view on DP Review > or wherever! > > David >> [] >> [quoted text clipped - 24 lines] >> >> David Paul, You missed the second lens in the V570. From the press release: "Using proprietary KODAK RETINA Dual Lens technology, the elegant V570 camera wraps an ultra-wide angle lens (23 mm) and an optical zoom lens (39 - 117 mm) into a small, sleek package less than an inch thin." Daivd >>> http://www.dpreview.com/reviews/specs/Kodak/kodak_v570.asp >> [quoted text clipped - 7 lines] > camera wraps an ultra-wide angle lens (23 mm) and an optical zoom lens > (39 - 117 mm) into a small, sleek package less than an inch thin." Oh cool, an interchangeable prime built in! >>>> http://www.dpreview.com/reviews/specs/Kodak/kodak_v570.asp >>> [quoted text clipped - 10 lines] > > Oh cool, an interchangeable prime built in! .. or perhaps a dual-sensor camera! http://www.dpreview.com/news/0601/06010201kodakv570.asp says: "Kodak EasyShare V570, dual CCD" David > Question 2: Why are lenses for 4/3 sensors (about half the linear size of > 35mm film), not half the linear size and an eighth of the weight? My guess is that the mirror box is in the way and that the shortest distance from the sensor to the back element of the lens is not half of that for a 35 mm equivalent. That is I assume the most important reason. The next reason is that you can not scale down all mechanics, and the handling can not be scaled down by a factor of two. (Or you have to scale down your hands to operate the lens.) (For point and shoots, no mirror box, you can't handle de lens at all, so the above constraint aply not or less.) ben > [sweet talk] > The related question I have is, if we started with some goals for the > glass and worked toward a sensor what kind of system would we end up > with? The prior posts are mostly about sensors...is that because we are sensor-centric or...what? > Is a 50mm 1.8 35mm lens a great lens optically, and for moderate cost, > because its in the sweet spot for the 'physics of glass' or is just > the most engineered / largest selling lens and market size reduced the > cost? Still interested in physics of the "Normal Lens". As a D50 owner, a fast 35mm lens will currently cost me 2x - 3x the cost of a 1.8 50mm. Will we see fast, good, cheap "Normal" lenses for the 1.5X format? > For wide angle for a given field of view, is a 24m easier to get > 'good' (distortion, flair, etc) vs an 18mm lens? So body and sensor > wars aside, if we could start over and pick a format driven by optics > what would it be? Same here, just a matter of time for good 18mm glass at 24mm prices...or no? >> [sweet talk] >> The related question I have is, if we started with some goals for the [quoted text clipped - 20 lines] > >Same here, just a matter of time for good 18mm glass at 24mm prices...or no? Yes, I noticed everyone leapt in on the sensor question and rather ignored the optics part. To some extent the answer to your question is that 50mm on 35mm represents the easiest specification for lens design. The angle of view - typically 46 degrees diagonal on 35mm film - is about the least challenging to perfect. The face that, historically, they tended to be the most popular may or may not have resulted in earlier perfection of designs, and cheapness through larger production runs, but after several decades of sophisticated computer design and of the almost complete dominance of zooms in the mass market I would be surprised if this is still a major factor. Wider lenses tend to suffer from a rapid increase in some design problems: vignetting, curvilinear distortion (i.e. barrel or pincushion), coma and astigmatism all get worse as FoV increases. The solutions include: restricting maximum aperture, using aspherical elements and floating elements, and in any event many *more* elements; all this results in lenses which are larger than you would expect, and much more expensive. Longer lenses tend to suffer a different array of problems. Chromatic aberration, especially lateral CA, gets worse as relative focal length increases. This can be reduced or eliminated by the use of (very expensive) fluorite or extra-low dispersion glass, and more complex design. Also, temperature effects start to become significant (focus point shifts as the lens barrel expands). And of course the size of glass elements required to give a decent f-number goes up as f goes up. To illustrate this, just look at a few specifications. These are from a Canon book as that's what I happen to have, but I have no doubt a similar story exists in other makers' lists. 50mm f/1.8 --- 6 elements in 5 groups 50mm f/1.4 --- 7 elements in 6 groups This shows that even at 50mm you have to use a more complex design to get good results at a 0.5 stop wider aperture. 28mm f/2.8 --- 5 elements in 5 groups Similar to the 50/1.8 - but notice we lost 1.5 stops to allow simplification 28mm f/1.8 --- 10 elements in 9 groups To get the wide aperture back we have to have a much more complex design 14mm f/2.8L --- 14 elements in 10 groups (including 1 aspherical element) About as bad as it gets at the wide end.. And going the other way: 100mm f/2.0 --- 8 elements in six groups Bit more complex, smaller aperture. 200mm f/2.8 --- 9 elements in 7 groups More complex and a stop down 400mm f/5.6 --- 7 elements in 6 groups Small aperture keeps it fairly simple (though still requires 2 ULD elements). But now see: 400mm f/2.8L --- 17 elements in 13 groups (including 1 fluorite and 2 ULD elements) I was quite surprised, looking this up, to see just how complex the design of these lenses are. If you look at the price lists you will see a similar (but more extreme) story - and again, probably not because of scale of production as the 50mm fixed focal length has fallen out of favour. When I had my first SLR, a 400mm lens typically had 4 or 5 elements - no fancy glass - but the performance was very poor, low contrast and low resolution, for the reasons I mentioned above. Way below the simple 58mm f/2 Helios I had as a standard lens. The 17/13 sophistication (and £4000+ price) is required just to get performance back to the same level as the £80 Canon 50mm f/1.8 - and still at 1.5 stops less aperture. Obviously if you change the film/sensor format then the focal lengths change - it is (mostly) the FoV that affects design, apart from mechanical things like material strength and size of controls, which as someone else pointed out will limit the benefits of a linear scaling down. There is no obvious reason why a 35mm f/1.8, or even a 17mm f/1.8, should not be available given a format which requires a 40-55 degree FoV at that focal length. Indeed, look at cine/video cameras and you can see it has been done. David SignatureDavid Littlewood David Littlewood: > 50mm f/1.8 --- 6 elements in 5 groups > 50mm f/1.4 --- 7 elements in 6 groups Not to forget the old standard designs: 50/4 --- 3 elements in 3 groups (tessar) 50/2.8 --- 4 elements in 3 groups (modifierd tessar) > This shows that even at 50mm you have to use a more complex design to > get good results at a 0.5 stop wider aperture. Correct. Concerning the focal length, one approach is to just "enlarge" a design and use it as a longer focal length (maybe on a larger format). But there are two problems with this: 1. This also enlarges all flaws in the design (CA, spheric aberations etc). With the same format, a longer lense is used to resolve more details, so a better design is necessary. On a larger format the flaws may be acceptable up to a point. 2. Size and economics. Make the design twice as big means 8 times as much glass and weight. By chosing a more sophisticated design (and maybe better glass), the weight and cost can be reduced to an acceptable value. Reducing the aperture size also helps :-) On the other side, shrinking a design also has problems: 3. If a smaller format is used, the effect of diffraction increases. For many compact digital cameras with tiny sensors, this is very relevant. 4. If the same format is used, the lense has to cover a (relatively) larger area. At an SLR there also has to be enough clearance for the mirror. Both factors can make lense design very difficult. Again, reducing the aperture helps. So in conclusing I think that a "normal" lense (focal length = format diagonal) is always going to be the "easiest" design with the best optical properties. Using a bigger format will also improve the quality of the picture, but it comes at a cost (bigger, heavier, more expensive, less DOF). Thomas >>> [sweet talk] >>> The related question I have is, if we started with some goals for the [quoted text clipped - 45 lines] >aberration, especially lateral CA, gets worse as relative focal length >increases. No, it gets worse as lens speed increases. The way colour error happens is due to the edges of the lenses acting like prism wedges. The faster the lens, the steeper the curves, the greater the prismatic effect. A 1000mm lens at f16 has virtually zero colour but a 1000mm lens at f6 has noticeable colour, provided both designs use the same glass. This can be reduced or eliminated by the use of (very >expensive) fluorite or extra-low dispersion glass, and more complex >design. Also, temperature effects start to become significant (focus [quoted text clipped - 7 lines] >50mm f/1.8 --- 6 elements in 5 groups >50mm f/1.4 --- 7 elements in 6 groups That is basically "old school" lens design using all spherical surfaces. Molded aspherics (at least in shorter lenses where the optical errors of such products can be tolerated) allow for a decrease in the number of spherical elements to be used. In fact, some specialized, highly corrected and flat field lenses suitable for medium format film coverage have only 3-4 elements. What this does is eliminate (lens surfaces) and the contrast and light loss associated with them. However, where lenses need higher optical correction (telephotos) molded aspherics would be impossible to use owing to the problems of making error-free large molded elements. The only solution in this case is the old spherical solutions or very expensive ground aspherical elements. Aspherics also eliminate some of the "trade off" abberation control e that characterizes all spherical lens designs where you would put up with (for e.g.) 5 waves of spherical aberration in order to control a wave of chromatic aberration, lesser of two evils. The invention of multicoatings has (IMO) hampered lens design because it has kept the cheap to fabricate spherical lenses as the mainstay for camera lens design by eliminating much of the problem with multiple element design which are internal reflections and light loss. -Rich >>Longer lenses tend to suffer a different array of problems. Chromatic >>aberration, especially lateral CA, gets worse as relative focal length [quoted text clipped - 6 lines] >lens at f6 has noticeable colour, provided both designs use the same >glass. I'm afraid every part of the above statement is incorrect. Lateral CA (or Transverse CA, or Lateral Colour, or Chromatic Difference of Magnification) is progressively worse at longer focal lengths, and is absolutely unaffected by stopping down. It is the hardest aberration to cure for long lenses, and is the one that limits their performance. > This can be reduced or eliminated by the use of (very >>expensive) fluorite or extra-low dispersion glass, and more complex [quoted text clipped - 31 lines] >with multiple element design which are internal reflections and light >loss. This sounds equally incorrect to me, but I haven't time to refute it line by line. If aspherics were the magic bullet for long lenses, there are plenty of places in modern designs where the elements are quite small and could easily be aspherical. In fact, since most of the problem with long lenses is lateral CA, which is (despite the incorrect statements you made above) not solved by aspherics, then I think the rest of what you say must be regarded as tosh. David SignatureDavid Littlewood >>>Longer lenses tend to suffer a different array of problems. Chromatic >>>aberration, especially lateral CA, gets worse as relative focal length [quoted text clipped - 12 lines] >absolutely unaffected by stopping down. It is the hardest aberration to >cure for long lenses, and is the one that limits their performance. No. Colour error (all types) is dictated by: -The glass used. -The aperture of the lens. -The f-ratio of the lens. If two lenses are constructed using the same basic design and glass, colour error will increase either because of an increase in aperture or a decrease in focal ratio. This has been known for about 350 years. Check out this page: http://www.vanwalree.com/optics/chromatic.html >> This can be reduced or eliminated by the use of (very >>>expensive) fluorite or extra-low dispersion glass, and more complex [quoted text clipped - 41 lines] > >David I can only relate this to high-end telescope production. The goal is fewer surfaces and they've done it to the extend of using index-matched oils to space lenses to eliminate surfaces because they know that even with the best multi-coatings, each surface reduces contrast in an image and light throughput. http://www.astro-physics.com/products/telescopes/curves/curves.htm If you look at this page and note the colour corrections, apertures and focal ratios of the instruments, you'll see what I've said is borne out. -Rich |
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