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#21
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EF 50/1.8 AF Experiment?
Wilba wrote:
Chris Malcolm wrote: Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? The AF sensors pay no attention to the aperture at which you're going to take the picture. They do their work before the lens is stopped down. Their construction gives them an effective aperture of their own. Often this is around f6. That means that when the largest aperture of a lens is smaller than that they can't get enough light to work properly. That's why generally speaking you can't make reflex lenses autofocus, because for technical reasons their best compromise aperture is often smaller than that, e.g. 500mm f8. More expensive DSLRs will also have larger aperture AF sensors at the central position, e.g. around f3, with which they'll be able to get focus in lower light with lenses which with max apertures which open that far. It also improves the focus on very fast lenses with spherical aberration and corresponding aperture related focus drift, such as the old spherical type of 50mm f1.4 lenses. Since the DOF gets very thin indeed at wide apertures and close portrait type distances, which is often what is going on in a dimly lit interior, the slightest error in AF will leave the image blurred at the point you wished to focus on, and sharp nearby. For example in a portrait you might have focused on the eyes, and find that the eyes aren't in focus, but the tip of the nose, or the ears, are. The reason for that is often that when DoF gets so sharp it becomes smaller than the small residual error in the AF of your camera, i.e. your camera has a slight front or back focus in the AF sensor plane calibration which is larger than the DoF at these wide apertures. If you find a systematic error of this type in your camera than you either must switch to manual focus, or compensate yourself, e.g. by holding down focus on the eyes and then simply moving your head back or forwards a few cm to take up the systematic error. Usually the more expensive DSLRs have better AF sensors so they can focus better in lower light. The wider aperture AF sensors are also able to get a tighter focus for wide aperture low light work because the AF sensor itself has effectively a shallower DoF. That will also rein in some of the aperture related focus drift of wide aperture spherical lenses. The more expensive DSLRs are also sometimes able to read lens-specific focus compensation factors from the lens, and use that to trim out systematic errors in autofocus for that specific lens. The most expensive DSLRs go one better than that. They have user trimmable tables of focus compensation for specific lenses in order to get better focus with the more awkward lenses in the more awkward situation, in which the AF will have slight lens-specific systematic focus errors. Thanks for your effort. Unfortunately, there is no answer to my question within it. :- ) Your qestion doesn't make sense because you don't understand enough about what might be going on with your specific camera and your specific focus difficulties. Your questions need to be revised in the light of an improved understanding, part of which must come from doing some experiments of your own to discover exactly what with your specific camera and focus problems the most important problems are. For example, your problems could be simply not enough light for your camera's AF sensors to work properly. If so there's nothing you can do with the way you use your lens to improve that. But there are several ways of helping your AF to work better in poor light. On the other hand your difficulties could be due to a small AF calibration error which starts to matter under those circumstances with your lens. If so there are a number of things you can do about that. But it's not worth going into all the details of all the possibilities until you have found out more about which particular limitation of the several possibilities your camera and lens are coming up against. -- Chris Malcolm |
#22
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EF 50/1.8 AF Experiment?
On 09-12-21 0:40 , Wilba wrote:
Alan Browne wrote: Wilba wrote: Those shots were focussed using contrast detection. Any error you might perceive is of the order of the typical shot-to-shot variation. That softness may fall outside the AF hysteresis band. More information please. In order to avoid hunting near focus, the system is designed with an error band such that once the focus lies within that band the focus action (control loop) can stop. Otherwise there could be hunting. For most lenses, the sharpest focus falls somewhere inside the hysteresis band. With your 50, the AF might determine the focus as in there or slightly to either side making the focus error range larger. This due to the softness wide open. The control loop must have several stop (exit) conditions including 'best case' (to cover this case as well as dim light). There may also be more mechanical slop built in to the lens to allow for a smaller motor to drive it. That's harder to pinpoint. |
#23
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EF 50/1.8 AF Experiment?
Chris Malcolm wrote:
Wilba wrote: Chris Malcolm wrote: Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? The AF sensors pay no attention to the aperture at which you're going to take the picture. They do their work before the lens is stopped down. Their construction gives them an effective aperture of their own. Often this is around f6. That means that when the largest aperture of a lens is smaller than that they can't get enough light to work properly. That's why generally speaking you can't make reflex lenses autofocus, because for technical reasons their best compromise aperture is often smaller than that, e.g. 500mm f8. More expensive DSLRs will also have larger aperture AF sensors at the central position, e.g. around f3, with which they'll be able to get focus in lower light with lenses which with max apertures which open that far. It also improves the focus on very fast lenses with spherical aberration and corresponding aperture related focus drift, such as the old spherical type of 50mm f1.4 lenses. Since the DOF gets very thin indeed at wide apertures and close portrait type distances, which is often what is going on in a dimly lit interior, the slightest error in AF will leave the image blurred at the point you wished to focus on, and sharp nearby. For example in a portrait you might have focused on the eyes, and find that the eyes aren't in focus, but the tip of the nose, or the ears, are. The reason for that is often that when DoF gets so sharp it becomes smaller than the small residual error in the AF of your camera, i.e. your camera has a slight front or back focus in the AF sensor plane calibration which is larger than the DoF at these wide apertures. If you find a systematic error of this type in your camera than you either must switch to manual focus, or compensate yourself, e.g. by holding down focus on the eyes and then simply moving your head back or forwards a few cm to take up the systematic error. Usually the more expensive DSLRs have better AF sensors so they can focus better in lower light. The wider aperture AF sensors are also able to get a tighter focus for wide aperture low light work because the AF sensor itself has effectively a shallower DoF. That will also rein in some of the aperture related focus drift of wide aperture spherical lenses. The more expensive DSLRs are also sometimes able to read lens-specific focus compensation factors from the lens, and use that to trim out systematic errors in autofocus for that specific lens. The most expensive DSLRs go one better than that. They have user trimmable tables of focus compensation for specific lenses in order to get better focus with the more awkward lenses in the more awkward situation, in which the AF will have slight lens-specific systematic focus errors. Thanks for your effort. Unfortunately, there is no answer to my question within it. :- ) Your qestion doesn't make sense because you don't understand enough about what might be going on with your specific camera and your specific focus difficulties. Your questions need to be revised in the light of an improved understanding, part of which must come from doing some experiments of your own to discover exactly what with your specific camera and focus problems the most important problems are. For example, your problems could be simply not enough light for your camera's AF sensors to work properly. If so there's nothing you can do with the way you use your lens to improve that. But there are several ways of helping your AF to work better in poor light. On the other hand your difficulties could be due to a small AF calibration error which starts to matter under those circumstances with your lens. If so there are a number of things you can do about that. But it's not worth going into all the details of all the possibilities until you have found out more about which particular limitation of the several possibilities your camera and lens are coming up against. I'm not having focus difficulties. The question is stated in the 5th paragraph. All the rest is preamble. |
#24
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EF 50/1.8 AF Experiment?
Alan Browne wrote:
Wilba wrote: Alan Browne wrote: That softness may fall outside the AF hysteresis band. More information please. In order to avoid hunting near focus, the system is designed with an error band such that once the focus lies within that band the focus action (control loop) can stop. Otherwise there could be hunting. For most lenses, the sharpest focus falls somewhere inside the hysteresis band. With your 50, the AF might determine the focus as in there or slightly to either side making the focus error range larger. This due to the softness wide open. The control loop must have several stop (exit) conditions including 'best case' (to cover this case as well as dim light). Thanks, no worries with all that, I just hadn't heard the term before. What you say fits with my thinking, the problem is finding a way to test the theories. There may also be more mechanical slop built in to the lens to allow for a smaller motor to drive it. That's harder to pinpoint. It sure feels and sounds like a crude sloppy mechanism with the 50/1.8, and I'm sure that has a lot to do with people inferring that that causes the focus performance they observe. |
#25
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EF 50/1.8 AF Experiment?
On Mon, 21 Dec 2009 14:04:56 +0800, "Wilba"
wrote: snip Yeah, we're not all called Bruce. Only the philosophers. |
#26
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EF 50/1.8 AF Experiment?
Wilba wrote:
The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? If you are using the center AF point, sharpness shouldn't be a problem. In fact the shallower DOF should improve accuracy. One factor could be the phenomenon where focus shifts when stopping down, but I suspect they've included corrections for that in the firmware. There could also be issues with curvature of field, if you are concerned about something off to the side, etc. -- Paul Furman www.edgehill.net www.baynatives.com all google groups messages filtered due to spam |
#27
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EF 50/1.8 AF Experiment?
Paul Furman wrote:
Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? If you are using the center AF point, sharpness shouldn't be a problem. I can interpret that to mean several different things. What do you mean exactly? In fact the shallower DOF should improve accuracy. I think that's part of why the "crude mechanism" theory is so popular, but I can't think of any way to test it. I'm not interested in theory or speculation except that which leads to an experiment which proves something. One factor could be the phenomenon where focus shifts when stopping down, but I suspect they've included corrections for that in the firmware. If that were a factor I would expect to see a consistent mis-focus, but that's not what I get. There could also be issues with curvature of field, if you are concerned about something off to the side, etc. I'm not. :- ) |
#28
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EF 50/1.8 AF Experiment?
Wilba wrote:
Paul Furman wrote: Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? If you are using the center AF point, sharpness shouldn't be a problem. I can interpret that to mean several different things. What do you mean exactly? Sharpness shouldn't be a problem in the center. I was doing some lens tests with macro stacking software and just for curiosity tried some really fast primes, much closer than they were designed for, not reversed or anything... and was amazed that the center performance was excellent wide open. That quickly degraded to 'hideous' outside the middle third though. http://www.flickr.com/photos/edgehil...7603231101723/ If you have extension tubes or a bellows, try it at extreme closeup. That's a good way to make flaws much more obvious. It may not apply to normal operation though. In fact the shallower DOF should improve accuracy. I think that's part of why the "crude mechanism" theory is so popular, but I can't think of any way to test it. I'm not interested in theory or speculation except that which leads to an experiment which proves something. Lots of ways to test. Do a series at the same spot but defocus manually, first several starting from infinity, then several from closest focus, then from just slightly off in each direction. Do that for something up close then something 30 feet away. You could try your stopped down theory by putting a piece of black paper with a hole punched on the front, but might try testing first that it doesn't make things less sharp due to non-optimal aperture placement. Put the paper aperture in the right place in back and the lens becomes telecentric, meaning light rays travel straight out the front and the field of view is no wider than the front element. One factor could be the phenomenon where focus shifts when stopping down, but I suspect they've included corrections for that in the firmware. If that were a factor I would expect to see a consistent mis-focus, but that's not what I get. The pattern could be complex though... depending how much you are stopped down, maybe depending if it's close up or infinity... There could also be issues with curvature of field, if you are concerned about something off to the side, etc. I'm not. :- ) -- Paul Furman www.edgehill.net www.baynatives.com all google groups messages filtered due to spam |
#29
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EF 50/1.8 AF Experiment?
Paul Furman wrote:
Wilba wrote: Paul Furman wrote: Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? If you are using the center AF point, sharpness shouldn't be a problem. I can interpret that to mean several different things. What do you mean exactly? Sharpness shouldn't be a problem in the center. I was doing some lens tests with macro stacking software and just for curiosity tried some really fast primes, much closer than they were designed for, not reversed or anything... and was amazed that the center performance was excellent wide open. That quickly degraded to 'hideous' outside the middle third though. http://www.flickr.com/photos/edgehil...7603231101723/ No, it is a problem for the nifty fifty. You can see it in the shots I posted a while back - http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.6MB) http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.4MB) If you have extension tubes or a bellows, try it at extreme closeup. That's a good way to make flaws much more obvious. It may not apply to normal operation though. In fact the shallower DOF should improve accuracy. I think that's part of why the "crude mechanism" theory is so popular, but I can't think of any way to test it. I'm not interested in theory or speculation except that which leads to an experiment which proves something. Lots of ways to test. Do a series at the same spot but defocus manually, first several starting from infinity, then several from closest focus, then from just slightly off in each direction. Do that for something up close then something 30 feet away. What consequence of which theory would this test, and how? You could try your stopped down theory by putting a piece of black paper with a hole punched on the front, but might try testing first that it doesn't make things less sharp due to non-optimal aperture placement. How could I do that? Put the paper aperture in the right place in back and the lens becomes telecentric, meaning light rays travel straight out the front and the field of view is no wider than the front element. You lost me somewhere there. :- ) Put the external aperture _behind_ the lens? And light comes out the _front_?! I don't get it. :- ) One factor could be the phenomenon where focus shifts when stopping down, but I suspect they've included corrections for that in the firmware. If that were a factor I would expect to see a consistent mis-focus, but that's not what I get. The pattern could be complex though... depending how much you are stopped down, maybe depending if it's close up or infinity... Yeah, let's just stick with what happens wide open. |
#30
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EF 50/1.8 AF Experiment?
Wilba wrote:
Paul Furman wrote: Wilba wrote: Paul Furman wrote: Wilba wrote: The Canon EF 50mm f/1.8 II "Nifty Fifty" has a reputation for two shortcomings, 1) softness at wide apertures (OK from f/2.8), and 2) erratic focus under difficult conditions (low light, shallow DOF). Many people claim that 2) is a result of the crudeness of the cheap focussing motor and electronics in the lens, that those components are not able to provide the required accuracy and control of motion of the focus ring. But I wonder if 2) is actually a result of 1) - if the AF sensors have fuzzy images to work with, how /could/ the system nail the focus in difficult conditions? It would be interesting to see what happens when the AF sensors have sharper images to work with (e.g. at f/2.8 or f/4), but my 450D refuses to AF when the DOF preview button is pressed, so I can't test that. External aperture perhaps? Any ideas for how these competing hypotheses could be tested? Is there a consequence of either hypothesis that could be disproved empirically? If you are using the center AF point, sharpness shouldn't be a problem. I can interpret that to mean several different things. What do you mean exactly? Sharpness shouldn't be a problem in the center. I was doing some lens tests with macro stacking software and just for curiosity tried some really fast primes, much closer than they were designed for, not reversed or anything... and was amazed that the center performance was excellent wide open. That quickly degraded to 'hideous' outside the middle third though. http://www.flickr.com/photos/edgehil...7603231101723/ No, it is a problem for the nifty fifty. Huh? "No" what? It's not sharp in the center? You can see it in the shots I posted a while back - http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.6MB) http://www.users.on.net/~alanw/Usene...ewFullsize.jpg (3.4MB) If you have extension tubes or a bellows, try it at extreme closeup. That's a good way to make flaws much more obvious. It may not apply to normal operation though. This will tell you if it's sharp in the center. In fact the shallower DOF should improve accuracy. I think that's part of why the "crude mechanism" theory is so popular, but I can't think of any way to test it. I'm not interested in theory or speculation except that which leads to an experiment which proves something. Lots of ways to test. Do a series at the same spot but defocus manually, first several starting from infinity, then several from closest focus, then from just slightly off in each direction. Do that for something up close then something 30 feet away. What consequence of which theory would this test, and how? Testing the 'slop' theory. To see if it stops at different positions on repeated runs of the same movement or different movements. You could try your stopped down theory by putting a piece of black paper with a hole punched on the front, but might try testing first that it doesn't make things less sharp due to non-optimal aperture placement. How could I do that? Just side by side comparison shots to see if the new aperture makes things softer. Put the paper aperture in the right place in back and the lens becomes telecentric, meaning light rays travel straight out the front and the field of view is no wider than the front element. You lost me somewhere there. :- ) Put the external aperture _behind_ the lens? And light comes out the _front_?! I don't get it. :- ) Heh, sorry, off on a tangent. The idea is that adding an aperture like that can do weird things, so maybe not sharper than stopping down the lens' own aperture and who knows, maybe softer than wide open. One factor could be the phenomenon where focus shifts when stopping down, but I suspect they've included corrections for that in the firmware. If that were a factor I would expect to see a consistent mis-focus, but that's not what I get. The pattern could be complex though... depending how much you are stopped down, maybe depending if it's close up or infinity... Yeah, let's just stick with what happens wide open. But you wanted to test if things improved stopped down. If I understand correctly, the view in the optical viewfinder and presumably what the AF sensors see, is already stopped down to maybe f/2.8; it doesn't benefit from faster lenses, due to obstructions I guess. So there is already some potential for focus shift there... but as you say, that should be consistent, so again this points toward slop in the mechanism. If you tested various final taking apertures, at least that's some kind of data pointing to whether that might be playing a role. The simplest test is that focus chart manually focused then stop down to various amounts & see if the focus shifts. It will get wider DOF but might not be centered if you count from each side. Then see if the AF places the focus differently from the manual fixed focus test at different apertures. If there is focus shift and the camera firmware is correcting for it, there should be little differences in where the camera decides to focus. PS I still don't get how the AF calibration thing works on the cameras that have that. I can see one master set screw for coordinating the distance of the AF sensors to match the distance to the picture sensor and or viewfinder ground glass but calibrating differently for different lenses would seem to me to have to be relying on this idea of focus shift when stopping down. If so, that's going to have to be checked at various apertures to establish a curve for the correction, not just one test shot. Does that make sense? -- Paul Furman www.edgehill.net www.baynatives.com all google groups messages filtered due to spam |
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