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#11
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Reason for so many focus errors we see today?
John O'Flaherty wrote:
On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer wrote: RichA wrote: Plastic? Thermal expansion of plastic is much greater than metal and it could very well be why we are seeing focus issues that need "lens re-calibration" at service depots or that we see the need for in- camera focus fine-tuning. Even cameras and lenses that appear to be metal today may have plastic cells holding lenses, components in cameras. The cameras are produced in a control temp environment but that isn't real life use where temps can vary by 10's of degrees. I don't remember all metal AF SLRs needing focus fine-tuning (or having that facility) in the film days. I can see that in open loop focusing, where you estimate the distance and dial that distance on lens. However, in any closed loop operation that source of error would not lead to a focus error. Also, there are a some plastics that have a thermal expansion less than many metals. So one cannot use generalities on this. Focusing with a phase contrast system isn't closed loop with regard to the picture sensor, since it uses a separate, simple sensor array for focusing. Thus the focus system can get out of calibration. I was under the impression that the OP was talking about lens cell materials. The structure holding the image chip and the focusing chip is in the body of the camera, so a movement of something within the lens itself should affect both chips the same, shouldn't it? |
#12
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Reason for so many focus errors we see today?
On Wed, 24 Jun 2009 09:42:20 +0100, bugbear
wrote: John O'Flaherty wrote: On Tue, 23 Jun 2009 16:20:03 +0100, bugbear wrote: John O'Flaherty wrote: Focusing with a phase contrast system isn't closed loop with regard to the picture sensor, since it uses a separate, simple sensor array for focusing. Thus the focus system can get out of calibration. (jargon as per http://en.wikipedia.org/wiki/Autofocus ) Interesting - since contrast measurement can be done in software, a camera with phase detection (which can, as you say, get out of calibration) could use contrast measurement to self-calibrate. That sounds like a good idea - you'd just have to take a special shot of something with detail after each lens change. Also, I wonder if it would be possible to make a main sensor chip with special data paths to some small subset of the pixels, so they could be digitized really fast, doing phase contrast The article speaks of "phase detection" and "contrast measurement"; what do you mean by "phase contrast", which sounds like a mixture? Sorry, "phase contrast" is a term from microscopy which was floating around in my head. I meant phase detection. -- John |
#13
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Reason for so many focus errors we see today?
On Wed, 24 Jun 2009 09:04:14 -0500, Don Stauffer
wrote: John O'Flaherty wrote: On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer wrote: RichA wrote: Plastic? Thermal expansion of plastic is much greater than metal and it could very well be why we are seeing focus issues that need "lens re-calibration" at service depots or that we see the need for in- camera focus fine-tuning. Even cameras and lenses that appear to be metal today may have plastic cells holding lenses, components in cameras. The cameras are produced in a control temp environment but that isn't real life use where temps can vary by 10's of degrees. I don't remember all metal AF SLRs needing focus fine-tuning (or having that facility) in the film days. I can see that in open loop focusing, where you estimate the distance and dial that distance on lens. However, in any closed loop operation that source of error would not lead to a focus error. Also, there are a some plastics that have a thermal expansion less than many metals. So one cannot use generalities on this. Focusing with a phase contrast system isn't closed loop with regard to the picture sensor, since it uses a separate, simple sensor array for focusing. Thus the focus system can get out of calibration. I was under the impression that the OP was talking about lens cell materials. The structure holding the image chip and the focusing chip is in the body of the camera, so a movement of something within the lens itself should affect both chips the same, shouldn't it? My impression of the o.p. was that it was about the camera in general, anything that could require recalibration of focus. Anyway, I think a phase system is open loop in general, since it moves the lens focus in a direction and amount indicated by the comparison of two adjacent sensor strips (at least for a first pass). So, maybe changes in the lens itself will affect the accuracy. -- John |
#14
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Reason for so many focus errors we see today?
In rec.photo.digital.slr-systems John O'Flaherty wrote:
On Wed, 24 Jun 2009 09:04:14 -0500, Don Stauffer wrote: John O'Flaherty wrote: On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer wrote: RichA wrote: Plastic? Thermal expansion of plastic is much greater than metal and it could very well be why we are seeing focus issues that need "lens re-calibration" at service depots or that we see the need for in- camera focus fine-tuning. Even cameras and lenses that appear to be metal today may have plastic cells holding lenses, components in cameras. The cameras are produced in a control temp environment but that isn't real life use where temps can vary by 10's of degrees. I don't remember all metal AF SLRs needing focus fine-tuning (or having that facility) in the film days. I can see that in open loop focusing, where you estimate the distance and dial that distance on lens. However, in any closed loop operation that source of error would not lead to a focus error. Also, there are a some plastics that have a thermal expansion less than many metals. So one cannot use generalities on this. Focusing with a phase contrast system isn't closed loop with regard to the picture sensor, since it uses a separate, simple sensor array for focusing. Thus the focus system can get out of calibration. I was under the impression that the OP was talking about lens cell materials. The structure holding the image chip and the focusing chip is in the body of the camera, so a movement of something within the lens itself should affect both chips the same, shouldn't it? My impression of the o.p. was that it was about the camera in general, anything that could require recalibration of focus. Anyway, I think a phase system is open loop in general, since it moves the lens focus in a direction and amount indicated by the comparison of two adjacent sensor strips (at least for a first pass). So, maybe changes in the lens itself will affect the accuracy. I think rather than open loop "in general" you mean "at first". It's difficult getting at the technical details of phase detection autofocus, since it's both highly technical, and at least in some cases the details are regarded as a trade secret by the manufacturer. However, having read the workshop repair manual for one set of Canon lenses, which in passing mentioned various aspects of autofocus as relevent to a service engineer, the impression I was left with was that in those particular lenses, which were made for specially fast focusing, what happens is as follows. The AF phase detection makes a first measurement of the direction and approximate distance to focus. The focus motor control then sets up the parameters for the focus movement. If the distance is long enough to warrant it this is a four phase movement. The first phase is an acceleration of the lens up to its maximum speed of focus travel. This is terminated automatically when the motor is spinning at maximum speed, and the second phase begun. The second phase travels at maximum speed until it gets to the deceleration position, at which point it automatically starts the third phase, the deceleration ramp down to the final focus approach speed. That final fourth phase approach speed is slow enough to allow a closed loop approach to focus detection by the AF sensor which given the inertia of the lens etc.. can be stopped close enough to exact focus that there will be no overshoot. If the lens overshot the focus point it would have to reverse. Reversal of direction is expensive in time, so like a car approaching a stop light it decelerates at a speed at which it can reliably stop close enough to the line. In the case of these lenses "close enough" is a permissible error parameter previously read by the camera from a table in ROM in the lens, and which is alterable by service engineers in the lens focus calibration procedure. That parameter may also be adjusted by the camera depending on the chosen aperture. In that way smaller apertures can focus the lens a bit faster. That parameter may also incorporate an early stop factor to take account for the delays, inertia, etc. in the system, which mean that it will always stop a little bit further on than where it was when the software loop decided to stop the motor. When the deceleration phase finishes the lens is now not quite at the required focus point, and moving slowly enough to operate in closed loop manner, closed on good enough focus detection by the AF sensor. That'a the fourth final closed loop phase. It drives on at that speed reading the sensor, and stops when it detects near enough focus. Of course sometimes it won't detect focus, because the squirrel may have moved away or the light changed enough to lose the necessary contrast in the sensor while the lens was moving. So there is also a timing watchdog to terminate this loop if it goes on for too long. Other lenses/AF systems don't use a watchdog and simply keep running the lens until it hits the end stop. Simpler, but slower. Termination on the final focus approach loop on a focus find failure condition will automatically initiate another complete focus from scratch loop. This is how "hunting" happens, and whether it runs from end to end of complete focus travel, or dithers about the focus point, depends on whether the system uses timer watchdog or end stop failure, and what kind of outer loop control system is operating. Of course if the camera is doing a more complicated kind of focus such as predictive focus of a moving object, or approaching a compromise focus between the readings from a number of previously selected AF sensors, a more complicated procedure is used. I'm describing here the simplest long distance high speed AF procedure. If the focus point is close to where the lens is of course it will only use the final approach phase. The manual also made it clear that not all Canon lenses were sophisticated enough to use this sophisticated long distance high speed focus method. Simpler modelds have only a single speed focus system. I also get the impression that there is another two speed AF system used by some lens/body combinations, where the high speed checks the AF sensor only every several motor steps, and when it gets near slows down to checking every step. I think it's an oversimplification to talk of THE phase detection AF system of DSLRs, since in the search for highest speed and highest accuracy the camera makers have kept improving the sophistication of both their AF sensors and the focus control loops using them. In some cases we've seen a camera maker offer a camera model software upgrade which has improved focus performance not by fixing a bug but by introducing a more sophisticated control system. There are a number of different phase detection AF systems out there, of different levels of sophistication and accuracy. Some, I suspect these days at least most, always operate a closed loop approach of monitoring the AF sensor during the final approach to the focus point. Some camera makers have caused confusion on this question of whether their DSLR AF systems are open or closed loop by describing them as open loop because the closed loop they operate is closed around the AF sensor and not the image sensor, whereas their compact cameras operate a loop closed around the image sensor. In terms of control theory it's technically correct to describe DSLR phase detection autofocus systems as open loop, because they don't close the loop by measuring the achievement of the purpose of the system, which is an image focused on the image sensor. Instead they close the loop around the proxy of the AF sensor. Hence all the focus calibration problems of DSLR autofocus. Unfortunately that technically correct description of DSLR autofocus as "open loop" without qualification has misled a lot of camera users unfamiliar with the technicalities of control systems theory into supposing that that "open loop" means that no DSLRs use the AF sensor in a closed loop fashion. -- Chris Malcolm |
#15
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Reason for so many focus errors we see today?
Chris Malcolm wrote:
I think rather than open loop "in general" you mean "at first". It's difficult getting at the technical details of phase detection autofocus, since it's both highly technical, and at least in some cases the details are regarded as a trade secret by the manufacturer. However, having read the workshop repair manual for one set of Canon lenses, which in passing mentioned various aspects of autofocus as relevent to a service engineer, the impression I was left with was that in those particular lenses, which were made for specially fast focusing, what happens is as follows. The AF phase detection makes a first measurement of the direction and approximate distance to focus. The focus motor control then sets up the parameters for the focus movement. If the distance is long enough to warrant it this is a four phase movement. The first phase is an acceleration of the lens up to its maximum speed of focus travel. This is terminated automatically when the motor is spinning at maximum speed, and the second phase begun. The second phase travels at maximum speed until it gets to the deceleration position, at which point it automatically starts the third phase, the deceleration ramp down to the final focus approach speed. That final fourth phase approach speed is slow enough to allow a closed loop approach to focus detection by the AF sensor which given the inertia of the lens etc.. can be stopped close enough to exact focus that there will be no overshoot. If the lens overshot the focus point it would have to reverse. Reversal of direction is expensive in time, so like a car approaching a stop light it decelerates at a speed at which it can reliably stop close enough to the line. In the case of these lenses "close enough" is a permissible error parameter previously read by the camera from a table in ROM in the lens, and which is alterable by service engineers in the lens focus calibration procedure. That parameter may also be adjusted by the camera depending on the chosen aperture. In that way smaller apertures can focus the lens a bit faster. That parameter may also incorporate an early stop factor to take account for the delays, inertia, etc. in the system, which mean that it will always stop a little bit further on than where it was when the software loop decided to stop the motor. When the deceleration phase finishes the lens is now not quite at the required focus point, and moving slowly enough to operate in closed loop manner, closed on good enough focus detection by the AF sensor. That'a the fourth final closed loop phase. It drives on at that speed reading the sensor, and stops when it detects near enough focus. Of course sometimes it won't detect focus, because the squirrel may have moved away or the light changed enough to lose the necessary contrast in the sensor while the lens was moving. So there is also a timing watchdog to terminate this loop if it goes on for too long. Other lenses/AF systems don't use a watchdog and simply keep running the lens until it hits the end stop. Simpler, but slower. Termination on the final focus approach loop on a focus find failure condition will automatically initiate another complete focus from scratch loop. This is how "hunting" happens, and whether it runs from end to end of complete focus travel, or dithers about the focus point, depends on whether the system uses timer watchdog or end stop failure, and what kind of outer loop control system is operating. Of course if the camera is doing a more complicated kind of focus such as predictive focus of a moving object, or approaching a compromise focus between the readings from a number of previously selected AF sensors, a more complicated procedure is used. I'm describing here the simplest long distance high speed AF procedure. If the focus point is close to where the lens is of course it will only use the final approach phase. The manual also made it clear that not all Canon lenses were sophisticated enough to use this sophisticated long distance high speed focus method. Simpler modelds have only a single speed focus system. I also get the impression that there is another two speed AF system used by some lens/body combinations, where the high speed checks the AF sensor only every several motor steps, and when it gets near slows down to checking every step. I think it's an oversimplification to talk of THE phase detection AF system of DSLRs, since in the search for highest speed and highest accuracy the camera makers have kept improving the sophistication of both their AF sensors and the focus control loops using them. In some cases we've seen a camera maker offer a camera model software upgrade which has improved focus performance not by fixing a bug but by introducing a more sophisticated control system. There are a number of different phase detection AF systems out there, of different levels of sophistication and accuracy. Some, I suspect these days at least most, always operate a closed loop approach of monitoring the AF sensor during the final approach to the focus point. Some camera makers have caused confusion on this question of whether their DSLR AF systems are open or closed loop by describing them as open loop because the closed loop they operate is closed around the AF sensor and not the image sensor, whereas their compact cameras operate a loop closed around the image sensor. In terms of control theory it's technically correct to describe DSLR phase detection autofocus systems as open loop, because they don't close the loop by measuring the achievement of the purpose of the system, which is an image focused on the image sensor. Instead they close the loop around the proxy of the AF sensor. Hence all the focus calibration problems of DSLR autofocus. Unfortunately that technically correct description of DSLR autofocus as "open loop" without qualification has misled a lot of camera users unfamiliar with the technicalities of control systems theory into supposing that that "open loop" means that no DSLRs use the AF sensor in a closed loop fashion. Thanks for that Chris. There's a lot of stuff there I didn't know. Is that "workshop repair manual" available online? I have a copy of the "EF 50mm 1.8, 28mm 2.8, and 15mm 2.8 Service Manual", but AFAIK that doesn't go into the operation like you described. |
#16
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Reason for so many focus errors we see today?
On 25 Jun 2009 09:40:09 GMT, Chris Malcolm
wrote: In rec.photo.digital.slr-systems John O'Flaherty wrote: On Wed, 24 Jun 2009 09:04:14 -0500, Don Stauffer wrote: John O'Flaherty wrote: On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer wrote: RichA wrote: Plastic? Thermal expansion of plastic is much greater than metal and it could very well be why we are seeing focus issues that need "lens re-calibration" at service depots or that we see the need for in- camera focus fine-tuning. Even cameras and lenses that appear to be metal today may have plastic cells holding lenses, components in cameras. The cameras are produced in a control temp environment but that isn't real life use where temps can vary by 10's of degrees. I don't remember all metal AF SLRs needing focus fine-tuning (or having that facility) in the film days. I can see that in open loop focusing, where you estimate the distance and dial that distance on lens. However, in any closed loop operation that source of error would not lead to a focus error. Also, there are a some plastics that have a thermal expansion less than many metals. So one cannot use generalities on this. Focusing with a phase contrast system isn't closed loop with regard to the picture sensor, since it uses a separate, simple sensor array for focusing. Thus the focus system can get out of calibration. I was under the impression that the OP was talking about lens cell materials. The structure holding the image chip and the focusing chip is in the body of the camera, so a movement of something within the lens itself should affect both chips the same, shouldn't it? My impression of the o.p. was that it was about the camera in general, anything that could require recalibration of focus. Anyway, I think a phase system is open loop in general, since it moves the lens focus in a direction and amount indicated by the comparison of two adjacent sensor strips (at least for a first pass). So, maybe changes in the lens itself will affect the accuracy. I think rather than open loop "in general" you mean "at first". It's difficult getting at the technical details of phase detection autofocus, since it's both highly technical, and at least in some cases the details are regarded as a trade secret by the manufacturer. However, having read the workshop repair manual for one set of Canon lenses, which in passing mentioned various aspects of autofocus as relevent to a service engineer, the impression I was left with was that in those particular lenses, which were made for specially fast focusing, what happens is as follows. The AF phase detection makes a first measurement of the direction and approximate distance to focus. The focus motor control then sets up the parameters for the focus movement. If the distance is long enough to warrant it this is a four phase movement. The first phase is an acceleration of the lens up to its maximum speed of focus travel. This is terminated automatically when the motor is spinning at maximum speed, and the second phase begun. The second phase travels at maximum speed until it gets to the deceleration position, at which point it automatically starts the third phase, the deceleration ramp down to the final focus approach speed. That final fourth phase approach speed is slow enough to allow a closed loop approach to focus detection by the AF sensor which given the inertia of the lens etc.. can be stopped close enough to exact focus that there will be no overshoot. If the lens overshot the focus point it would have to reverse. Reversal of direction is expensive in time, so like a car approaching a stop light it decelerates at a speed at which it can reliably stop close enough to the line. In the case of these lenses "close enough" is a permissible error parameter previously read by the camera from a table in ROM in the lens, and which is alterable by service engineers in the lens focus calibration procedure. That parameter may also be adjusted by the camera depending on the chosen aperture. In that way smaller apertures can focus the lens a bit faster. That parameter may also incorporate an early stop factor to take account for the delays, inertia, etc. in the system, which mean that it will always stop a little bit further on than where it was when the software loop decided to stop the motor. When the deceleration phase finishes the lens is now not quite at the required focus point, and moving slowly enough to operate in closed loop manner, closed on good enough focus detection by the AF sensor. That'a the fourth final closed loop phase. It drives on at that speed reading the sensor, and stops when it detects near enough focus. Of course sometimes it won't detect focus, because the squirrel may have moved away or the light changed enough to lose the necessary contrast in the sensor while the lens was moving. So there is also a timing watchdog to terminate this loop if it goes on for too long. Other lenses/AF systems don't use a watchdog and simply keep running the lens until it hits the end stop. Simpler, but slower. Termination on the final focus approach loop on a focus find failure condition will automatically initiate another complete focus from scratch loop. This is how "hunting" happens, and whether it runs from end to end of complete focus travel, or dithers about the focus point, depends on whether the system uses timer watchdog or end stop failure, and what kind of outer loop control system is operating. Of course if the camera is doing a more complicated kind of focus such as predictive focus of a moving object, or approaching a compromise focus between the readings from a number of previously selected AF sensors, a more complicated procedure is used. I'm describing here the simplest long distance high speed AF procedure. If the focus point is close to where the lens is of course it will only use the final approach phase. The manual also made it clear that not all Canon lenses were sophisticated enough to use this sophisticated long distance high speed focus method. Simpler modelds have only a single speed focus system. I also get the impression that there is another two speed AF system used by some lens/body combinations, where the high speed checks the AF sensor only every several motor steps, and when it gets near slows down to checking every step. I think it's an oversimplification to talk of THE phase detection AF system of DSLRs, since in the search for highest speed and highest accuracy the camera makers have kept improving the sophistication of both their AF sensors and the focus control loops using them. In some cases we've seen a camera maker offer a camera model software upgrade which has improved focus performance not by fixing a bug but by introducing a more sophisticated control system. There are a number of different phase detection AF systems out there, of different levels of sophistication and accuracy. Some, I suspect these days at least most, always operate a closed loop approach of monitoring the AF sensor during the final approach to the focus point. Some camera makers have caused confusion on this question of whether their DSLR AF systems are open or closed loop by describing them as open loop because the closed loop they operate is closed around the AF sensor and not the image sensor, whereas their compact cameras operate a loop closed around the image sensor. In terms of control theory it's technically correct to describe DSLR phase detection autofocus systems as open loop, because they don't close the loop by measuring the achievement of the purpose of the system, which is an image focused on the image sensor. Instead they close the loop around the proxy of the AF sensor. Hence all the focus calibration problems of DSLR autofocus. Unfortunately that technically correct description of DSLR autofocus as "open loop" without qualification has misled a lot of camera users unfamiliar with the technicalities of control systems theory into supposing that that "open loop" means that no DSLRs use the AF sensor in a closed loop fashion. That's a lot more detailed information than I've ever read before - thanks for that. So, they are already decelerating short of the target point, then closing the loop. Do you think it would be possible to get quick end-point contrast readings from the image sensor itself, or from small parts of it, to make a hybrid system that would have the advantage of both speed and accuracy? -- John |
#17
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Reason for so many focus errors we see today?
Chris Malcolm wrote:
I think rather than open loop "in general" you mean "at first". It's difficult getting at the technical details of phase detection autofocus, since it's both highly technical, and at least in some cases the details are regarded as a trade secret by the manufacturer. However, having read the workshop repair manual for one set of Canon lenses, which in passing mentioned various aspects of autofocus as relevent to a service engineer, the impression I was left with was that in those particular lenses, which were made for specially fast focusing, what happens is as follows. There was a big patent fight between Honeywell and someone (I forget who right at the moment. I followed the case because I worked for Honeywell at the time. The lawyers even gave a noontime seminar one day for folks interested in the case). It was over the details of that particular autofocus method. The material disclosed in court is voluminous, but is at least part of public record. |
#18
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Reason for so many focus errors we see today?
In article , Don Stauffer
wrote: There was a big patent fight between Honeywell and someone (I forget who right at the moment. I followed the case because I worked for Honeywell at the time. The lawyers even gave a noontime seminar one day for folks interested in the case). It was over the details of that particular autofocus method. The material disclosed in court is voluminous, but is at least part of public record. minolta. |
#19
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Reason for so many focus errors we see today?
In article , Don Stauffer
writes I was under the impression that the OP was talking about lens cell materials. No, the OP was continuing a decade long rant about something he has little knowledge of but finds it a convenient subject to troll with. -- 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) |
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