I'm glad this extremely informative thread is still present and alive.
After reading and digesting everything, I'm still left with a question. Does anyone know what the effective "exposure" time is for the snapshot of focusing state that the AF sensors take prior to calculating the required lens movement? Camera shake or motion blur will smear the image captured by the AF sensors and reduce contrast. This could give a focusing advantage to IS lenses beyond their benefits toward final image capture or have implications for focusing accuracy as a function of focal length (beyond the implications stemming from the just depth of focus tolerance criterion).
drgreenberg wrote:
I'm glad this extremely informative thread is still present and alive.
After reading and digesting everything, I'm still left with a question. Does anyone know what the effective "exposure" time is for the snapshot of focusing state that the AF sensors take prior to calculating the required lens movement?
Interesting question. It's certainly quite short in good light--short enough for the camera to adjust focus (including lens movement) between exposures at 5fps and 1/250. We're only talking about the amount of time it takes the pixel arrays to react to the light and process a fairly simple signal to the computer--certainly no slower than the same number of pixels in the imaging sensor can do it.
Maping the focus sensors on my 20D, I figured out that the sensors were not aligned correctly with the marks. All seems to be shifted to the left, about one center mark width. My center focus mark isactually on the right end of the sensor. I believe this is the reason for the "sharp background, blury subject "problem I have been having with some photos. Now I use the imaginary focus parts and the AF works just like it should. Local Canon technical service confirms the problem but they could not supply a solution. Has anybody experienced anything similar? Does anyone know if it is possible to slide the focus sensor chip to the correct position?
Wow, awesome information, can't believe the original thread got "lost". This should be a sticky.
I'll do my tiny bit to try and add info to it and some half-baked theories
My first reaction (which some others had) was, how does it do this open loop? Won't it need to know the absolute focus position? Well, I got out the optics book and a pad and pencil, and guess what it needs to know?
Absolutely squat!!! Doesn't need focal length or aperature or absolute focus postion!!!
So basically if you make an generic ray trace diagram and do a little geometry and trig you will see that the physical distance the lens must be moved from/towards the film plane to achive focus given an arbitrary subject distance and starting position for the lens is:
E = P / (2*tan(A))
Where E is the distance to move the lens from/towards the film plane (i.e. the current focus Error), P is the offset of the image patterns on the Phase comparator arrays and A is the "look Angle" of the comparator array prisms from the optical axis (this is a constant determined by the shape of the prisms). This equation is *not* a function of: focal length, aperature, subject distance, or absolute positon of the lens from the film/imager plane. It basically says that for any lens the distance to move the lens is linearly proportional to the distance between the image patterns at the comparator. The phase measurements and focus commands can all be relative and the camera really needs to know nothing about the lens. The lens had just better move the amount the camera tells it and that's about it.
There is one place there is a little error in the above equation - it assumes the film plane is what moves. Unless you' ve got a really wierd camera its probably the lens that moves . This changes the lens to subject distance very slightly. This would introduce a small error if you made a big change in focus for a very very close subject (magnifications greater than 1:1). I don't think it's really possible for the camera to even get a phase measurement in this case, it would rack the focus until it got close enough to make a phase measurement and once it was this close the error would still be well within tolerance.
OK, so that was physics and pretty solid (though please jump in if I screwed up).
On to speculation...
Why does it want to know the focal length? Well, the exposure system certainly cares for calculating 1/f hand holding speed. And the flash needs to know how tightly in can focus its light cone. And well it looks nice in the EXIF. But I don't think the autofocus strictly cares. The depth of field exposure mode probably does though!
Why does it want to know the maximum aperature? Probably to tell whether it should auto-focus at all - for most systems if the aperature is smaller than f/5.6 the prisms on the phase comparators will just see black. For the snazy "high precision" models I'm guessing at 2.8 and faster it uses a different set of comparators with prims that have a steeper look angle (and thus can make a more accurate measure of phase, again see Doug's neat article). If the lens is slower than 2.8 the precision comparators will just see black so it won't want to use them. And of course the exposure system really needs to know this. But as far as the autofocus goes, besides deciding what mode and whether to try and focus at all it is probably not strictly necessary either.
And even more speculation...
So why do many third parties stink at focus? I suppose the servos could be garbage, but I wonder if it isn't more to do a little with the optics. In order for the above system to work when you move the focus it should do so in a well behaved linear fashion. For an "ideal" lens this is easy. But some of these really compact long zoom ratio things do some weird stuff. For example, the Sigma 18-125 while apparently parafocal (i.e. if you move the zoom ring the focus distance doesn't change) it does do a neato thing at least at the long end - when you change the *focus* the *focal length* changes - at infinity it is around 121mm and at 50ft it is more like 118mm (again, thanks to Doug Kerr post on dpreview). So something exciting is going on in there and the focus may not behave well in this "open loop" system. In fact, if in the above case the nodal points don't also magically move just right to compensate this would introduce overshoot into Canon's open loop system (e.g. if auto focus started from inifnity to a middle ground subject the command would over shoot and front focus). For reference, Sigma's SLR's apparently use a contrast detection method that is closed loop (and slow) - so it probably does a good job on their lenses even if the optics are wierd. So I wonder if Canon lenses are optimized for very well behaved focus so this really fast open loop system is accurate and the third parties sometimes don't? Just a guess!
The other (perhaps more likely explanation) is that the third party lens don't handle the second servo command correctly to fine tune the servo position - that'd also introduce error.
I wonder if everyone elses phase comparison autofocus systems are open loop?
All right, I've blabbed enough. Hopefully some one finds that useful or entertaining... or at least a cure for insomnia.
I noticed a while ago that my 20D center AF sensor seemed to be located just off the edge of the viewfinder square. My method was crude but repeatable, involving a near and far subject boundary. I'm guessing that the easier fix would be to move the viewfinder overlay, rather than the AF sensor array. I wonder if there is play in the overlay, as there was in the split prism viewfinder screen I installed.
I just got a XT/350D and it seems to always front focus a little. However, it is well within the specification (i.e. focus point should fall within the depth of focus for an XT). Mine hovers around focusing about 30% of the depth of focus forward. This is really only ever a "problem" with my 50 at 1.8. At first when I measured it I thought I'd send it in for service, but after doing a number of well lit tests and calculating what the "acceptable" range was for Canon's spec I determined that the camera was in fact performing to spec.
So make sure you determine by how much it is front focusing or yeah, you may in fact be too picky.
Oh, and of course try more than one lens. My 50/1.8 is from the early 90's and is clearly more eratic, probably on account of an old servo.
Absolutely squat!!! Doesn't need focal length or aperature or absolute focus postion!!! The phase measurements and focus commands can all be relative and the camera really needs to know nothing about the lens. The lens had just better move the amount the camera tells it and that's about it.
The camera does need to know the maximum aperture of the lens in order to determine the accuracy tolerance. But you're correct, the lens doesn't need to know anything except where to move.
There is one place there is a little error in the above equation - it assumes the film plane is what moves. Unless you' ve got a really wierd camera its probably the lens that moves .
There was a camera--the Contax AX, I believe--that did indeed move the film plane to autofocus. I suspect the Japanese company making the cameras couldn't get Zeiss to make any autofocus lenses, so they put it in the camera instead. I don't know how they did their measurements.
Why does it want to know the focal length? Well, the exposure system certainly cares for calculating 1/f hand holding speed. And the flash needs to know how tightly in can focus its light cone. And well it looks nice in the EXIF. But I don't think the autofocus strictly cares. The depth of field exposure mode probably does though!
In addition, it does not need to know the focal length to determine the focus accuracy tolerance because (according to Canon in Lens Work III) depth of focus does not change with focal length.
Why does it want to know the maximum aperature? Probably to tell whether it should auto-focus at all - for most systems if the aperature is smaller than f/5.6 the prisms on the phase comparators will just see black. For the snazy "high precision" models I'm guessing at 2.8 and faster it uses a different set of comparators with prims that have a steeper look angle (and thus can make a more accurate measure of phase, again see Doug's neat article). If the lens is slower than 2.8 the precision comparators will just see black so it won't want to use them. And of course the exposure system really needs to know this. But as far as the autofocus goes, besides deciding what mode and whether to try and focus at all it is probably not strictly necessary either....Show more →
As I mentioned, it needs maximum aperture to determine depth of focus.
So why do many third parties stink at focus? I suppose the servos could be garbage, but I wonder if it isn't more to do a little with the optics. In order for the above system to work when you move the focus it should do so in a well behaved linear fashion. For an "ideal" lens this is easy. But some of these really compact long zoom ratio things do some weird stuff. For example, the Sigma 18-125 while apparently parafocal (i.e. if you move the zoom ring the focus distance doesn't change) it does do a neato thing at least at the long end - when you change the *focus* the *focal length* changes - at infinity it is around 121mm and at 50ft it is more like 118mm (again, thanks to Doug Kerr post on dpreview).
...Show more → This is actually common in practically all lenses, though. The focal length is only true when focused at infinity.
The other (perhaps more likely explanation) is that the third party lens don't handle the second servo command correctly to fine tune the servo position - that'd also introduce error.
I think this is more likely. I suspect each 3rd party uses a common (to that company) comm language to their lens drive-control cpus across all their lens mounts, but they use a translator chip specific to each mount. For instance, Tamron's 28-75mm zooms would all have a drive cpu that speaks "Tamron," but the lenses with the Canon mount would have a translator that speaks "Canon" while the lenses with the Nikon mount would have a translator that speaks "Nikon." This would introduce, at the very least, a delay that OEM Canon and Nikon lenses don't have.
RDKirk wrote:
Tamron's 28-75mm zooms would all have a drive cpu that speaks "Tamron," but the lenses with the Canon mount would have a translator that speaks "Canon" while the lenses with the Nikon mount would have a translator that speaks "Nikon." This would introduce, at the very least, a delay that OEM Canon and Nikon lenses don't have.
I dunno. We're talking electronic speeds: a millisecond, if even that.
However, it's very likely that sigma has only partially reverse-engineered the command language, and miss-understands some commands.
It would be possible for a lens+body manufacturer to encrypt the command language, thus making it _impossible_ to reverse engineer it. The technology is cheap enough, and I'm suprised that the 4/3s consortium didn't do this, forcing sigma to license the spec.
jojohohanon wrote:
I dunno. We're talking electronic speeds: a millisecond, if even that.
However, it's very likely that sigma has only partially reverse-engineered the command language, and miss-understands some commands.
It would be possible for a lens+body manufacturer to encrypt the command language, thus making it _impossible_ to reverse engineer it. The technology is cheap enough, and I'm suprised that the 4/3s consortium didn't do this, forcing sigma to license the spec.
Canon has continually said that they have not licensed anyone to produce lenses using their mount. All of the 3rd party companies are reverse engineering.
It's not particularly possible to effectively encrypt something like lens movement commands. First, Canon can't introduce an encryption that would prevent old lenses from communicating with new bodies and vice versa. Second, the action itself is so simple and repeatable that any attempt at encryption would be transparent.
OTOH, even a couple of milliseconds of translation delay could result in a lens not receiving a "stop" command in time after it's been given the command to "rack forward" to find the focus point.
RDKirk wrote:
OTOH, even a couple of milliseconds of translation delay could result in a lens not receiving a "stop" command in time after it's been given the command to "rack forward" to find the focus point.
But if I understand the information above correctly, there is no such thing as a stop command; the lens is only told to move a certain distance, or even only to correct a certain phase difference. The AF system in the camera only looks once, and does not check the result while the lens is focussing or afterwards.
Still I do agree that introducing encryption now would be very hard for Canon, because they need to maintain backward compatibility.
But if I understand the information above correctly, there is no such thing as a stop command; the lens is only told to move a certain distance, or even only to correct a certain phase difference. The AF system in the camera only looks once, and does not check the result while the lens is focussing or afterwards.
Still I do agree that introducing encryption now would be very hard for Canon, because they need to maintain backward compatibility.
When the camera's sensor arrays take their bit-directional "look" at the scene, the camera calculates the distance and direction the lens needs to move to resolve the array conflict.
But if the image is so out of focus that the camera can't calculate a focus solution, it commands the lens to rack first all the way forward, then all the way backward. It is apparently "watching" and calculating focus solutions even as this racking is going on, and will stop the lens in mid-rack at the correct focus point. That means one command was sent to "rack" and another command was sent to halt the racking at the correct focus point.
I would agree this is probably not a "stop" command; it's probably the same kind of "move to this point" command the camera would normally give, except that the lens is in motion at the time.
That means there has to be some resolution in the lens of any latency between the point the lens was at when the camera calculated the focus solution, the time it issued the command, and the point the lens reacted to the command. It certainly isn't much time.
When we experience 3rd party lenses doing more "hunting" than OEM lenses in dim light (in which takes the camera longer to calculate the focus solution), my theory is that the lens isn't resolving the latency problem fast enough to obey the "move to this point" command while it's racking.
The OEM lenses don't have this problem, which suggests to me that the 3rd party lenses have a greater latency.
fruitpap wrote:
But if I understand the information above correctly, there is no such thing as a stop command; the lens is only told to move a certain distance, or even only to correct a certain phase difference. The AF system in the camera only looks once, and does not check the result while the lens is focussing or afterwards.
That is correct the lens command is to move a certain distance, not start/stop. Know somebody who had to reverse engineer this. I don't know exactly what the fine tuning commands at the end are (nudges the servo to get it in the right position, but apparently doesn't actually check focus again) - I'd think the lens servo would be able to do this by itself, but apparently not.
Oh, and thanks for the comments on my post RDKirk!
RDKirk wrote:
It's not particularly possible to effectively encrypt something like lens movement commands. First, Canon can't introduce an encryption that would prevent old lenses from communicating with new bodies and vice versa. Second, the action itself is so simple and repeatable that any attempt at encryption would be transparent.
Encryption that can't be replayed is a solved problem (search for the replay attack). To be specific: Let's assume the command is just a number between 0 and 99, representing how far to focus. Now, the camera thinks of a random number divisible by 100, adds that to the command, and encrypts the sum. The lens decrypts and ignores all but the last two digits. No two commands will ever be the same.
You're right about backwards compatibility, tho. That's partially why I chose the 4/3s std as my example: a completely new standard.
If canon had wanted to, they could have added additional pins to the EF-S mount: the AF commands could then be sent in the clear via the old pins and encrypted via the new. EF lenses listen to the old pins, EF-S listen to the new. Canon would then be able to release a camera that only had EF-S pin-outs; most likely a consumer one, as few pros would go for that. It would be incompatible with EF lenses (except in MF mode), but work with all EF-S lenses.
Note: I don't think they did that, but they could have, if they had intended to migrate to an incompatible standard over a period of time. Perhaps they considered something like that but decided that it would lose more sales from disgruntled customers than it gained from excluding 3rd party lenses. The 4/3s consortium had no such downside, and could have made some money licensing the spec to 3rd party manufacturers.
Here are my conclusions:
1. The view-finder makings are "conceptual". The rectangles on the view finder has very little to do with the real sensor size and shape. Only the positions matter.
2. The greatest confusion comes with the word "vertical" and "Horizontal". The Fuziness of the vertical line can be detected only horizontally. Thus, the vertically sensitive sensors are physically horizontal lines. The horizontally sensitive sensors are vertical lines. That's why the view finder markings are opposite in this regard.
3. The "image" formed on the AF sensors are NOT the same real image on the film plane. In the AF beam path, there is a set of lens group called "Secondary image formation lens". I speculated that they are to form the phase signals. So, a fuzzy unfocused line or dot can be extent over large area. Look at the center cross sensor. It comprises actually 8 sensors - 4 blues ones are cross type. 2 additional green ones further enhance the horizontal signal, the 2 red ones activated for F2.8 being able to detect faint vertical signals.
Pondria wrote:
Here are my conclusions:
1. The view-finder makings are "conceptual". The rectangles on the view finder has very little to do with the real sensor size and shape. Only the positions matter.
2. The greatest confusion comes with the word "vertical" and "Horizontal". The Fuziness of the vertical line can be detected only horizontally. Thus, the vertically sensitive sensors are physically horizontal lines. The horizontally sensitive sensors are vertical lines. That's why the view finder markings are opposite in this regard.
3. The "image" formed on the AF sensors are NOT the same real image on the film plane. In the AF beam path, there is a set of lens group called "Secondary image formation lens". I speculated that they are to form the phase signals. So, a fuzzy unfocused line or dot can be extent over large area. Look at the center cross sensor. It comprises actually 8 sensors - 4 blues ones are cross type. 2 additional green ones further enhance the horizontal signal, the 2 red ones activated for F2.8 being able to detect faint vertical signals. ...Show more →
If you go back to my Jul 19, 2005 at 11:09 PM post, I've got an annotated version of the sensor diagram. Each of the AF points marked on the focusing screen represents a pair of pixel arrays on the sensor board. It takes two arrays (an array is a line of pixels) to determine the point of focus because each looks at the light from the opposite angle. The arrays in each pair are oriented in the same direction (horizontally or vertically), in line with each other, and are positioned about as far apart as the length of a single array.
The shape of the AF markings on the screen does correspond with the orientation of the arrays--the vertical rectangles represent vertically oriented arrays and the horizontal rectangles represent horizontally oriented arrays. Like using a split-image rangefinder, you turn the orientation of the focusing aide perpendicular to the line you're focusing on for best results.
The central point, being a "cross-type" sensor, is represented by four sensor arrays (a vertical pair and a horizontal pair), however there is a second pair of vertically oriented arrays (seeing horizontal lines) that are swtiched on when an f2.8 lens is mounted that increases the accuracy of the center sensor.
Notice that the sensor board is smaller and narrower than the actual focusing screen, and there may be lenses or prisms that change the angle of the light as well.
