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Author Topic: How can lens focus ever be "off" in a way that AFMA fixes it?  (Read 5586 times)

Mt Spokane Photography

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #30 on: April 20, 2013, 12:47:51 PM »
Canon cameras contain information about all the EF lenses and the commands to send to them.  Third party lenses tell the camera that they are a Canon lens and then translate the command they receive to their lens.  This can cause even another step that adds to inaccuracy, but it can be adjusted by AFMA as well.
 
Its a lot more complex that a person might think.


Again, all of that would be solved by closing the loop. Then all you'd need is a correction for sensor/vs AF array, which would be body specific and programmed by Canon at the factory.

It is sounding like the whole phase detect AF system is fully open loop, which really surprised me. Is Nikon like this too? Do they also have an AFMA type feature on their bodies?

Wish there were a Canon engineer I could speak to this about, would be a fascinating discussion!


You should read Roger Cicala's LensRentals blog. He does extensive testing with lenses, and in his assessment, modern AF systems, including Canon's, ARE closed loop! Here's a good read:

http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

And to quote, from Canon's own patent:

Quote
In order to achieve this objective, this invention provides a camera system comprising: a first focus detection unit, a second focus detection unit, a stepping motor that drives a focusing lens, . . . or a rotation detector, which detects the rotation . . . of the motor. . . The control circuit performs closed-loop control, based on the output of the rotation detector to control the motor.



Yes, Canon patented that. But...did they actually implement it, and if so, in which lenses and bodies?

 
I think it may be used in the STM lens.  However, that only means that the internal lens stepping motor operates as a closed loop.
 
A Phase Detect System for all manufacturers operates as a limited closed loop, but that only assures that the lens tells the camera that it has moved where it was told to go.
 
A lens cannot measure the actual distance to the subject, its calibrated and the resulting values are stored in the lens.  The camera determines the distance.  As others have pointed out, the thickness of the lens mount, position of the sub mirror, position of the sensor all have tolerances.  The camera is calibrated at the factory to compensate for tolerance buildup as are the lenses.
 
However, things do go wrong and AFMA merely adjusts the offset of the focus + or -.  On the 5D MK III, 1D X it allows two adjustments for zooms, wide and telephoto, and then interpolates the values between the extremes.
It the best setting for a 24-70mm lens is -2 at 24mm, and +2 at 70mm, it will set AFMA to zero at approx 47mm.

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #30 on: April 20, 2013, 12:47:51 PM »

neuroanatomist

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #31 on: April 20, 2013, 01:07:26 PM »
Read the article I linked. According to Roger, all of Canon's newer lenses, dating as far back as the 70-200 f/2.8 L II I think, have rotation detectors.

Oh, I've read it - including the part where you seem to have missed the 10 year difference between when you suggest rotation detectors were incorporated, vs. when Roger does, or maybe that's a typo on Roger's part...but it makes me wonder what was going on for those 10 years...

Quote from: Roger Cicala
If this is the case, then the newer Canon lenses should definitely have a rotation detector built into them. We know there are rotation detectors in many lenses released after 2000, but if they are  in older lenses we can’t identify them, so this fits too. (As an aside, I am particularly skilled in finding them because usually if you touch them with your fingers the lens won’t focus anymore and the unit has to be replaced.)



A Phase Detect System for all manufacturers operates as a limited closed loop, but that only assures that the lens tells the camera that it has moved where it was told to go.

This was my point earlier - the closed loop is 'look-move-confirm' but not 'look-move-confirm-look'.
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LetTheRightLensIn

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #32 on: April 20, 2013, 01:11:57 PM »
Canon cameras contain information about all the EF lenses and the commands to send to them.  Third party lenses tell the camera that they are a Canon lens and then translate the command they receive to their lens.  This can cause even another step that adds to inaccuracy, but it can be adjusted by AFMA as well.
 
Its a lot more complex that a person might think.


Again, all of that would be solved by closing the loop. Then all you'd need is a correction for sensor/vs AF array, which would be body specific and programmed by Canon at the factory.

It is sounding like the whole phase detect AF system is fully open loop, which really surprised me. Is Nikon like this too? Do they also have an AFMA type feature on their bodies?

Wish there were a Canon engineer I could speak to this about, would be a fascinating discussion!


You should read Roger Cicala's LensRentals blog. He does extensive testing with lenses, and in his assessment, modern AF systems, including Canon's, ARE closed loop! Here's a good read:

http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

And to quote, from Canon's own patent:

Quote
In order to achieve this objective, this invention provides a camera system comprising: a first focus detection unit, a second focus detection unit, a stepping motor that drives a focusing lens, . . . or a rotation detector, which detects the rotation . . . of the motor. . . The control circuit performs closed-loop control, based on the output of the rotation detector to control the motor.



Apparently the rotation detector measures how much it spins and where it actually lands since the breaking of the rotation may not be perfect and it might overshoot a trace or not and then it applies a correction for how much over or undershoot it measured. Apparently the newest Canon lenses such as the 24-70 II, 24 IS, 28 IS and a few others have a higher resolution rotation detector. Apparently the lens is not able to use its own rotation detector to apply corrections internally but must send the info back to the body which then sends out a new commands to rotate a bit (and hopefully with this smaller command the new amount doesn't overshoot or undershoot to as great a degree as during the initial rotation) so apparently you also need a new body that has higher precision to be able to take advantage of the higher resolution rotation detectors and apparently that is why only the 5D3 and 1DX make use of the new ultra-precision detectors in some of the new lenses.

To be honest the whole system sounds crazily over complex and you'd think some sort of much more direct closed loop would be less prone to all sorts of issues, at least for non-AI Servo AF, but I assume they know what they are doing so there are most likely very good reasons for it all (although perhaps some are only due to the tech avail back in the 80s??).

LetTheRightLensIn

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #33 on: April 20, 2013, 01:16:31 PM »
Canon cameras contain information about all the EF lenses and the commands to send to them.  Third party lenses tell the camera that they are a Canon lens and then translate the command they receive to their lens.  This can cause even another step that adds to inaccuracy, but it can be adjusted by AFMA as well.
 
Its a lot more complex that a person might think.


Again, all of that would be solved by closing the loop. Then all you'd need is a correction for sensor/vs AF array, which would be body specific and programmed by Canon at the factory.

It is sounding like the whole phase detect AF system is fully open loop, which really surprised me. Is Nikon like this too? Do they also have an AFMA type feature on their bodies?

Wish there were a Canon engineer I could speak to this about, would be a fascinating discussion!


You should read Roger Cicala's LensRentals blog. He does extensive testing with lenses, and in his assessment, modern AF systems, including Canon's, ARE closed loop! Here's a good read:

http://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras

And to quote, from Canon's own patent:

Quote
In order to achieve this objective, this invention provides a camera system comprising: a first focus detection unit, a second focus detection unit, a stepping motor that drives a focusing lens, . . . or a rotation detector, which detects the rotation . . . of the motor. . . The control circuit performs closed-loop control, based on the output of the rotation detector to control the motor.



Yes, Canon patented that. But...did they actually implement it, and if so, in which lenses and bodies?


My understanding is that it goes back quite a ways at the least.
However, my understanding is that only very recently did they up the precision for the rotation detector+camera body system and that only the 5D3/1DX are capable of sending back micro-precision rotation adjustments and that only lenses starting around the time of 70-300L or so release have the ultra-precision rotation detector in them. Lenses before that have a lower precision rotation detector and pre-5D3/1DX and even current but lower end bodies don't understand the extra bits of precision. I'm not sure if the lenses have always had rotation detectors or not.

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #34 on: April 20, 2013, 01:42:35 PM »
Thanks all for the info, lots over very good hints and links in this thread.

So, to summarize, in case I've got something wrong:

There are basically 2 major types of error when PD focusing:
1) Misalignment between the PD array and the sensor plane
2) Error in the lens with regards to responding to a position request, i.e. command "focus to 8.1ft" results in actual focus to 8.2ft

The first error is mostly lens independent, and should be fully accounted for during manufacture, but can result from physical changes in the body, i.e. dropping, wear, etc. Hence AFMA helps here.

The second error is almost exclusively lens dependent, and can result from a variety of factors, including error in position sensor in the lens, and physical changes in the lens, i.e. wear, dropping, etc. Here AFMA has the most important role in fixing the problem. It also explains how the 3rd party systems can correct for focus issues just by modifying firmware in the lens.

Being an engineer and having taken courses in control theory had me thinking that by "closed loop" that meant the PD array was used to close the loop, camera checks PD array, moves lens, checks PD array, moves lens, etc. until PD array reports focus.

I didn't consider that another type of closed loop is camera checks PD array, tells lens to move to a position and lens reports done. While technically closed loop, it isn't as simple as the first case, and exposes the system to more sources of error.

Engineering is all about compromise, so the fact that Canon does it the way it does points to there being a benefit to the more complicated method, my guess would be speed.

It's similar to how the contrast detect focusing in live view is "slow", contrast detect doesn't deliver as reliable a "your focus is off by this much" sort of error signal, so the loop is closed by the contrast detect, hence the hunting you see while in live mode.

So thank you all for clearing this up for me!

One last question: how does Nikon do it? Does it also have an AFMA type tool?

Thanks!

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #35 on: April 20, 2013, 01:52:38 PM »
Nikon is similar.  AFMA is called "AF Fine Tune".

I found this thread informative... my Nikon D600 is spot on wih my 50mm 1.8G and my 70-300... but is quite off at the wide end of my 24-70 f/2.8.  I'm sending it in for adjustment soon (after the next round of trips) but I could never work out how the focus could be perfect for all of my other lenses and be off for one.  This thread definitely gave some insight into that!

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #36 on: April 20, 2013, 02:47:06 PM »
It's easy.  You do need a stable tripod, and lots of light in the target (sun or tungsten/halogen, not fluorescent or LED).

Ideal and pretty easy is to put your tripod on a concrete pad like a sidewalk or your driveway and tape the target to a heavy wall like the side of your house. Do this in direct sunlight, and you've got the perfect setup for FoCal to work its magic.

Cheers,

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #36 on: April 20, 2013, 02:47:06 PM »

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #37 on: April 20, 2013, 03:18:19 PM »
Read the article I linked. According to Roger, all of Canon's newer lenses, dating as far back as the 70-200 f/2.8 L II I think, have rotation detectors.

Oh, I've read it - including the part where you seem to have missed the 10 year difference between when you suggest rotation detectors were incorporated, vs. when Roger does, or maybe that's a typo on Roger's part...but it makes me wonder what was going on for those 10 years...

Quote from: Roger Cicala
If this is the case, then the newer Canon lenses should definitely have a rotation detector built into them. We know there are rotation detectors in many lenses released after 2000, but if they are  in older lenses we can’t identify them, so this fits too. (As an aside, I am particularly skilled in finding them because usually if you touch them with your fingers the lens won’t focus anymore and the unit has to be replaced.)

I did see that they had rotation detectors in lenses dating back a decade. However I also only saw that it seems more recent AF systems (lens+camera) use a closed loop, or at least a partially open loop. Older systems still used an open loop, and I suspect some kind of partially open loop when a new camera is used with an older lens that isn't quite up to snuff (although were exactly the cutoff is I really can't say.)

A Phase Detect System for all manufacturers operates as a limited closed loop, but that only assures that the lens tells the camera that it has moved where it was told to go.

This was my point earlier - the closed loop is 'look-move-confirm' but not 'look-move-confirm-look'.

Hmm, that is not my experience. And in one of the articles in that series Roger posted, he clearly stated that the AF system seems to fully adjust the lens as necessary in a single press of the AF button in single-shot mode to fully lock AF. His expectation was that it the AF system only did "look-move-confirm" rather than "look-move-confirm-move-confirm" that multiple presses of the AF button would further refine focus. That was not the case. AF was fully and precisely locked the FIRST time, and all subsequent attempts to AF resulted in no change...but only with the latest gear...i.e. a 5D III with one of the new STM lenses, or one of the new Mark II Great Whites. Older cameras still seem to operate on a partially open loop.
« Last Edit: April 20, 2013, 03:25:02 PM by jrista »
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neuroanatomist

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #38 on: April 20, 2013, 03:49:30 PM »
I think the situation may be a bit more complex than that.  One observation I made when doing manual AFMA (LensAlign Pro) with the 85L II (a lens with AF slow enough that one can actually observe the focusing with the focus distance window) was that the 7D seemed to lock on faster (or at least in a different manner, I didn't time it) than the 5DII.  Specifically, when focusing from infinity or MFD, the 7D would move the focusing group in one direction then stop, whereas the 5DII would consistently overshoot by a little bit, then move slightly in the opposite direction.
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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #39 on: April 20, 2013, 05:47:30 PM »
I think the situation may be a bit more complex than that.  One observation I made when doing manual AFMA (LensAlign Pro) with the 85L II (a lens with AF slow enough that one can actually observe the focusing with the focus distance window) was that the 7D seemed to lock on faster (or at least in a different manner, I didn't time it) than the 5DII.  Specifically, when focusing from infinity or MFD, the 7D would move the focusing group in one direction then stop, whereas the 5DII would consistently overshoot by a little bit, then move slightly in the opposite direction.

Usually my 7D will focus in a single movement, but that is not always the case. There are times when it will make on big move, then one or two small moves, then issue AF lock confirmation in the viewfinder. It is usually VERY quick at it, but you can just barely feel the lag and see those last couple of movements. I have my AF button set to a rear button on the body, so it is separate from the shutter button, so there is no confusion about what is causing the movements.

If Canon still used an open/partial open AF system, I don't believe the secondary movements would occur. I have not used a 1D X, so I am not sure if it might do the same thing at times. It has a vastly superior AF system as well, so the chances that you even need a second or third movement are probably so low as to be practically unheard of...especially with newer lenses that have updated electronics and firmware to support a more accurate AF system overall.
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Mt Spokane Photography

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #40 on: April 20, 2013, 08:06:22 PM »


I didn't consider that another type of closed loop is camera checks PD array, tells lens to move to a position and lens reports done. While technically closed loop, it isn't as simple as the first case, and exposes the system to more sources of error.

Engineering is all about compromise, so the fact that Canon does it the way it does points to there being a benefit to the more complicated method, my guess would be speed.

It's similar to how the contrast detect focusing in live view is "slow", contrast detect doesn't deliver as reliable a "your focus is off by this much" sort of error signal, so the loop is closed by the contrast detect, hence the hunting you see while in live mode.

So thank you all for clearing this up for me!

One last question: how does Nikon do it? Does it also have an AFMA type tool?

Thanks!
Phase detect operates in basically the same way for all Cameras.  It is old technology.  A closed loop will still be inaccurate as well as slow.  Remember, the check done by the PD array does not mean the subect is in focus.
 
Contrast detect, which uses the actual sensor photosites to compute focus is the closed loop type of focusing.  Its painfully slow because of all the checking and back and forth searching.  It often starts searching in the wrong direction.
 
Some of  the newer Canon models now use a hybrid focusing method.  Phase Detect pixels are embedded in the sensor, and tell the contrast detection system where to look for sharpest focus.  The drawback for DSLR's is that the mirror must be up, so there is no visibility thru the viewfinder to be able to track the subject.
 
Mirrorless cameras get around this by using EVF technology which is improving with every new version.  They are still very slow to focus where large sensors are involved.  Small sensors might only use three to five lens positions to cover focus, so they are much quicker.  The very deep depth of field makes this work.

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #40 on: April 20, 2013, 08:06:22 PM »