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Author Topic: How can lens focus ever be "off" in a way that AFMA fixes it?  (Read 7669 times)

hgraf

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #15 on: April 19, 2013, 04:33:17 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!

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #15 on: April 19, 2013, 04:33:17 PM »

Drizzt321

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #16 on: April 19, 2013, 07:19:38 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!

Even with a closed loop, you may not be able to guarantee that it will always be correct since there's long term wear and tear, if the body is sent back to be cleaned and things shift slightly...plus it'll be different if you're using it in cold weather vs hot weather since parts expand/contract and at different amounts.
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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #17 on: April 20, 2013, 06:45:32 AM »

As for the execution of AFMA, nothing physical is shifted in the camera. It's an electronic correction factor - if the AF system would prompt the lens to move the focus elements a distance of x, the AFMA value modifies that command to x+n.
If its only an electronic correction, couldn't the manufacturers make some software that's kinda "idiot proof" for people like me who find AFMA a bit too tedious and complex? e.g. I shoot some text using the center focus point, with the camera connected to a computer and the software analyses if Af is accurate or not and makes AF adjustments accordingly? ... does that make sense?

Camera manufacturers could, but they haven't. Reikan did, it's called FoCal.  Worth every penny (or pence, in this case).
IC ... Thanks! will look at FoCal
neuroanatomist, I checked Reikan website for FoCal and the price sounds reasonable ... if I get the "Pro" version, how easy or difficult is it to work with?
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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 #18 on: April 20, 2013, 07:12:20 AM »
It's easy.  You do need a stable tripod, and lots of light in the target (sun or tungsten/halogen, not fluorescent or LED).
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kphoto99

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #19 on: April 20, 2013, 08:16:41 AM »
AFMA to my knowledge doesn't change anything in the lens, but rather tells the camera how to shift the sensor so the focal plane is aligned to different lenses. That's why you need to do it for different lenses.
Make sense?
Interesting ... so the chip in the lens does not make any adjustments to the lens, other than read command(s) off the sensor?
To my knowledge that's correct. The corrections are done in the camera body and not the lens. Auto-Focus Micro Adjustment would seem to be making sure the autofocus bits of the sensor are calibrated and not calibrating the lens. Mind you, this is just a possible scenario from a geek perspective how it would be the easiest way to implement AFMA in many camera bodies.

I have a Tamron lens and T4i, the lens was doing front focus. Tamron adjusted the lens to work with my body. So the lens can be modified.

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #20 on: April 20, 2013, 10:51:43 AM »
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!
Its not totally open loop, there is checking going on. There is information about how it works, but the fine details are not disclosed.
 
There are two possible types of errors.
 
1.   The Camera tells the lens to move to 8.2 ft, but due to errors in the camera, it should actually be in focus at 8.1 ft. 
 
2.  The lens receives a command from the camera to move to 8.2 ft but due to internal tolerances and errors, it moves to 8.3 ft.  It does tell the camera when it has moved, which closes the loop.
 
When both of these happen, there is a significant focus error.  AFMA Adjusts the camera command to compensate for this.  Sometimes a lens or body error compensate and the errors cancel out.  In the example above, the lens could have actually been in focus at 8.1 ft even though it told the camera that it had moved to 8.2 ft as commanded.  The image would have been in perfect focus and the owner boasting that his lens was perfect and needed no AFMA!
 
 
If you send your camera and lens to Canon, they check each separately and write new parameters in the internal firmware so that both are more accurate.
 

cocopop05

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #21 on: April 20, 2013, 11:44:36 AM »
Here are my thoughts, they may be wrong  :)

Contrast detection (Live view focusing, taken from image sensor itself) should be very accurate.

Phase detection (autofocus sensors, separate to image sensor) can have inaccuracies.  This is where AFMA can resolve issues.

All I know is that I bought a Canon 5D MIII with 24-105 F/4 L,  it was not pin sharp when using phase detection.  I used the EOS utility with the tethered method (http://ophrysphotography.co.uk/pages/tutoriallensmicroadjustment.htm) to set my AMFA.

I found at all focal lengths of the zoom lens a +5 adjustment meant the sharpest results as possible.

After setting that value, the autofocus is almost always dead-on accurate.

Whether you think AFMA can in principle work or not, in practice with my experience, it is an invaluable feature.



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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #21 on: April 20, 2013, 11:44:36 AM »

TrumpetPower!

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #22 on: April 20, 2013, 11:50:57 AM »
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,

b&

jrista

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #23 on: April 20, 2013, 12:18:38 PM »
Here's a question that's been bugging me.

Why does AFMA fix anything? As I understand modern phase detect focus systems, focus is a closed loop sort of thing The camera tells the lens to change it's focus, and when the camera body sees things are in focus (the phase detect sensors say focus has been achieved) it stops moving the focus on the lens.

How can this "go wrong" in a way that AFMA can fix it? The way I see it is the only way things can go wrong is if the sensor is physically in the wrong spot, meaning an "in focus" in the phase detect sensor doesn't correspond to in focus at the sensor.

Where the lens is shouldn't matter?

So what am I missing?

Thanks!

You pretty much have it. AFMA is purely an algorithmic thing, and all it does is tell the AF drive system to adjust focus by a small amount to ensure that when the mirror lifts and the image resolves, the focus plane is centered at the sensor plane.

The reason this is necessary is simply a matter of calibration. All devices are manufactured to certain tolerances. Higher end equipment that costs more is generally manufactured to tighter tolerances, but even then, it is extremely difficult and extremely costly to keep things "perfect". There are also three separate things in play when it comes to focus...the lens, the sensor, AND the AF unit. It can be difficult to perfectly align two interacting elements of a system, let alone more than two.

You also have to realize that even at its extremes of +/- 20, AFMA is making MICROSCOPIC adjustments to the focal plane. Reality is compressed within the mirrorbox, so a significant AFMA setting is likely only adjusting the focus plane by MICROMETERS. We aren't even talking about millimeters here (changing the focus plane by a millimeter would likely result in radical defocus)...we are talking about an incredibly FINE adjustment!  :D It is all just a matter of calibration, and it only actually changes FIRMWARE CONFIGURATION in the body. Making an AFMA change does not actually change any physical attribute of the camera body or lens. Making AFMA adjustments on multiple different bodies for the same lens simply reconfigures each body, so the lens can still be used on each one and it should work ideally on each one once properly "calibrated."

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #24 on: April 20, 2013, 12:20:49 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).


Neuro's comment about fluorescent and LED light sources is true for those that are equipped with old-style magnetic ballasts that operate at 60Hz.  The light output fluctuates at 120 Hz and at any shutter speed faster than 1/120 there is a high probability that the image will be captured between peak light levels and the exposure will be incorrect.    Fluorescent and LED lights with electronic ballasts operate at approximately 20,000 Hz, far in excess of any shutter speed you will be using.

jrista

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #25 on: April 20, 2013, 12:22:55 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.
« Last Edit: April 20, 2013, 12:26:38 PM by jrista »

skitron

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #26 on: April 20, 2013, 12:30:02 PM »
I've often pondered the same question as OP, so dug around and found this post:

http://photographylife.com/how-phase-detection-autofocus-works

Easy read, but enough info to understand the technical.

Basically there are known and expected phase detection errors at the individual phase detect cell level and those are dealt with in code when the camera is manufactured. AFMA allows you to "mop up" at a global level.

The phase detection process as a whole is a hybrid of open and closed loops. Basically if a phase error is detected beyond what has already been documented in code during manufacturing, instructions are sent to move the lens to a specific focus. It keeps checking the results and sending further instructions to the lens until it confirms. So in this respect it is closed loop.

However, the Phase Detection sensor and the Image Detection sensor are two entirely different physical entities operating on two distinct and separate light paths. By definition, using the PD sensor to derive an ID sensor focus state is open loop in nature. An "in focus" state on the PD sensor results in an "unknown focus" state for the ID sensor evey single time. And it is only by making assumptions about the physical relationship between the PD and ID light paths and sensors that the ID sensor focus state is derived and in fact *assumed*.

LOL, after seeing how this works I'm surprised it functions as well as it does.

The way to close all process loops is to place the PD cells in the same physical light path and focal plane as the ID cells. Seems like I heard something along those lines recently.
« Last Edit: April 20, 2013, 12:34:36 PM by skitron »
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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #27 on: April 20, 2013, 12:31:14 PM »
.

You also have to realize that even at its extremes of +/- 20, AFMA is making MICROSCOPIC adjustments to the focal plane. Reality is compressed within the mirrorbox, so a significant AFMA setting is likely only adjusting the focus plane by MICROMETERS. We aren't even talking about millimeters here (changing the focus plane by a millimeter would likely result in radical defocus)...we are talking about an incredibly FINE adjustment!  :D It is all just a matter of calibration, and it only actually changes FIRMWARE CONFIGURATION in the body. Making an AFMA change does not actually change any physical attribute of the camera body or lens. Making AFMA adjustments on multiple different bodies for the same lens simply reconfigures each body, so the lens can still be used on each one and it should work ideally on each one once properly "calibrated."

Older DSLR's like the Canon XTi had a small set screw that let you change the position of the sub mirror.  This was a mechanical adjustment for AF that was easy to do.  I judt turned the screw 2 clicks with a tiny allen wrench and adjusted my AF system to be accurate.  After the XTi, Canon went to all electronic adjustments on the Rebel line.
 
http://www.astrosurf.com/buil/autofocus/adjust.htm

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #27 on: April 20, 2013, 12:31:14 PM »

neuroanatomist

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #28 on: April 20, 2013, 12:33:26 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?
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jrista

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #29 on: April 20, 2013, 12:39:12 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?

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. The new generation of Canon bodies, at least the single-digit ones, seem to have improved firmware to support closed-loop operation. That includes the 1D X and 5D III at the very least, both of which score PDAF marks as high as CDAF. I am not sure if the 7D has the necessary drive firmware...there was a recent firmware update, but I don't know if that included better drive. In my experience, the 7D with the new firmware focused WAY better with the 300, 500, and 600mm Mark II lenses I rented than the 100-400mm lens. That is largely anecdotal, and it could just be the relatively poor quality of the 100-400mm lens. I couldn't say whether the 6D had closed loop operation, although I don't see why it wouldn't.

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Re: How can lens focus ever be "off" in a way that AFMA fixes it?
« Reply #29 on: April 20, 2013, 12:39:12 PM »