Comments on MFA

[Colin500]

Hi Everybody,


a couple of weeks ago I finally took the time to perform some micro-focus adjustments for my lenses, after all, being able to do an MFA was one reason why I upgraded from the 500D. Here's my thoughts on the subject:


- Every lens required some adjustment, unsurprisingly the 50mm f/1.8 II with +12 the most, the others a lot less, but all in the same direction, perhaps the body is off.


- There is a remarkable improvement, even a +3 or +4 makes a world of difference in AF accuracy with thin DOFs, I am only now really happy with my glass.


- It's a scandal that MFA isn't standard on _every_ DSLR (hello, I'm looking at _you_ 60D), how many millions of people shoot unnecessarily crappy images with their Rebels just because the AF is bad?


- The procedure is relatively easy, at least I made it so, just printed out some detailled pattern, hung it on the wall, and played a bit with magnified live view.


- That said, I don't understand why the camera can't do it automatically, once it's set up with an appropriate target, using the normal contrast-detect method to check the real AF, that's exactly the same thing that I did "manually".


(I'll check the Canon SDK to see whether I can programmatically set the MFA, and force a focus-to-infinity, the rest shouldn't be that hard.)


Regards, Colin


PS: Nice to have the forum back up-and-running, the nightly server problems were translating to "forum offline in the afternoon" for me; I've also been busy with many other things...

[scalesusa]

I do think that its a worthwhile feature, and for wide aperture lenses with a shallow depth of field, it makes a huge difference with just a small adjustment.


Out of the dozens of lenses I

[HDNitehawk]

A few Saturdays ago it was raining and I decided to go through most of my lens (all the primes) and check the MFA to see if I could improve. With both 5D Mark II the only lens that really seemed to improve with adjustment were the 24mm F1.4L and the 35mm F1.4L. But what I found was that if I did the MF at a certain distance, then backed up several feet and did it again, the adjustment would be off by a bit. After playing with this for hours I ended up with +-0 on both lens.


Maybe others might have thoughts on this, but it seemed that it only fine tuned to the distance you might be shooting. (with the 24 and 35mm). Other distances and conditions would vary and the adjustment would change. Another thing I noticed with the 5D MarkII is that it would be less likely to hit the exact same mark depending on the position the focus was at on the lens at the time you begin to auto focus. This was true with both 5D

[Daniel Browning]

I prefer the acronym AFMA (AutoFocus MicroAdjustment) over MFA (Micro Focus Adjustment).

[bob williams]

I prefer the acronym AFMA (AutoFocus MicroAdjustment) over MFA (Micro Focus Adjustment).
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Daniel, Please explain? You usually have valid/technicalreason for your thoughts and opinions, But I can't figure this one out.


Thanks,


Bob

[Daniel Browning]

No big deal, really; it

[HDNitehawk]

Yes Canon calls this AF Microadjustment


and then this note follows "Normally, this adjustment is not required. Do this adjustment only if necessary. Note that doing this adjustment may prevent correct focusing from being achieved."


and here is another one I find interesting "The registered AF microadjustment will be retained even if you use the Custom Function to clear all settings. However, the setting itself will be (0: Disable)"


So if I read the manual correctly I set a lens and say it is +5 and I clear the settings it stays at +5 but shows 0 disabled. So this would not leave a option to return it to the factory default setting, if my +5 was not what it should be. If that is the case I wonder if the 24-70mm I sent off for service still has +12 in the camera since I have reset all the settings after it was sent off?


Well they did tell me at the start I could mess up the AF of a lens.

[Richard Lane]

and here is another one I find interesting "The registered AF microadjustment will be retained even if you use the Custom Function to clear all settings. However, the setting itself will be (0: Disable)"


So if I read the manual correctly I set a lens and say it is +5 and I clear the settings it stays at +5 but shows 0 disabled. So this would not leave a option to return it to the factory default setting, if my +5 was not what it should be. If that is the case I wonder if the 24-70mm I sent off for service still has +12 in the camera since I have reset all the settings after it was sent off?


Well they did tell me at the start I could mess up the AF of a lens.

Hey Hawk,


The adjustments below are for the 7D and 5D.


The camera will retain the +/- settings that you set prior to the clear all settings selection, but the feature will be ( 0: Disabled). To turn the feature back on just go into the C.Fn III 5 (for the 7D) and C.Fn III 8 (for the 5DII) and select ( 2: "Adjust by lens" (now this feature is enabled again) then press info, and you can individually adjust the lens +/- that is currently attached to the camera. You can turn the micro adjustment dial to zero to reset that particular lens back to the original factory setting.


If you would like to clear all of the AFMA settings for "all of your lens at one time", then turn the dial and highlight ( 2: Adjust by Lens) then instead of pressing info, press the Garbage Can Icon "Clear All" and all of your lens should be set back to factory settings of Zero AFMA.


I hope that helps!


Rich

[HDNitehawk]

Hey Hawk,


The adjustments below are for the 7D, but I'm sure the 5D is probably similar.


The camera will retain the +/- settings that you set prior to the clear all settings selection, but the feature will be ( 0: Disabled). To turn the feature back on just go into the C.FnIII 5 and select ( 2: "Adjust by lens" (now this feature is enabled again) then press info, and you can individually adjust the lens +/- that is currently attached to the camera. You can turn the micro adjustment dial to zero to reset that particular lens back to the original factory setting.


If you would like to clear all of the AFMA settings for "all of your lens at one time", then turn the dial and highlight ( 2: Adjust by Lens) then instead of pressing info, press the Garbage Can Icon "Clear All" and all of your lens should be set back to factory settings of Zero AFMA.


I hope that helps!


Rich

Thanks Rich, reading the manual I interpreted it wrong. It just means it turned the function off not that it retained and reset everything to zero (except the AF settings will remain and can be reactivated).

[neuroanatomist]

Normally, this adjustment is not required. Do this adjustment only if necessary. Note that doing this adjustment may prevent correct focusing from being achieved.

First off, I suppose it's important to know what AFMA does... It corrects for bias in the AF system, or put another way, it improves the accuracy of the AF system. The terms accuracy and precision are sometimes (improperly) used interchangeably - accuracy is 'closeness to true' whereas precision is repeatability. Here's a diagrammatic example:





AFMA corrects for accuracy but does nothing for precision. Chuck Westfall has told me that, "The AF precision for the standard precision sensors is within the depth of focus for the maximum aperture of the lens, while the AF precision for the high precision sensors is within 1/2 or 1/3 the depth of focus for the maximum aperture of the lens, depending on the camera model under discussion." Note that depth of focus is measured at the image plane of the sensor, and is different from (but proportional to) the depth of field. I interpret that to mean that although the precision is specified, for example with the f/2.8 sensor all the shots will be focused within a region that is 1/3 of the depth of focus deep for that lens' maximum aperture, focusing could still be inaccurate (i.e. it would look like case C above).


AFMA moves the center point - the 'average' focal plane of multiple measurements. But it's important to remember that precision plays a role, so if you test your AF system with just one or a couple of tries, random chance says your test won't be 'spot on' due to imprecision. So, when testing AF performance, you need to perform multiple tests for a given lens, and fewer tests would be needed for an f/2.8 or faster lens due to the higher precision. I suspect this is what's behind Canon's statement that AFMA can prevent correct focusing - if you base an adjustment on one or two test shots, and those shots were relatively far off the correct focal plane, then on average most of your subsequent shots will miss focus.


Now, that's all fine. But, how do you know if AFMA isrequired? In relatively severe cases, it's obvious even if you don't look that hard. Here's an example:


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This was shot with a 7D and 70-300mm DO before I did the AFMA, using the 7D's spot AF over the frog's eye. The camera/lens combo was apparently back-focusing, as the plane of critical focus is running through the frog's back legs. After testing, I ended up with a -7 for the adjustment (the most I've needed to apply to any lens).


Even with a given precision, there will still be an occasional miss - Canon doesn't publish the details of their precision measurements, but I'd assume that the specification 'within 1/3 of the depth of focus' does not mean it's in that range 100% of the time, but rather some defined cutoff ignoring the tails of the distribution (e.g. it's within 1/3 of the depth of focus 95% of the time). But, at thinner DoF's that you get with fast lenses, even a small AFMA will improve the % of hits.