No big deal, really; it
No big deal, really; it
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.
Originally Posted by HDNitehawk
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
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).
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.
Great thread, folks.
My experience with AFMA was similar to the one of Hawks. I actually wound up adjusting only my 1.4 and 1.8 primes, as it turns out, likely because they are my most precise lenses and I did not exceed 1-2 frames per each AFMA setting.
Thanks Neuro for methodical explanation.
Great post, Neuro. That illustration is great for describing the difference between precision and accuracy.
Chuck misspoke here slightly. The maximum aperture of the taking lens doesn't matter at all -- all that matters is the f-number rating of the activated AF sensor. (Aside from the obvious fact that which AF sensor will be activated does depend on the f-number of the taking lens, of course.) His statement implies that since an f/1.0 lens has much thinner depth of focus than an f/2.8 lens that it will have higher autofocus precision. In fact, the precision is exactly the same as the f/2.8 lens, because the autofocus sensor only sees a tiny sub-aperture from inside the annular ring corresponding to f/2.8. That's only the center point of course; most autofocus sensors on non-1D bodies are f/5.6, so the precision is much lower.
Thanks, Daniel. In fact, if he misspoke, it was more than slightly! I specifically asked that question of him in the same email: "Is the relevant ‘depth of field’ for those statement the DoF at the maximum aperture for the lens currently mounted? If so, how are lenses with apertures wider than the AF point sensitivities handled – e.g. is an f/4 lens accurate within its f/4 DoF or its f/5.6 DoF, and likewise, what is the case for an f/1.2 lens at either the center f/2.8 or an off-center f/5.6 AF point?"
His response was: "The fact that the AF points are functional with apertures as small as f/5.6, f/4 or f/2.8 respectively depending on the camera model and AF point under discussion does not imply that their measuring precision is limited to the depth of focus at those apertures. The AF detection system has the capability of calculating depth of focus based on the maximum aperture of the lens, whatever it happens to be."
By the way, depth of focus is different from depth of field.Depth of field deals with object space, such as the person that is 10 feet away with 5 inches DOF, while depth of focus deals with image space, such as the tiny fractions of a millimeter in front of and behind the sensor. The difference is important when it comes to autofocus for several reasons. One is because imperfect collimation of the autofocus has a much stronger effect on wide angle(e.g. 24mm f/1.4)than telephoto (e.g. 100mm f/5.6), even if the depth of field is the same. There are a few calculators around the web that will give you the depth of focus numbers, such as Bob Atkins'.
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Very interesting. Wouldn't be the first time he was wrong, but I'm surprised he's so specific and adamant about this. I would suggest that he reads Canon's 1D3 white paper. Page 17 clearly illustrates why f-numbers faster than f/2.8 have no effect.In fact, you could literally cut off the outer parts of an f/1.0 lens, remove the inner parts (e.g. the f/5.6, f/8, etc. annular rings), and even remove the non-sub-aperture parts of the f/2.8 ring and the f/2.8 autofocus sensor would still function exactly the same. The extra glass that makes an f/2.8 lens into an f/2 or faster lens has absolutely no effect on the f/2.8 autofocus sensor.
Here is an analogy. Imagine you are looking at the world through a 3cm cardboard tube. There a 1cm hole in a wall, so when you look through the tube your view is restricted by the hole, not the tube. If the hole is expanded to 3cm, you'll have the maximum 3cm view possible. If the hole is expanded to 10 cm, the view is restricted by the tube, not the hole, so you can still only see 3cm.
The high precision f/2.8 autofocus sensor can only look at the world through an f/2.8 tube. If there is an f/5.6 hole in the wall, AF wont work. If the hole is expanded to f/2.8, then it will have an unobstructed view and work normally. If the hole is expanded to f/1.0, it makes no difference because the view is restricted by the f/2.8 tube. However, unlike a cardboard tube put up to your eye, the camera autofocus sensor can never move to look at different parts of the hole -- it is always firmly fixed in the exact same direction.
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Daniel and Neuro
Ok, I have a meeting to go to and limited time to digest all this. But I read Neuro's post on the fly this morning.
So possibly to break this down to laymans terms that maybe easier to understand tell me if this sounds correct.
That the AF point, will vary each time we engage the AF function of the camera. The amount it varies will be within the 1/3 to 1/2 depth of focus. Depth of Focus would be releated to Depth of Field but less depth because of the proximity it is measured from the camera. Because it is not accruate to the point, it produces a group of shots similar to say a rifle and scope would. But by adjusting this "group" foward and back with the AF microadjustment we are looking for more of an average of shots that are "Spot On" rather than pin point accuracy each shot.
(This would explain why it seemed harder to tune the 24mm and 35mm than the tele's. It would also expain why as you go further from your target, it might appear less adjustment is neccessary because your Depth of Field increase andit wouldincreases the area of Auto Focus ).
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