Technical Discussions

Autofocus Reality Part 3B: Canon Cameras

Published August 1, 2012

Some days it’s good to be a geeky gear-head. This is one of them because a) I finished testing autofocus on all of the Canon camera bodies we had, b) I actually found out some interesting stuff, and c) I got worked up about camera marketers while doing it, so I have my next post in the works already.

Because I’ve been writing this series on the fly (telling you what we found as we found it), I’ll keep going in that fashion and keep the conclusions for the end of the article.

What We’ve Found So Far

In the first Autofocus Reality article, we demonstrated two things:

1. Phase-detection autofocus (even using still targets and center-point only) wasn’t nearly as accurate as contrast detection.

2. The contrast-detection autofocus was about as accurate as the most careful manual focusing.

Part two of the series showed that a few newer lenses did focus as accurately as contrast detection on 5D Mark III cameras but not on 5D Mark II cameras. The third article (part 3A) showed that the newest Canon lenses (40mm f/2.8, 24mm f/2.8 IS, 28mm f/2.8 IS, 70-300mm L IS and 300mm f/2.8 IS II) focus more accurately when mounted to 5D Mark III camera but not on 5D Mark II cameras.

The Canon 70-200 f2.8 IS II, oddly enough, seemed not quite as good as the above lenses, but more accurate than the older ones. Whether this was oddness in my measurements or a real finding, I wasn’t sure.

Since we knew that the newest lenses autofocused accurately on 5DIII but not on 5DII cameras, the next step was obviously to compare an accurate AF lens on different camera bodies to see with which ones it was capable of accuracy.

We expected the 1Dx (which has the same AF system as the 5DIII) would be accurate. We weren’t sure about the others.

Today’s Contestants on The Focus is Right!

We know the Canon 28mm f/2.8 IS II had accurate autofocus on the Canon 5DIII so we chose one copy to be our test lens.

We AF microadjusted each camera to the lens prior to shooting. If the camera did not offer AF microadjustment, we checked the body with the test lens and exchanged it for another copy if the lens back or frontfocused at the test distance.

(Microfocus adjustment makes absolutely no difference in the shot-to-shot variation in AF–it only improves the average value of the group. But I got tired of explaining that to people in the previous articles. It was easier to just do it than to answer another 50 emails.)

We then tested it in our Imatest lab using one copy of each Canon camera we carry. To save you from running amok on the internet, finding out which cameras are how old and have what kind of autofocus, I’ve listed that information in the table below.


Camera Year released Year firmware AF description
1DsIII Dec-07 12/16/09 45 point, 19 cross, 26 assist, f/4 at center, dedicated AF processor
5D II Dec-08 2/28/12 9 point, f/2.8 cross center
50D Dec-08 2/28/12 9 point cross, dual diagonal center cross
7D Aug-09 4/25/11 19 point, all cross, center dual diagonal cross (advanced algorithm), first “zone AF” and “spot AF”
1DIV Dec-09 3/29/12 45 point, 39 cross (f/2.8-f/5.6, f4 at center), first AF expansion camera
60D Aug-10 6/19/12 9 point, f/2.8 cross center
T3i Jan-11 1/30/12 9 point, f/2.8 cross center
1Dx Mar-12 2012 61 point reticular, 41 cross type, 5 dual diagonal cross, including center
5D III Mar-12 2012 61 point reticular, 41 cross type, 5 dual diagonal cross, including center
T4i Apr-12 2012 9 point, f/2.8 dual cross center, hybrid CMOS AF Live View


I suspected that the autofocus improvement we’ve seen had more to do with hardware than firmware, but I listed both year of release and year of latest firmware upgrade for completeness. I also listed the basics of the camera’s AF system, as well as any marketing comments made about it at release like the 7D having “the most advanced AF algorithms.”

My thought going in was the difference would be in hardware not firmware. I don’t think any amount of firmware is going to make the AF sensor on the lower left behave like the one on the lower right, even in center-point, single-shot mode.


The Results

In the lens variation article, we used the standard deviation (SD) to measure how accurately the camera focused: Softer focus results in lower Imatest values. As an example, we’ve put up a graph of Imatest values for the 5D Mk II versus the 5D Mk III below.

 As you can see, the 5D Mk III shots (red square) are all very similar. The 5D Mk II shots (blue diamonds) are more spread out: The variation in shot-to-shot focus is greater.

In this example, the SD of the 5D Mk III samples was 17 lp/ih, while the SD of the 5D Mk II was 38.5. Those are similar to the numbers we’ve seen over and over—accurate focusing combinations have SDs in the teens while less accurate ones have SDs in the 30s.

Rather than clog up the post with a lot more graphs, I’ll list the SDs of the various cameras with the 28mm f/2.8 IS lens in the table below.

Camera SD
1DsIII 29
5D II 38.5
50D 34
7D 41
1DIV 22
60D 34
T3i 41
1Dx 17
5D III 17
T4i 29


It’s a little confusing. There’s a range of variation, of course. But clearly the 5DIII and 1Dx do better than the other cameras, while the 1D Mk IV seems to be a bit between those two and the rest of the pack.

It seems a little clearer to me if we graph the standard deviations and separate the cameras by type (more expensive at the top, less expensive at the bottom.) The oldest cameras in both groups are on the left, while the newest are on the right.


A couple of points are worth making.

The graph of the more expensive cameras seems to show a pretty logical progression. The 1Ds III is by far the oldest, having been around since 2007. But it included every possible AF technology of the day, including a separate AF processing chip. The 5D Mk II, even on its release, was known to have “consumer-grade” autofocus.

Despite my well-recognized modesty, I will also point out that when the 5D Mk III was first released, and Canon fanboys were dropping off cliffs right and left, I said “the 5D III is no minor-upgrade camera; it’s an entirely new camera using the old camera’s name”. Its autofocus system is certainly not a minor upgrade–it’s moved over to the big-boy camera side.

I had hoped the T4i might be more accurate than it was, at least with new lenses. It does seem more accurate than the other consumer / prosumer cameras in phase detection, but it’s not nearly as good as the 1Dx or 5D III.

I assume that it’s new hybrid LiveView system does not carry over to create phase-detection AF. I will say, in it’s defense, that when focusing in LiveView it is obviously faster than and just as accurate as any of the other cameras, including the 5DIII and 1Dx.

So Why Could This Be?

All this autofocus stuff 1) gave me a headache and 2) made me rather curious and uncertain.

I started doing a simple demonstration of what I already knew: Phase-detection AF isn’t as accurate as contrast-detection AF. But then I got results that indicated sometimes it is just as accurate as LiveView. But you have to have a certain camera and a certain lens or it doesn’t happen.

This didn’t make much sense to me.

I would have understood if each generation of newer cameras and lenses got a little better. Or if a new camera or lens was dramatically better. But why a rather sudden change, and why did you need both a new camera and a new lens?

I spent a fair amount of time emailing with Dave Etchells of The Imaging Resource  and SLRGear.com who was kind enough to bounce ideas back and forth with me. His thoughts sent me on the right track for figuring this out.

I started off by trying to find out how long it took for an autofocus improvement to go from idea to released-to-the-public. I found one answer in a Canon patent from 2003 describing combined phase / contrast-detection AF in the camera mated to a lens using a stepper motor.

Which we saw  . . .  oh, yeah. Now. With the Canon T4i / EOS-M sensors and new STM motored lenses.

One thing of interest in this patent application, tucked away in the background section, is the following statement:

. . . though the prior art indicates a method for realizing high speed and high precision of autofocusing at the same time, it does not accompany a lens drive control for realizing this and thus does not adequately realize [. . .] high precision of autofocusing.

In other words, back in 2003, Canon recognized that a high-accuracy AF system in the camera required a more accurate lens drive to yield precise autofocus. You can’t have one without the other.

Suddenly the need for both a new camera and a new lens to get accurate autofocus began to make sense. But wait, the patent was talking about using a stepper motor to achieve accurate AF. We did find the 40mm pancake (stepper motor) was more accurate. But we also found that the new 24mm, 28mm f/2.8 IS and 300mm f/2.8 IS II lenses were more accurate. Yet they don’t have stepper motors.

Then I read on a bit further in the patent:

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.

The wording of this patent, back in 2003, suggests that closed-loop was not how AF worked at that time. It was largely open loop. The camera took a measurement and told the lens where it should go. Done.

This is backed up by a lot of other information, including statements in Canon’s “EF Lens Work III” and quotes made by a lot of people who would have known.

It’s clear that later AF systems were closed loop (the camera double checked where the lens had gone), although exactly when and where that change occurred I don’t know. But this probably explains the lengthy, and now largely meaningless, debates about whether systems were open or closed-loop: They used to be open. Now some (probably most) are closed.

More to the point, though, is the comment that a rotation detector would be needed in lenses driven by ultrasonic motors to increase accuracy of the focusing movement.

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.)


From Left: A Canon and Nikon magnetic rotation detector.


Just to be certain, I asked Aaron to find an excuse to take apart one of the new lenses. Or failing that, I told Aaron to take one apart and not tell Tyler or Drew, who never really believe we can put them back together correctly.

The result: Aaron checked and the newer lenses did have rotation detectors.

There was one other big hint hiding in plain sight in Canon’s resource article on precision cross-type AF sensors. The article states that the farther apart each pair of AF sensors are (each pair compares phase from opposite sides of the lens) the more accurate the sensor. It also states only the 5D Mk III and 1Dx have them. I italicized some of the quote for emphasis:

[…]simply by reading which pixels on each sensor line are being struck by light, the AF system can tell instantly what direction to move the lens in for proper focus, and by how much to move it. . . Canon EOS SLRs with high-precision AF sensors simply move the pairs of sensors much farther apart, and accordingly, the AF information can be more finely broken-down and reacted to.

The 5DIII and 1DX have f/5.6 cross-type sensors (most previous cross-type required f/2.8 or faster lenses). The central AF sensors in these cameras also have diagonal cross sensors. The article continues:

For pros who prefer to use the center AF point, simply manually selecting it [. . . ] gives the user the highest precision AF possible with these cameras (assuming an f/2.8 or faster lens is being used). And, with their unique diagonal cross-type layout, the AF points are much more likely to latch-on to typical horizontal or vertical subject details.

In Summary

As it turns out, my little tests just confirm things that were already published (if obscurely.)

The two newest Canon cameras have more accurate phase-detection sensors than their previous cameras. The newest lenses have more accurate focus movement (or provide more accurate focus movement feedback, or both) that takes advantage of those sensors.

Older cameras don’t have accurate enough AF sensors to take advantage of the new lenses’ capabilities.  Older lenses can’t move their focusing elements with enough accuracy to take advantage of the new cameras’ accurate sensors.

It’s rather sad (given the amount of other work that’s piled up during this little series) that I didn’t find this information until after I’d done all this testing, but it’s rather a testament to our times. There’s so much marketing drivel and useless verbiage thrown about that the marketing noise drowns out the actual useful information the camera makers offer us.

But that’s the topic of another blog post in which I will take personal offense at the marketing crap that’s been shoveled our way at an ever-increasing rate.

I don’t believe we photographers are nearly as stupid as the marketers seem to think we are. In the meantime, while I’m on this rant, I highly recommend Thom Hogan’s very funny look at camera icons “A Different Kind of Focus.” Thom writes better stuff sitting on the tarmac waiting for takeoff than I do at my desk.

And I guess, given all the marketing fluff, I wouldn’t have really believed “better autofocus” if I hadn’t seen it with my own tests.

I’ve heard it before and it wasn’t so. Not to mention I doubt seriously we’d have seen the marketers say, “better autofocus…but not with the lenses you already have.” So maybe this is worthwhile after all.

Otherwise I might have put my trusty old 85mm f/1.8 on a 5D III and thought “not better at all, they’re lying to me again.”


Roger Cicala (with thanks to Aaron Closz and Dave Etchells)


August 2012

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Technical Discussions
  • Rodrigo


    It was very interesting and enlightening to read your posts on this subject. It must have been quite a job to run all those tests.

    I disagree with one of your statements however: “I would have understood if each generation of newer cameras and lenses got a little better”

    If yopu bear in mind Canon has different camera segments, you can see clearly a trend to a better accuracy in your graphs:
    1st segment is the flagship 1D series, each generation gets better.
    2nd segment the 5DII / 5DIII and the 7D (5DII and 7D were released with only some months separation and approx geared to the same semi-pro users but for the APS-C/Full frame feature), again a betterment.
    3rd segment EOS 50D and 60D, this time approx the same, would be interesting to see the 40D performing
    and last, 4th segment wich is the entry level T3i and T4i, showing again a betterment

    With the exception of 2nd and 3rd segs which are quite similar, you see a trend that each segment is better than the following which also corresponds to the price tags of the cameras.

  • Daniel

    Thanks, Roger, for a *very* interesting article. Would you expect all new EOS cameras released from now on to have the new AF system? Or would you think the new AF won’t “trickle down” to cameras like the “entry level” FF rumored to be coming soon (not to mention future “Rebels”)? Thanks a lot for all insights!


  • Roger Cicala


    As best I can tell, the 24 and 28mm f/2.8 IS, 40mm f/2.8, and all of the Mk II supertelephoto lenses, as well as the 70-300 IS L are in the ‘new lens’ category. I’m not certain about the 70-200 f/2.8 IS II. New copies of older designs should not be different than the older copies. They aren’t redesigned. But I haven’t tested them all to be certain.


  • Mattias

    great job finding out that the 5DM3 needs the new types of lenses to show it’s full AF capabilities.
    That leads me to two questions:
    1 – What lenses of this “new type” do exist currently? I understand that the 24-70 2.8 M2 is one of those, but do there exist others today?
    2 – What about lenses of older design, but newly maufactured, like e.g. a 50 1.4 or a 24-105 L bought today, are they still of the “old type” or did they have some kind of unpublished “face lift” to fit the 5DM3 AF capabilities?


  • Denny

    The first two written reply from Canon HK stated the models have a f/5.6 “+” cross-type AF and a f/2.8 “+” cross-type AF. But the third verbal reply confirmed they have f/5.6 “+” cross-type AF and a f/2.8 “X” cross-type AF. She mentioned that they are different from the those on 5D3/1DX which features an Offset Array Sensor (with staggering AF point arrangement)

  • Denny


    Thanks for your update. The previous reply from Canon HK stated these models do not support dual-cross center AF point with the quotes seems to be from the manual. Let me check with them again and let you know the result when I got the reply from them.

  • Greg

    @Denny — That would be exceptionally strange, especially since Canon’s Hong Kong website says the 7D, 60D, and 650D have dual cross center AF points:

    From http://www.canon.com.hk/ Home > Product Catalog > EOS Digital SLR Camera > EOS Digital > EOS 7D, EOS 60D, and EOS 650D Features:

    7D) The newly developed AF System features all cross-type19-point AF sensors which detect both horizontal and vertical lines for better focusing performance. They evenly spread out across the viewfinder, allows easier focus on off-center subject. All 19 points are f/5.6-sensitive for f/5.6 or larger aperture lenses, while the center AF point adds high-precision diagonal cross-type sensitivity for f/2.8 and larger aperture lenses for enhanced accuracy. Inherited from the EOS-1D series, a dedicated microcomputer for AF calculations is featured in EOS 7D, not only achieve high-speed continuous shooting at approximately 8 fps, but also improve AF accuracy and reliability.

    60D) The 9-point AF system on the new EOS 60D features f/5.6-sensitive cross-type focusing on all 9 focusing points. The focusing points are widely spread out across the image plane and clearly displayed through the viewfinder, making it easier to focus off-center subject. The center AF point is a hybrid of standard cross-type and special diagonally-shaped “X” cross, with high-precision sensitivity for f/2.8 and larger aperture lenses. EOS 60D also adopts the AI Servo AF II algorithm as found in EOS-1D Mark IV and EOS 7D. It offers enhanced focus tracking accuracy by fast and correct prediction of movements, free from the short-term effect caused by intervening objects. AI Servo AF II also optimize auto focus performance in macro photography, to further enhance the ratio to obtain sharp macro images with auto focus.

    650D) The 9-point AF system on the new EOS 650D features f/5.6-sensitive cross-type focusing on all 9 focusing points. The focusing points are widely spread out across the image plane and clearly displayed through the viewfinder, making it easier to focus off-center subject. The center AF point is a hybrid of standard cross-type and special diagonally-shaped “X” cross, with high-precision sensitivity for f/2.8 and larger aperture lenses.

    Greg H.

  • Denny

    In the US, the 7D, 60D, T4i has a dual cross center AF point, but in Hong Kong, the 7D, 60D, 650D only has a single cross center AF point. Strange! Why would Canon do that?

  • BengtS

    Having just updated my 7d to firmware version 2.0, it seems to me that AF is now working more consistently than before.

    Would it be possible to rerun your tests with the new firmware to verify if that is actually the case?

    Thanks for your very thorough analysis.

  • I was just going to ask if you were going to test nikon.. and you answered it.
    I’ve been using some software (FoCal) to automatically set my AF muicroadjust values, and i see the variability in the AF, i’ve wondered why, and you’ve answered that very well.
    I’d be interested if there was a way to select improved precision via a custom function, sort of a firmware double check, without relying on rotation detection, this would open up more accurate AF for older lenses too.
    I’m expecting this may be an even bigger issue with older nikon lenses and cameras with high resolution such as the Nikon D800.
    I look forward to reading your results and conclusions with eagerness.
    Thanks again for the article(s)

  • Mike S

    Thank you for bringing together your experimental design skills, your statistical analysis background, and your long time interest in photography, to define Canon’s current AF outcomes – in the context of variational data. It has been a while since I could go through some of your writings, and, on vacation this week returned to your website to see what is new. Wow, the development of results in terms of averages around variation is a great way to define focus outcomes for lenses and cameras, and, your data and analysis are excellent. The reverse engineering of the lens rotation sensor, and publishing it, is probably causing Canon some consternation (although their competitors probably do read, carefully, all of their patents). The patent examination, and correlation to the new 40mm lens design was also outstanding. In addition to your business enabling mere mortals to test lenses we cannot afford, we have access to data that no other site publishes. Nice job. Thanks for starting the analysis with Canon.

  • Roger Cicala

    Not anytime soon. It’s extremely unlikely the third-party lenses could have whatever the new algorithms or hardware that the new Canon lenses do. We may look to see if they’re significantly less accurate after we do the Nikon tests. It’s my suspicion that third party lenses are more or less accurate on different bodies and we’ll look into that. Certainly anecdotal evidence seems to suggest that.


  • Jerry

    Are there any data or planned tests of 3rd party lenses with the newer EOS’s? I’d like to see how the Tamron 18-270 PZD compares when used on the 4Ti.

  • Roger Cicala

    Thank you, Greg: as always, a well thought out point well made. I always appreciate your contributions and input.



  • Greg

    Roger, thanks again for some terrific research and write-up. The data is very interesting and the info about rotation detection in the lenses very pertinent. But I think you muffed your reading of Rudy Winston’s article and the conclusions you drew from it, though it still provides insights.

    Rudy employs an historical approach in explaining cross-type AF points and “High-precision” AF points, not just strictly info about the 5D Mk III and 1D X. He does NOT state that ONLY the 5D Mk III and 1D X have “precision cross-type AF sensors” but that Canon has offered “high-precision” AF sensors since the 1990s. Indeed, we see a cross-type sensor in the EOS 1, marketed 9/1989, and a high-precision sensor in the EOS 1N, marketed 11/1994 (a hybrid cross — f/5.6 horizontal and f/2.8 vertical). Rudy also says (re: 5D Mk III and 1D X) “These aren’t the first EOS cameras to use a diagonal cross-type, high-precision arrangement, but they are the first to offer it with multiple points in the central area.” The EOS 40D (marketed 9/2007) introduced a sensor with a dual cross-type center point — a f/5.6 “+” and f/2.8 “X”, like the 5 in the center column of the 5D Mk III and 1D X.

    Instead, allow me to speculate on clues in Rudy’s article. The traditional “High-precision” (f/2.8) sensors use longer lines, with 2x-3x more sensor pixels, spaced further apart than standard-precision (f/5.6) sensors. While the lens aperture and the size of its cone of light remains a constraint, the AF optics could be improved to focus on smaller, more densely packed sensor pixels.

    While this is just speculation, it could also be applied beyond the 5 “High-precision” diagonal cross-type sensors to improving the precision of all 61 points. This would help explain the improvement of the EF 70-300mm f/4-5.6L IS USM which, tested at f/4, would have used the “standard” f/5.6 “+” sensor.

    Returning to the AF sensor introduced in the 40D — according to Canon, it has also been used in the 50D, 60D, and T4i. You tested the last 3 and found 50D and 60D equal but the T4i improved by a value of 5. To me this indicates that even without a new, higher precision sensor design, more modest improvements can be made via the AF algorithms (which are usually coded in firmware). I hope you re-test the 7D with the new firmware — the test results with the existing firmware are appalling.

    So there are my speculations: a) substantial design precision improvements in the new AF sensor; and b) modest improvements via the algorithms (that could be applied to older AF sensors).

    I hope someone can pin a Canon person down at Photokina for some real answers!

    In investigating all this I also noted: 1) The 5D Mk III and 1D X both have separate dedicated AF processors; and 2) The exposure light metering sensor on the T4i is the image sensor.

    Thanks again,
    Greg H.

  • Very helpful review. One spelling tip (that I used to give to my 3rd and 5th grade students): If you don’t know the difference between ‘its’ and “it’s”, always fully write out “it is,” and that means you NEVER need to use “it’s.”

    Sorry for being so pedantic, but it’s the English teacher in me . . . it just hurts to see really basic spelling errors. 🙂

  • Kai


    Thanks for the effort. Any comments as to why the 7D actually has a larger standard deviation (41) than both the 50D and 60D (34)?
    Bashing SD’s can be fun, but what about the average resolutions? If the 7D is 1000 +/- 41, I’ll take it over a 700 +/- 34 50D any day thank you 😉

    Also, picking some nits: the 50D was not released in December 08, but September or October (mine is from mid October). DPR lists the 50D as announced on August 26, 2008. Likewise, the 5D2 is listed as announced on September 17, 2008.

  • Zak

    I’m sure that 3rd party lenses will have a disadvantage but it would be interesting to see whether there are models that will provide the same type of feedback to the body. I imagine it’d be difficult for the manufacturer to include such a system-specific thing in a lens. But if they manage to do it anyway, it would be a shame if it went unnoticed.
    Regarding the open-loop/closed-loop story: I’d agree that the AF procedure _must_ include a feedback of some form if you want reproducible results with different lenses (i.e. the body must re-check the focus and correct the lens until it’s “good” (for whatever “good” means). But with a sensor in the lens and/or a step motor, the feedback contains a lot more information so the autofocus loop will need fewer iterations because the result of a signal given to the lens can be predicted better, and therefore Focus can be achieved quicker.
    Only quicker, not more accurately? Well, depends. In control engineering you can always trade one for the other: If you move quickly, you arrive sooner, but might miss the target a bit. If you want to be extra-precise, you need to move slower, or check back more often, and in general apply smaller inputs in between getting feedback. I’ve seen this in the completely unrelated example of a hydraulic robot arm once. In one extreme it’s lightning fast but can’t really sit still, in the other it is rock-steady and precise but sloooooow. The difference is made by how the control circuit is designed.
    Now if you get more feedback from the lens, you can determine the required dose of lens movement quicker (in the case of step motors you can also execute more precisely), and that gives you an advantage for either speed or accuracy, or a bit of both, whichever the engineer designing the system prefers.

  • Roger Cicala

    Hi Mike,

    We are using Imatest targets with 80% 20% gray target to background variation, lit with diffused halogen lights to keep our exposures at ISO 400, f/2.8 to between 1/250 and 1/800. I think your points are completely valid. Given the time involved, and my desire to limit variables, I just want to do one thing at a time. I see this as a year’s worth of effort, at least.


  • Mike Mullen

    Roger, thanks for all the info – very useful.

    I assume you used Imatest charts as the AF target?
    I am curious under what light levels your testing was perfomed? My testing of the 7D and 40D (using various fabrics as AF targets) suggested that center point phase detection AF consistency and accuracy took a dramatic hit as light levels fell below EV 6 or so when testing lenses of f/2.8 or faster. However, this fall-off was not noticed when using an AF point off center (non-HP AF point) or a lens slower than f/2.8. The difference was so dramatic it appears the high precision AF point causes the lens to make an erroneous correction under light levels that are not typically thought of as causing AF issues. I imagine using a test chart with more contrast than typical, real world AF targets could mask this phenomenon. That is why I use fabrics to test AF (because I do not use high contrast test charts as my AF targets under actual shooting conditions).

    It would be interesting if you could duplicate these results. I know testing is time consuming but I think this is an area that needs further exploration because photographers often use the center (HP) AF point under low light conditions with apertures f/2.8 and larger and with AF subjects that are not high contrast black and white test images. They do this expecting higher AF precision. If this is not the case, then they would be better off selecting one of the regular precision AF points (surrounding the center AF point).

    Thanks again for sharing your results so generously.

  • Roger Cicala

    If you actually read the articles, you’ll see the graphs are in Line Pairs / Image height.

  • Gozo Kiona

    What’s the point of putting graphs of everything with numbers, with no units at all? “Yes, the car was running at 1902”,… feet per hour? Very non-technical for a dslr review…

  • Roger Cicala

    Well, I guess it was a “II” in my own mind. . .

  • Edwin Herdman

    Something odd I noticed after the “contestants” header in blue:

    “We know the Canon 28mm f/2.8 IS II had accurate autofocus on the Canon 5DIII so we chose one copy to be our test lens.”

    28mm f/2.8 IS II?

  • Roger Cicala

    ARthur, I don’t think this little study would show anything about the Sigma 120-300. It’s not the fastest autofocusing lens, that’s for sure, but it does bring a lot of advantages to the table.


  • Roger Cicala


    It’s always a possibility, but the only difference I could imagine would be in a group between full=frame and APS-C, I think. The target has standard lighting and all shots are at ISO 400 so shutter speeds vary a tiny bit by focal length, and a lot by aperture, but slowest are around 1/400 and fastest about 1/1000. In theory that should be out of the range of significant effect.


  • Tim

    What is the possibility that mirror-flop and shutter vibration could influence these results? What shutter speed were these shot with? Obviously for the 1DX/5DMKII on a 300mm f2.8 it looks like mirror-flop and shutter speed had no effect…but what if that is a factor with other bodies?

  • Arthur

    very interesting – thanxs a lot for that great efford. Could you write or say something about using Sigma/Tamron for example a Sigma 120-300mm 2.8 OS on Canon Cameras 5d2 / 5d3? For sigma/tamron the whole reeingenering must be a nightmare – but i am seriously interested in that lense – there is hardly a alternative in the Canon Catalog. Your article makes me really thinking about it. ( i do not have $$$$$ to find out) also i am from switzerland, no returning in if you don’t like – and very high prices compared to the US)

  • Alan

    Sounds like some 7D owners are hoping the new firmware will address the AF issues described ad nauseum. Based of the Canon update info it seems like most of the changes are either for movie makers or add some useless on camera functionality that most shooters like myself don’t care about. I take pictures with my camera and make pictures with my computer.

  • Roger Cicala

    Thank you Farid. That makes a lot of sense and fills in some blanks very well.

    One of the best things about writing articles like this is a lot of people take the time to broaden my knowledge in the comments, and I do appreciate it! As do all of the others who read them.


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