Autofocus Reality Part 3B: Canon Cameras

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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

174 Responses to “Autofocus Reality Part 3B: Canon Cameras”

Doug said:

Great series of articles Roger, as always! I always enjoy reading your work as you are so thorough with every aspect of what you are doing.
So, I'm noticing that the 7D is tied for the highest standard deviation in your Imatest values chart - when you say "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" does that include the 7D as one of the 'older cameras'? And if so, what does this mean for 7D users - is it worth upgrading/using any of the newer lenses as a result of this focus inaccuracy? Or are we still going to be far better off with the improved glass, despite the inability to take advantage of the newer focus capabilities?

Maiaibing said:

Fits my experience with the 300 f/2.8 IS L I, 35L and 135L which do not seem to focus notably better with the 5Diii than the 5Dii.

LensRentals Employee

Roger Cicala said:


I wish I understood the 7D system better. I'm not sure if it was an experimental dead end, or maybe is improved and just not in ways that this kind of test (center point AF on still subjects) detects. Certainly what I read from 7D users seems to show that there's a learning curve with it's AF system, but otherwise opinions are pretty wide spread. A few people love its AF, a lot don't. I expect that means it's good for certain situations more than others, but I don't know what those are.


Andre said:

Roger, this is really fantastic stuff --- thank you! Any plans to check on the accuracy of mirrorless cameras' CDAF?

LensRentals Employee

Roger Cicala said:

Andre I'll do Nikon next, and then go on to mirrorless.

markb3699 said:

As a former marketing writer (now a full time photographer) I'd like to point out that the reason for all the marketing hype may be due to the fact that the people writing Canon's marketing content may simply not fully understand the products. I don't think many product managers don't understand product lines they're responsible for with great depth. It didn't bother some of the large companies that I worked for that I didn't ucomprehend their products in fine detail, only that I could explain them to a reasonable degree and make brochures and data sheets read smoothly.

Maji said:

Great series Roger. You are not only technically gifted (an euphemism for techno geek), but you write well too. Not many techno geeks can claim that. You maybe in the cross hair of the marketing consultants as you are trying to cull out their babble. Keep this up... I mean keeping us educated and informed photographers :)

LensRentals Employee

Roger Cicala said:

Mark, I couldn't agree more. My favorite example, now mercifully pulled off of their website, was a Tamron blurb for an f/5.6 Vibration Control lens that said "Vibration Control allows you to freeze motion, making it perfect for athletic events", or something very similar.

Jos said:

...It’s a little confusing. There’s a range of variation, of course. But clearly the 5DII 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...

5DII must be 5DIII

Will said:

Thank you very much for this very interesting review . I keep my eyes on your future stuff on Nikon and mirorless gears !

hwyhobo said:

Good stuff, Roger! Having had to make sense out of technical publications for most of my life (I work in technical training, so I actually have to understand it, not just memorize keywords), it is a pleasure to read a straight-shooting article.

Jon said:

Any hope for a Sony test?

LensRentals Employee

Roger Cicala said:

Hi Jon,

I'll certainly do NEX when I try to test mirrorless, but I don't have a good comfort level with alpha autofocus. I'm afraid I might make technical errors since I've never done this with SLT mirror cameras.

David said:

Great article! I'm sure the engineers from Nikon, Sony et al. are just as interested in this as we photographers are.

Shane Pope said:

very interesting and informative Roger well done!!!!

Roger do you happen to have a list of Canon lens that have the Rotation detector or can you point us to a list if one is already published else where?

warm regards


Mel Gross said:

This is interesting, As it follows CNC machine tool practice, something that all camera and lens manufacturers are very familiar with.

There are two types of CNc machines. The least expensive ones use stepper motors to move the table, quill, or forth or fitth motion. These are accurate enough for those machi ES, and can drive 0.001" accuracy. On very well made machines, it can even go down to 0.0005". But that where it ends. And today, a half thou. Isn't all that great. But this is an open process, no checking of where things are.

Better machines use servo motors. As can be understood from the name, these motors use a feedback loop going back to the computer, which knows where they are at all times. These machines are also made more precisely. They can have an accuracy to 0.000005", or even twice that—one hundred thou. Inch! That's accurate! A compromise has arisin more recently that uses steppers with feedback, and is I between the two, though closer to traditional steppers. Still, it can get to a "tenth", as we call it, which means a ten thousandth.

So what Canon is saying here makes great sense, but requires a different way of thinking, and a higher level of manufacturing reliability, something we've seen hit within the mid, 90's. It's too bad that it takes so long, but figuring out how to do this without breaking the bank isn't easy. In addition, sensor making has hit a new high during the middle of the last decade, which has allowed this from the other end.

It's good to see it in action in cameras, though too bad that all of our lenses will need replacing (assuming that the mis focussing really has any effect most of the time in the real world of shooting.).

Mel Gross said:

Oops! Sorry for the typo's. I'm typing on my iPad which is on my lap, in a moving vehicle, and didn't notice the errors until

Michael B said:

Thanks for the insights and your article!

I always wondered why my 40D needs 2 or 3 AF actions (manually induced by half-pressing the shutter butten) before further AF trials gave the same result (e.g. with f/2.0 100). Since I observed this I trigger the AF several times until I do not hear any noises from the USM drives.

I think that I induced something like a feedback loop manually (by changing my brains "software" for photographing) - perhaps this might be an idea to recheck the older cameras and compare the AF consistency with that of the newer ones.

Best - Michael

LensRentals Employee

Roger Cicala said:

Hi Shane,

I do not have such a list. We only happen to know if we've looked inside and seen it. And it's possible for us to look inside and miss it if we haven't completely disassembled the lens.

Tern said:

"The 5DIII and 1DX have f/5.6 cross-type sensors (most previous cross-type required f/2.8 or faster lenses). "

Any chance you'd test other brands, such as Nikon/Pentax whom have had f5.6 cross-type sensors for generations now, and most interestingly with the new f8 cross-type sensors in the D800?

It'd be interesting to see if the theoretical choice of 'accuracy (if you have f2.8)' (Canon) vs 'availability (cross point with any lens)' (Nikon/Pentax) actually matches to reality.

CarVac said:

I thought that most ring-USM lenses have at least a coarse sort of distance sensor for flash use, but I guess that's not the same as this more precise sensor for autofocus feedback.

Murray Scott said:

Quick question. I suspect your manual focus tests were done using either live view, or the standard Canon-installed focus screen...which I find to be pathetically poor for manual focusing. Do you think manual focusing would improve significantly, using an after market focus screen such as that supplied by KatzEye? I'm a 7D user. The work you folks put into these articles is greatly appreciated!

Zak said:

First: Thanks a lot for (not just) this very enlightening series of posts. I know no camera or lens review site whose operators could not learn a lot from your systematic approach to pretty much anything.

Second: I'd imagine that the standard deviation values are difficult to compare across different cameras because their sensors have different resolutions. If the lens manages to outperform the sensor, resolution-wise, there's some space for the focus motor to move without imatest reporting much (or at least not as much as otherwise). I'd also be interested in knowing if the resolution figure is actually properly linear with the focus error.
Not that I want to cause you more work, but it would seem that the proper way of testing which camera supports the feature would be to compare two lenses (an old one, a new one) with similar optical properties (i.e. best achievable center sharpness) on each camera and see which bodies will show a difference. That way you can separate the difference in the autofocus procedure from the difference in the sensors. ... having said that, it might also be overkill :)

Another data figure I'd be interested in: Is the new autofocus procedure faster or slower than the old one, or neither?

My background is in science and engineering (including optimization), and the computationally cheapest algorithm for minimizing a function is the Newton method, where you have the value of the function (offset on the AF sensor) and the gradient (how much movement gives me which change in the offset?), or some approximation of it. You compute how fare you need to go, then re-evaluate and so on until you're satisfied. As you've shown, most cameras seem not to re-evaluate too much in favour of being done faster (or their AF-sensors are not that accurate?). But if you can re-evaluate on the fly, while the lens is still moving, and if you get feedback from the lens about how far it moved in reaction to the last input and what that did to the AF sensor offset -- you have more data on your hands and can move from first-order approximation (Newton) to second-order or even some much fancier algorithms to know much more precisely how to correlate input and output.

Zak said:

oh, oooh!
One more thing! I'd love to see whether 3rd party lenses can also do this trick (but suspect it will take them some time to do so, if they manage at all). Even if they can't: Is there a significant difference in the phase detect AF scatter between Canon and third party lenses?

Ethan said:

It's not totally clear to me how much of a difference we're talking about here. Is the variance noticeable shot-to-shot? How much of it is related to your focusing methodology (spin ring, refocus - I know you mentioned better results just by hammering the autofocus button a few times in a row)? If I have a 5DMIII, is it worth waiting for a version III of the f2.8 70-200 w/ IS or is that just silly?

Wilba said:

Interesting stuff, great to see rigorous testing in the AF area.

It's important to identify which control process you're talking about. The Ishikawa patent is about the control system _within_the_lens_, which interprets commands from the control system _within_the_camera_. It's the second system which has been debated at length and proven to be closed-loop from at least the 20D onward (see http://www.dpreview.com/articles/5402438893/busted-the-myth-of-open-loop-phase-detection-autofocus).

Whether the control system in the lens is open or closed-loop doesn't make any difference to the question of whether the control system in the body is open or closed-loop, and it's invalid to make conclusions about the second from an old patent about the first.

adrian said:

Good job! Thanks!

RP said:

Great job Roger. I always realized my 7D, and my 5DMKII too, were not so precise to auto-focus. I think inter shot variation should be software rather than hardware dependent. The only thing I was surprised was to know that T4i does not focus so precise as we should expect from the acquisition of the new sensor technology!

Robbie said:

The older AF lenses definitely have a rotational position sensor. It may not be as precise as the newer ones but it exists. I have seen it inside a Canon 50 1.4

It looks like the below image, with brushes riding against traces along the barrel (measuring resistance that varies as it is turned, ie a potentiometer).

The EOS control scheme must be closed loop on all the lenses... when the lens is installed or switched from MF it knows where the focus is rotationally. Otherwise it would need to drive to MFD or infinity focus first to establish a known starting position, which from experience doesn't occur. (Similarly for focus distance for flash in MF mode.)

It is likely the newer lenses have sensors with more angular resolution.

K D Sandmann said:

Thank you for yet another great article.

To some extent it seems people using older glass are now faced with updating the glass to justify & fully benefit from buying new bodies.

It would be nice if Canon would give us a list of optimal combinations but I doubt that would help their sales. O well. :-)

mantra said:

thanks for this very interesting article, rally an excellent investigation!
i shoot canon but may i ask a question?
did you compare the canon and nikon Phase-detection autofocus?
thanks a lot!

Nqina Dlamini said:

You've put in a bit of a pickle, I was ready to pull the trigger on a 7D.
From what I can deduce, I can either save up for a used 1D MKIV or go the 5D III route.
Thanks for this.

Dave Coombes said:

Great article. I have been confused about what Canon has been doing with their releases over the last few years but this makes some quite dull products like the 28mm 2.8 seem much more interesting.

I do wonder whether third party manufacture will be able to use this at all and whether they even know Canon have been doing this.

Cheers, Dave.

tenaiko said:

thank you so much for this fantastic write-up! I am particularly intrigued by the results for the 7D, which appears to be something of a black horse in Canon's line-up: it uses an AF system that was something of a departure for Canon at the time (and supposedly designed to be best the 'consumer-grade' AF they had just put into the 5D II), but in your testing its (first-generation) 'improved' AF system seems to be neither here nor there. As you are probably aware, Canon is set to release a major firmware upgrade for the 7D this month that is not only supposed to improve the camera's performance in various ways (making it faster in some circumstances for example), but also (a rarity, at least for Canon) adds features that were inherent in the hardware (such as audio levels) but that Canon decided to reveal, dare I say Magic-Lantern-style, only now.

Now here's the rub: If I read your assumptions about open-loop and closed-loop AF correctly, it would seem that changing AF performance from one to the other could well be achieved with a firmware update, at least with a camera body whose processors are fast enough pull off such a feat. Said firmware update would allow the camera to
1) recognize the ability of the new-generation lenses to provide the position of the USM motor with its rotation detector and
2) if such a lens is detected, change the camera's AF system so as to provide a 'closed-loop' measurement: AF - send data to lens - read rotation data back - AF again - send data to lens etc.

At least at first glance, a firmware update should suffice to add that capability to the camera - provided that Canon wants to provide that feature to the users of a specific camera model in their line-up and that the camera is in fact fast enough to deal with the back and forth.

It might be worth checking your results with the 7D once that big firmware upgrade has landed—what do you think?

Barnett said:

Roger, you are confusing closed loop AF with closed loop motor control. There is a HUGE difference between the two. Let me try to explain.

Consider an electric motor driving the lens focus position. When you apply power to this motor it starts running at a speed that is difficult to predict. So after some period of time you would have no idea how far the motor has moved. So you need some form of sensor to tell you (ie measure) the current position of the lens. Using this feedback sensor you can adjust the power of the motor (faster/slower/forwards/backwards) to accurately position the lens where you want it. This is called closed loop motor control.

A stepper motor on the other hand works differently. It has discrete "steps". The size of these steps are known and fixed. So if you want the motor to move 10mm, and you know each step is 1mm, you tell the motor to take 10 steps, and it will take 10 steps, and you know exactly how far it moved. This is open loop because you don't need to measure.

There are various advantages and disadvantages to these two types of motors. One advantage of the stepper motor is that it is a much simpler open loop system. One disadvantage is that you cannot position it in fractions of a step, so the accuracy is always limited to the step size. There are some other advantages/disadvantages too, but I won't do into that now.

The point I am trying to make is that the patent you mentioned simply listed these two types of motor control. It has NOTHING to do with the AF control loop which has to be closed loop. Always.

Armis said:

Well, I thouroughly enjoyed this series of articles. Some things are actually counter-intuitive: in those tests, the 7D's AF I've heard so many people gush about doesn't seem to out-perform the 5D2's old, consumer-grade AF (though I understand the 7D's AF strengths are in tracking subjects in motion, so the test may not be fully representative); in addition, my 5D2 focuses way better with my 70-200 f/4 IS (where it can't use the center cross type) than with my 50 f/1.4. Weird.

Raoul J said:

Well done !
I like your scientific mind ! Experimentation, intuition and theory... Great !

Now several thoughts about it
1. I guess you realize everybody is now expecting you to list all lenses that actually focus better with 5DIII and 1DX ?

2. Isn't that the same list than the list of lenses where EXIF data for distance is available ? (= lenses compatible with E-TTL II flash metering, which make use of distance).
I know 50mm 1.4 and 24mm 2.8 doesn't provide the distance but 85mm 1.8 does... as does 70-200 2.8L IS (I). So your answer will probably be "no".

3. If I understand you correctly, your results mean that the focus will actually be better on 1DX and 5DIII with lenses without feedback if you double click. Am I right ? Could you test it ?

Raoul J said:

About 7D's AF results against 7D's AF common feeling :

Of course, your articles are just part of the story :
AF absolute accuracy (what you speak about here) is different than AF capacity to acquire and keep focus on a moving subject, where the 7D is percieved as (and probably is) much better than 5DII.

Jim said:

Thanks for this VERY informative article. I am just an entry level DSLR user with an entry level model and consumer zooms. As your test shows, my shots shows quite a bit of sharpness variation. Given the apparent huge phase AF inconsistency, and for amateurs like myself, should we just go to mirrorless and get that fast reliable CDAF? Will I will a significant change in hit rate?

Chris P said:

Brilliant, easily understood explanation. I have now bookmarked your site and am looking forward to the report on the Nikon system.

dominique said:

Thank you so much for bringing us that information, Roger! I share your point of view that most photographers would be educated enough to bear more true and useful information about a new camera than all that marketing blabber that even seems to ever become worse.

What marketing people are trying to hide, others have to unveil afterwards - what a waste of time :-)

Anders said:

There are several different AF sensor types used in Canon's cameras.

The most basic is the short base line, single line sensor type. They are used in all peripheral points in the older cameras, and in some in the newer ones too. Due to the short distance between them, they aren't too accurate, but on the other hand can focus at f/5.6. Sometimes they are made so short, thus inaccurate, that they even can focus at f/8.

Then there's the long single line sensor. Due to the longer baseline, it gives higher accuracy, but on the other hand usually requires lenses with at least f/2.8.

There are also dual line, zig-zag type sensors. To increase resolution of the AF sensors, it's favorable to have more pixels/mm, but smaller pixels gives more image noise in the sensor. By using two normal resolution sensors side by side, one shifted half a pixel to the other, and then combining the read-out, you can get both higher resolution and good signal to noise ratio.
The dual line sensor can have the same baseline as a single one, so this can offer focus at f/5.6, but still with better precision than the most basic type, although not as good as the high precision sensor, which has its two elements further apart.

Older cameras offering high precision focus, at f/2.8, typically achieved this only with their center point. The 1D Mark III is one example. However, that camera had high precison in one direction only. The other direction used the standard precision type, at f/5.6. Hence when testing focus with such a sensor it makes a big difference if the test object has a vertical or horizontal line.

The 40D was the first Canon camera to have high precision, at f/2.8, cross type sensor in the center. That's the diagonal type of sensor which we also can see in the 1DX and 5D Mark III.

A camera like the 7D has short base line single type cross sensors in all positions except center, top and bottom.
The top, center and bottom have sig-zag short base line sensors in cross arrangement. Thus a bit better precison and better ability to detect large de-focus than the standard sensors.
The center sensor is also augmented by the diagonal high-precision cross type sensor, which requires f/2.8 to work.

To detail this for every single Canon camera is of course outside the scope of this comment, but it should be sufficient to see that exactly how you arrange the test target is very important for some of the camera models. In real world examples, a lot of other things influence the result too.

rf-design said:

Technical PDAF is a closed-loop system because in the end the system decide AF is ready. The focus motor have a mass to accelerate, have slip, limited resolution to control forces and finally limited precision. If the system works with a limited number of optical measurement control samples the final step before AF confirmation could be without optical measurement. That could be defined as some manner of open-loop.

What possible blame the hole combination of marketing and product definition is that there is a possible tradeoff of PDAF speed versus accuracy. That seems very natural and is simply different parameters in the driving SW to control the motor more slowly but to have more accurate focus. That is simply nonexistant in any PDAF camera and shows the ignorance of the product defintion against the user. I anticipate that the lens protocol should be backward compatible but system cameras should be run optimum with an existing system and not enforce technical unnecessary system component replacements.

The best possible motors are moving coils which fit better in nonsystem cameras but there the CDAF is the possible limiter.

Paul said:

This is a really interesting article. The 5DII's autofocusing really is hard work so it's great to read that the 5DIII has that sorted. I've noticed a few quirks too. Comparing our 600D "go anywhere" camera to one of our 5DIIs sometimes the 600D is better. Looking at the graphs it looks like there's actually not much difference between the two!

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Roger Cicala said:

Hi Jim,

I don't have any information about mirror less versus SLR - I think all we've shown is for still subjects contrast detection is more accurate with a few exceptions on Canon cameras. I would guess that using Live View AF on an SLR would be as accurate as using a mirrorless. But we'll find out in a couple of weeks.


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Roger Cicala said:

Ethan, I think waiting is silly. We've all been getting by pretty well with what we've had. Not to mention these are lab tests - the real world doesn't have a test target on it, and things aren't as neat and clean as the lab was. My 'keeper rate' is about 80% out in the real world. Maybe with this new equipment it will be 90% or maybe 82%. I wouldn't drop good lenses over it.


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Roger Cicala said:

Zak, I don't know if they are worse (maybe, or more likely some but not others). I can't think of any way they'll be better.


LensRentals Employee

Roger Cicala said:


The manual focus were all using 10X Live view. I'm not very accurate through the viewfinder, although I'm a lot better with a focusing screen. I don't think I'd be as accurate as with Live View even with a screen, but I could be wrong. And my eyes are old, others would be better than me.

LensRentals Employee

Roger Cicala said:


In the second article we tried double click and never found it to help. In the third we listed the lenses better with the 5D III, it was only a few of the newest ones, most of the ones that give distance data were not more accurate.

Richard Cowan said:

Very interesting comparison, I wonder how other systems are going to perform.

Can I make a suggestion however. Presenting SD values like this, particually when comparing different bodies of different resolutions and AA strength is a little confusing.

Can I suggest that you first normalize all the tested MTFs to the average MTF for that camera, lens and AF method. Then we can make more direct comparisons between 21 MPix and 15 Mpix (for example) bodies, without worrying about the effect of different resolutions.

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