Autofocus Reality Part 1: Center-Point, Single-Shot Accuracy

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Once upon a time I wrote an article attempting to explain phase-detection autofocus and its limitations. That was a lot like trying to explain what happened to the Mary Celeste or why Stonehenge was built.

Actually we probably have a better handle on the Stonehenge thing than we do on the Mary Celeste or phase-detection autofocus. So rather than rehashing and speculating, I thought I'd instead do some simple demonstrations and explorations.

Obviously there's a lot of ground to cover, so rather than making a 162-page blog post, I thought I'd break this up a bit. This first part will be very simple. We'll compare the accuracy of phase-detection AF, contrast-detection AF, and Roger-detection MF (that would be me manually focusing using 10X LiveView) on still targets using just center point AF. While we're here, we'll demonstrate just what AF Microadjustment does, and doesn't, accomplish.

The Setup

We use Imatest to demonstrate AF variation using Canon full-frame cameras and 50mm f/1.4 and f/1.2L lenses. The reasons are pretty straightforward: Our setup involves shooting a still test target with star, vertical, and horizontal focusing aids in the center from a tripod, so we should be getting very accurate AF. The setup we're using involves shooting at a 12-foot distance so the depth of field is narrow. If focus is off by just a bit, Imatest will detect the decrease in resolution.

Phase-Detection AF Accuracy with an Older Design

We started with a Canon 50mm f/1.4 lens mounted to a 5D Mk II camera. The first thing we did (deliberately a bit out of focus so it would show up easily) was set the lens on manual focus and take eight repeated images without touching anything. This gave us a baseline (represented by blue diamonds) of the variation in testing procedures if nothing else changed.

Then we took eight repeated shots using LiveView and manual focus (represented by red squares), spinning the focus ring between each shot to either infinity or absolute close up. The red squares demonstrate Roger Units (how well Roger can manually focus given all the time in the world on a tripod with a perfect test target.)

Finally we did the same thing but let the camera autofocus in LiveView (represented by green triangles), resulting in contrast-detection autofocus. In theory this should be as accurate as Roger is, perhaps more so.

The graph below shows the results of those shots. For those of you who are not familiar with our Imatest graphs, the numbers reflect the sharpness of the image in Line Pairs / Image Height. The sharpness in the center is shown on the X-axis and average sharpness on the Y. Higher is better and, in this test, better focus equals higher sharpness.


The graph shows that there's a little bit of variation when we take several shots without changing anything (blue diamonds) which probably reflects tiny movements from mirror slap, minute changes in lighting, or possibly even tiny fluctuations in sensor performance. Remember the blue shots are purposefully a bit out of focus so they'd show up separately in the graph.

When we tried to manually focus accurately (red boxes) or let the camera do it with contrast detection there is a bit more shot-to-shot variation. But the variation is still small, as is the difference between the camera and Roger. To put some numbers to it, the standard deviation of the baseline shots was 5.5; for the Roger-focus shots 11.5; and for the LiveView AF shots 8.4.

Now let's throw away the repeated shot results from above since we've made that point, and replace them with standard (phase-detection) autofocus shots. These are taken in exactly the same way as the live view AF shots: take the image, spin the focus ring to one extreme, let the camera refocus, save the image.

 Two things become obvious:

1) When phase detection AF gets it right, it's every bit as in focus as contrast detection or Roger detection.

2) Four of these 10 shots aren't quite as accurate as the other six. That sounds like a ridiculously high miss rate but let's put the numbers in a bit of perspective.

If we believe in Subjective Quality Factor (SQF) then an SQF difference of about five is needed to see a difference in a reasonable print or pixel peeping at 50 percent, given that the two lowest blue diamonds (phase AF shots) are going to show up as a bit soft or out of focus if we're really critical. The third lowest might or might not look different from the "good shots"--it's right on the edge.

Still, that gives us a "missed focus" rate of 20 percent in this little test, and that's using center point AF with a still target. That's not a big sample size obviously, and we repeated it with several other 50mm f/1.4 lenses and got "missed focus" rates of between 10 and 20 percent for all of them.

That's not horribly out of focus, but it's definitely pixel-peeping out of focus. For those of you who like numbers, the standard deviation for phase-detection AF ranged from 25 to 44 on different runs (always greater than the 5 to 15 we got with LiveView AF.)

Demonstrating Microfocus Adjustment

Next on our trial list, we thought we'd compare with a newer designed lens. The Canon 50mm f/1.4 doesn't have a real USM motor and is known to be a bit difficult to focus. Not wanting to make the test too easy on the camera (and because we were set up at 50mm), we grabbed a few Canon 50mm f/1.2 lenses. These lenses have a modern design but again are known to be a bit "focus challenged", to use the politically correct term.

One of the first copies we shot gave us the opportunity to demonstrate microfocus adjustment. We took our usual LiveView autofocus and manual focus charts, followed by a set of phase detection autofocus shots. When we graphed the results, we had to change the axis range because the phase-detection AF shots were so out of focus. Some of the shots still don't show up; they're even worse than the expanded axis shows. Unlike the above example, every single one of this is obviously out of focus at a glance--no pixel peeping necessary.


What happened was readily obvious: This copy of the 50mm f/1.2 was backfocusing badly on this camera. We did a quick microfocus adjustment (12 points) and reshot the lens with the results below.

Here is a superb example of what microfocus adjustment accomplishes. After adjustment phase detection AF is now very accurate, although there still is going to be shot-to-shot variation. For the two or three of you who like to scream, "I don't want to us microfocus adjustment! The lens should be perfect out of the box!", we went ahead and put the same lens on a different body with no microadjustment.

On camera No. 2 the same lens autofocuses accurately. The other 50mm f/1.2 lenses we tested all autofocused accurately on the first camera. The lens is perfect, just not with the first camera. The first camera is fine with all the other lenses we tested. Sometimes a given lens that's fine doesn't match up with a given camera that's fine. So it goes.

The conclusion is pretty obvious: If you want to shoot wide aperture prime lenses and you don't want to use microfocus adjustment, you just refuse to cope with reality.

We did several runs with 50mm f/1.2 lenses and found similar results all the way through. Standard deviations for sets using LiveView AF were 11 to 20, while phase detection ran from 20 to 35. These results are similar to what we saw with the 50mm f/1.4.


This really wasn't a "conclusion" post; our intention was to just demonstrate some autofocus basics that most people know already.

  • LiveView (contrast-detection) AF on a still target is more accurate than phase-detection AF. It should be so. Contrast detection is using the actual sensor to determine focus; phase detection is not. Overall we found about one shot in 10 was out of focus with phase detection.
  • LiveView AF is as accurate as Roger View MF. You may be better than this, or you may not.
  • Phase-detection AF has more shot-to-shot variation than contrast detection. It's not huge, but it's real. This shouldn't surprise anyone. Phase detection was developed for fast AF speed and to detect subject movement. It wasn't developed to be more accurate.
  • Microfocus adjustment pulls good phase-detection AF results up to a par with LiveView, but it doesn't eliminate the small amount of shot-to-shot variation that phase-detection AF has.

I'll be expanding on this first article in some subsequent posts. Next I will be comparing the "two beep" AF method (where you push the shutter halfway down to focus then repeat before taking the shot) to the "single beep" method we used here. (I think the "two beep" method is more accurate for no reason other than when I started people told me it was so.  We shall see.)

We'll certainly do some comparisons with different lenses on the Canon cameras and try to see if newer designs, STM, or other lens changes reduce the AF variation. Obviously we'll have to get a larger database to be able to detect subtle changes, but that's just a matter of repetition. (What we showed today were just examples. We've done a larger number of these lenses already.)

We will also look at different camera systems' AF to see if they seem to have less phase-detection variation. I had thought about comparing contrast-detection systems in other cameras, but it's obvious we'll struggle to find one that's better than Canon's.

I'm not saying Canon's is the best, but it's as good as what I can do, so it may be difficult to detect "better" in this situation. We might be able to detect worse, of course.

Finally, within limits, I'm asking for suggestions about other things we can look at.

Remember when you make suggestions, that this model requires a fairly narrow depth of field to detect differences. Very long focal lengths (over 200mm full-frame equivalent) test at longer distances which may increase depth of field too much for us to detect small differences. Narrow maximum apertures (f/5.6 or so) would have the same effect.

Given those limitations, though, I'm open to suggestions!


Roger Cicala


July 2012

49 Responses to “Autofocus Reality Part 1: Center-Point, Single-Shot Accuracy”

Steven said:

I think it would be interesting to rig up a moving target, towards the camera (on a motorised Dolly), and getting the camera to take 5 successive photos, timed intervals (so that there was sufficient movement) - and plotting the results on an above chart.

Suggestions for lens - 85 1.8 please :)

Ed said:

Roger, the next obvious test would be to look at the other focus sensors away from the center. After that, compare cross-type to non-cross-type sensors.

As usual, your posts are first rate.

Bret Douglas said:

Please also test the Back Button Focus accuracy vs. the Shutter button halfway down approach.

Tim said:

Thanks for the information, especially on the phase vs constrast autofocus.

Siegfried said:

I think it'd be interesting to see the DOF within which different camera+lens combinations assume themselves to be in-focus when using phase-detection AF method. I'm no way a canon boy and don't exactly know how it really looks like in Canon, but I believe that there're some AF assistance from camera when you are manually focusing (like beeping a beep or lighting up a dot in the viewfinder). Obviously, there is a range with-in which camera's PDAF engine assumes that we're in-focus. And that's what I'm curious about - that very range vs different camera+lens combinations (if you're stuck with Canon brand, then let them be 5D MkII as the baseline, MkIII to see the progress, 7D as comparison with amateur line and something pro-level to see what pro's are paying for).

You could move the target to measure that range in distance units (feet or centimeters) or per cents (range / focusing distance). Or you could twist the focusing ring and refer to distance scale and focusing ring angle play.

I believe you've got the idea.


jim thomson said:

1. My next camera will definitely have MFA.

2. Test some of the third party lenses OSM, USD. See how they perform on the different camera makers AF systems.

3. How good is the contrast detect focus on your OM-D?

Tom said:

Great article with very interesting observations.

I would be more interesting to see a comparison between something like Nikon 50mm f/1.4 AF D and Nikon 50mm f/1.4 AF-S G. Thus, they have the same f-value but different focus motor.


A said:

Thankyou for confirming my feelings/experience with actual data!

I'd be interested to see whether a 5d3 would do a better job with the same lenses - or a recent 1D-series body for that matter.

I'm also mildly curious about the lower end cameras Rebel/Kiss/nn0D too (or whatever it is Canon chooses to name them in the various parts of the planet).

I do wish the camera manufacturers would stop changing product names every few yards around the world. Seriously guys, one product, one name; it's not that difficult!

Tom said:

BTW, your data also supports the fact that lenses with bigger apertures are more difficult to get focused accurately.


Chris K said:

High precision versus low precision AF points might be an interesting test. Back when I was paying attention to such things I remember Canon spec'ed high precision AF points as "within one third of the DOF" while low precision points were "within the DOF". I don't know what their definition of CoC is... I suspect they are using the same CoC they used with their film SLRs, which isn't appropriate for a camera like the 5D2, IMHO.

I'd also love to see a Canon versus third-party EOS lens comparison. The Sigma 50/1.4, perhaps? I finally lost my faith in PDAF when I bought an EP1 that nailed focus every time, but my 5D, 40D, and 7D were all over the map. At first I thought it was my Sigma 50/1.4 and its (presumed) reverse-engineered AF algorithms, but I discovered the same lack of consistency in my Tamron 28-75/2.8 and various Canon lenses. Some more than others. Some people say Canon lenses are more accurate than third-party lenses. Is that true?

I think there are so many questions surrounding AF that people tend to bury their heads in the sand on the issue. It's very difficult to test AF accuracy well, but there are no lack of testers willing to submit (often flawed) data. We've ended up with mythology and traditions surrounding AF because of this complexity. Demystifying AF a bit is a noble cause. Thanks for all your work you do for the community, Roger!

asad said:

@Bret Douglas -- testing BBF is pointless. BBF vs. halfway pressing the shutter button is just a change in what button tells the AF system to do its thing, not in actual hardware. Why would you even think BBF is more or less accurate?

Nate said:

Macro extension tubes would probably let you use longer focal lengths. And possibly get you insanely shallow DOF, depending what combination you ended up choosing.

I don't think that this would change the overall characteristics and render the whole exercise pointless...?

Walter Freeman said:

Two things:

1) Some people have suggested that the high speed of the contrast AF on newer Micro Four Thirds cameras, particularly the EM-5, make them less accurate than the "slow deliberate" contrast AF on the Canons (or whatever). You might repeat this test with the an EM-5 and see what happens.

2) Does "AI Servo" give the same accuracy as single-shot AF on Canons? (My father has never been able to figure this out with his 7D.)

3) Olympus claims to use an "interleaved/hounds'-tooth" layout of AF-sensor pixels that yields greater accuracy in their 4/3 SLR's. In my experience, it definitely does seem to be pretty good, with less variability in some cases than Canon AF. I guess I am one of eight people in the world that still cares about legacy 4/3, though. :P

4) I notice that my macro lens (Zuiko 35/3.5, a great little lens otherwise) can have serious problems with phase-detect AF accuracy at long distances. Are macro lenses consistently worse in this regard since they have a longer range to worry about, or different gearing?

Thanks again for all your hard work!

Budi said:

Roger: it'd interesting to take the raw data and make a statistical distribution plots to analyze whether there are other influential factors or even interaction factors, that contribute to the event referred to as missed focus. If we remove all the nuissance factors (operators, etc) and all the related ones, Lenth could be used to calculate which factors are dominant (focal length, motor type, aperture, contruction, temperature, etc).

Back to fun, what tools/software would you recommend for a proper and easy to use AF Micro Adjustment?
Imatest sounds like a good tool, but not many of us could justify the cost of owning one.

Stv said:

Found out about your blog just recently. I really love it. All the details, but easy to follow. I started with the postings about the new mFT lenses, your Canon lens tests. And now this series about autofocus types is super exiting.

Volker Herrmann said:

Why is there no funktion in our cameras, that do an lens auto-calibration between the focus difference of phase and contrast AF shooting a given testchart?
I could imagine this is only missing software.

Thanks for your intersting articles!

Armis said:

I'll echo Ed's and Walter's requests: I'd be really interested to know how bad the focus accuracy drops ou outside points, and I'd be hugely interested to see if there's any statistically significant accuracy drop when using AI Servo.

I'd actually probably be ready to pay for a database that plots, say, 100 AF shots (one-shot and AI Servo, if one can dream) on each cameral-lens pair. I've been looking for a 50 (or 50-ish) with a large aperture and AF performance that doesn't suck, and after trying Canon's f/1.8 and f/1.4, let's just say I'm still in the market. And I'd really prefer not having to actually try all of them.

Brilliant post regardless, I'll be waiting for the follow-ups with baited breath.

Gary Rowe said:

Just bought/downloaded the Fo-Cal system to test out my camera (5d mk ii) & lenses - it seems to do testing akin to yours for phase-detect, and comes up with the best MA setting, all auto-magically. It was therefore really interesting to read this article as it crystallised many of my thoughts I'd had doing it ...

Everything is going OK so far (haven't calibrated the longer focal lengths yet as the weather has been so bad), but interestingly enough it was concerned over my Sigma 70mm macro and said it was too inconsistent! It was happy with my short Canon L zooms, and I'm looking forward to see what it makes of my old 50mm 1.8, which works amazingly well for copying artwork as it's so sharp out to the edges (using MF & liveview x10).

Would be very interested to hear your views on the Fo-Cal system,

Thanks again,


intrnst said:

'm joining the chorus, is there any (easy) way to test predictive PDAF?
If so, how better is the efficiency against a very fast CDAF system, bearing in mind this Subject Factory... Quality... thingy.

Just keep writing.

Max said:

It would be very interesting to see how the same camera/lens behave at short vs. long distance. Example: trying to shot a target ad 10 foot, and then the same target at 100 foot.

And then check what kind of difference autofocus microadjustment can make.
If I microadjust focus for a lens using a 10 foot target, what happen when I shot the same target ad 100 foot? does the lens, microadjusted at 10 foot, retain its accuracy even at 100 foot?
And what happen if I microadjust a lens at 100foot, and then shot a target at 10 foot?

And what happen when I microadjust a zoom lens? Is better to microadjust zoom lens at their widest position? or ah their tele position?

And what happen if I'm using a real macro lens (like the sigma 180 macro)?
Does the focus microadjustment retain it's precision even when I'm using the lens for shotting closeup/macro subject? (i know that real super-macro lenses (M-PE 65) dosen't have focus ring... but I'm not going to setup a tripod and a camera rail just to shot an apple :-) )
And what happen when I'm using a near-macro zoom lens? (like the sigma 17-70mm)

Thanks for writing these kind of article, I find them very-very-very interesting :-)

LensRentals Employee

Roger Cicala said:

Hi Gary,

I like Fo-Cal very much, although I find it better if I actually am looking at what it does, rather than just putting it on full autopilot.

One thing that's important to remember, though: it's going to set your lens up for perfection at one focusing distance (and one focal length if it's a zoom). But the results will vary a bit at other focal lengths or focusing distances. The bottom line is it's best to adjust to the distance and focal length you tend to shoot at most.


LensRentals Employee

Roger Cicala said:


I like what you have in mind. I'm hoping we'll end up with a bit of a database that shows us how accurate different lenses, cameras, etc. are. I may be off the mark, the 'tool' we're using may not be accurate enough. Or perhaps there isn't a difference to detect. But I want to know that, too.

I think FoCal is a really good choice: it's not perfect but it's pretty accurate and very easy to use.


Brian Leighty said:

Volker Herrmann, I like your suggestion of auto alignment considering Roger's article shows that contrast focus is pretty close to the best you can get.

In regards to suggestions, I'm surprised nobody's brought up the Canon 5D mark ii. I see so many forum postings about it having awful focusing. I think the test I'm thinking is regular light vs low light accuracy, comparing it to something with a comparable middle focus point like the 50D I believe. Also people talk about the the assist focus points and I'd be curious to know if those actually do anything to help or hurt. I guess one other possibility would be testing if it's mostly the camera that affects the focus accuracy, the lens or a combination of the two. That's all I got.

Budi said:

Roger: as long as the tools being used are calibrated/consistent, they don't have to be 100% accurate. That would allow correct statistical analysis. Since you test every lens you have, it'd natural to know what you could get from that huge database. :)
I'm no expert, but I know a little about experiment design and reliability life calculation, email me if you want more info - gratis! :)

Arun said:

A while back the FoCal proprietor had tweeted that the 5d3 had one of the most consistent AF his software had ever encountered. So, if you could repeat the tests you did with the 5d2 with the 5d3, I'd much appreciate it.

Esa Tuunanen said:

Max, at least Olympus offered possibility for separate focus adjustment value for zoom's widest and narrowest focal length with camera interpolating values for in between focal lengths.
That feature must have been too advanced for other makers if they don't have similar...

Integration of PDAF into sensor would solve at least inaccuracies coming from SLR PDAF measuring focus in elsewhere than actual focal plane and beyond rather complex optical path with lots of parts needing very precise positioning to have any changes for correct focus.

CDAF's accuracy comes from measuring focus from real focal plane/imaging medium.
But unlike PDAF which has from the first sample good idea how much and to which direction focus must be moved CDAF can only see that image is either probably blurred or maybe not blurred (even with spot on focus it can't know it from first sample) which slows things down because it needs lens to move focus in tiny steps while sampling focus constantly to know if things are going for better or worse untill highest contrast is found.
Faster sampling rate (more images per second from sensor) allows giving focus commands at faster pace if lens can handle it.
Also from enough consequent samples more complex focus algorithms can estimate how much further focus can be moved directly to get close to correct focus speeding process by skipping over number of smaller focus movement steps and also lessens focus overshoot.
That's how m4/3 cameras have increased their focus speed.

But of course all that speeding of CDAF focus process by estimations flies out of window if target keeps moving.
Which is why Nikon 1 is only mirrorless which can properly track moving target because its phase detection can tell instantly from next focus sample at what rate and to which direction target is moving.

Applets here show very well operating principles of imaging sensor CDAF and SLR PDAF.

Samuel Hurtado said:

* Longer focal lengths don't alter DoF in a significant way. They affect perspective, so the out-of-focus background seems more out-of-focus, but with what you're doing here there should be no extra issues (you're shooting a flat surface, there's no background).

* I'd like to see if in-plane phase detection helps (it doesn't require AFMA, and may be constrast-detection-enhanced) or is a problem (having to be buried between the photosites may make some tradeoffs arise).

Lester said:

You say that Canon's live view PDAF is as good as it gets, but I'd like to join the (small) chorus that asks for some runs using an Olympus (or Panasonic) mFT system -- they've bet the farm on PDAF, and it would be very interesting to see whether your tests might show a difference... Great blog!

Lester said:

Argh, what were my fingers thinking, that should read, "bet the farm on CDAF"...!

lexvo said:

Great article!
I think it would be great do to the same test with the Canon 5DmarkIII to show how much AF has improved.

LensRentals Employee

Roger Cicala said:

I'm definitely going to test the Oly and other cameras. I'm afraid that my test tool here is too blunt to tell any difference. I didn't mean Canon is as good as it gets, I meant it's as good as my test can detect.

photodummy said:

How do you get the same camera to use contrast detect vs. phase detect? Is it as simple as the camera uses contrast detection when in "one shot" focus, and uses phase detect when in "AI servo"?

LensRentals Employee

Roger Cicala said:

5D bodies use contrast detection for Live View autofocus, Phase Detection for standard AF.

Wayne S. said:

Thanks Roger for doing all this testing and posting it.
I didn't see you mention what apertures you were shooting at but I assume at or near wide open.
I ask because as you know fast lenses 1.2 or 1.4 usually suffer from some amount of focus shift which will cause the normal PDAF focusing to be much more inaccurate vs. the stopped down LV mode focusing.
Also, I wish Canon and Nikon would allow more adjustment variables which can be tweaked liked the Conurus converted Contax N lenses. Not only is there a base micro-adjust value for a lens that can be specified but also parameters which affect the focusing value as function of distance from subject. These adjustments allowed me to get accurate near and far focus with my Contax N 50/1.4 lens.
As Thom Hogan mentions in his blog, these AF databases will be interesting between Canon and Nikon and will hopefully add more pressure for these manufacturers to improve their AF accuracy. Most of the time I use MF Zeiss lenses and mostly use LV to get accurate focus where I want it.

LensRentals Employee

Roger Cicala said:

hi Wayne,

It was all at completely wide open, hoping to avoid focus shift if possible. I agree with Thom (because Thom's almost always right) - I'm a cynical person and wonder, since the manufacturer's don't seem to want us to know about this, if knowing people are looking may make them a bit more willing to discuss it.


Kelvin said:

Was RogerView manual focusing done using the focusing screen or the TFT screen? Just wondering whether careful manual focusing with the pentaprism focusing screen can be trusted as well.

LensRentals Employee

Roger Cicala said:


It was live view manual focusing on a star chart - just because that's where I'm most accurate and I wanted a good baseline. I'm not sure how accurate pentaprism would be in comparison: in theory it should be as good, but if the focusing assembly is shimmed improperly it can front focus or back focus.


Kelvin said:

Thank you Roger for the feedback :) I have recently been looking at a pile of my old 35mm negatives from last century and scanning them into the computer at around 20 megapixel equivalent. In hindsight I am amazed at how many of my old shots haven't achieved critical focus. Most of them were taken with a Nikon F4 or FM2.

In comparison, a lot more negs and slides taken with a rangefinder (Leica etc) had pin sharp focus.

My critical focus hit rate on the negs / slides taken with a Hasselblad 500-series was abysmal.

I am suspecting that SLR focusing has always been a compromise thanks to mirror box, mirror, and focusing screen alignment tolerances, but that they haven't really come to prominence until high resolution digital sensors have come to the fore.

Joaquín said:

Hi Roger,
I've just come back from a trip in Vietnam and Camboya. I've shot more that 5000 frames in 3 weeks. I've used a 5DMKII.

30% of my shoots are out of focus. A third of them (10%) are due to my wrong using the camera ( after checking exif data) but still 20% of my shoots are simply out of focus, even using the new 70-200 f/2.8L IS II

Trying to find an explanation, this site crossed my path. Roger, this is the clearest and most interenting reading I've found by far. Thanks a lot

Steve Runyan said:

coincidently, i've been trying to find a better Canon 50 1.4 than my old one which has been improved by 2 visits to Canon service but is still disappointing wide open. i've tested 2 additional copies using Imatest. One appeared to be slightly better both with micro-adjusted pdaf and cdaf, one is much better with cdaf than pdaf, and the old lens is MUCH better with pdaf than cdaf. one of the tested lenses has focus shift in the opposite direction of the old lens.

am i wasting my time trying to find a copy of this lens that will autofocus wide open with decent resolution (all of the lenses clean up nicely between f2.8 and f4)- i'm looking for decent (if not great) resolution off center for subject isolation and autofocus wide open for street shooting in low light.

Paul said:

Very nice post. Thanks.

Now i have issues with my camera and AF all lenses behave differently. My short question is: if say Nikon 50 1.4 works perfectly on all distances and all focus points in LV, but has many AF issues in PDAF on the same camera as other lenses works well. Can I conclude that it's bad lens or problems with camera?

Thank you.

LensRentals Employee

Roger Cicala said:


I don't think you can make any conclusion just by that. PDAF isn't going to be as accurate as contrast detection. But if you have a problem with consistent front or back focus then a microadjustment is in order.


Carl said:

Roger, another great series of articles! (I haven't read parts 2 and 3 yet).

I think my camera has issues with the shim spacing (or whatever) of the focusing screen, because I can almost never focus accurately via the viewfinder (no matter what lens I use...telephoto, wide angle, fast aperture, etc...and I do have the finder's focus adjusted for my eye). It's possible this can only be so accurate, at least with lesser-than-pro series "crop" bodies, but I can only guess. Mine does have the "pentaprism" and not "pentamirror".

On an unrelated front, have you thought of doing noise test comparisons, like what they did here? http://www.techradar.com/reviews/cameras-and-camcorders/cameras/digital-slrs-hybrids/canon-eos-1dx-1091200/review/page:4#articleContent

I find this particular test even more interesting than what DXOmark has done so far. Especially surprising is that the 5D3 they tested, outperformed all others in a narrow range from ~ISO 25k to 50k...besting even the D4 and 1DX. I'm sure there are logical explanations and counter arguments, but still it's nice to look at a graphed comparison of noise and dynamic range directly comparing the latest and "best" DSLR's.

If you've already done a similar test, I apologize for not seeing it. I'm less familiar with your new website's layout.

Rick N said:

Excellent. You have confirmed my observations with clear concise data.
Someone needs to start a business verifying the performance characteristics of each lens copy to a measurable standard that would instil confidence in the product. A few extra hundred for a certified lens would be worth it. This service could also provide an independent evaluation of a lens when repair by the manufacturers service centre fails to resolve a perceived issue.

G.cavitori said:

From my 3 years test i have got the same identical results of the dear Roger, author of this realistic article. Simply the phase detect is something that should be strictly used ONLY in particular situation where speed is more important of accuracy. Otherwise, for still, landscape or slow scene it is simply a shit.
3 travels i come back home angry due to lackness of tack sharp focus on lot of very stupid day shots.....with live view never had problems
And all this happened in different bodies with different lenses....
I sent back for calibration but no luck, everytime if you take the same identical still shot 10 times, you can bet 10000000 dollars 3 or 4 over 10 pictures will be badly focus
Good bye phase detect, end of the story. I revert back to good old days of compact cameras contrast detect system.

Frank Kolwicz said:

I've just been through hell getting Canon to fix the soft focus problem on my 600/4L IS (I) and while it was giving me soft focus due to a misaligned element I noticed that phase detect AF was very erratic, sometimes way in front or behind the subject, even ones well isolated from potential competition for focus. Contrast detection in LiveView did give the best possible focus, as poor as that was, under the same conditions where phase detect failed.

It seems possible to me that, when the AF systems can't get very sharp focus, they just guess as best they can and that's not very good. So, very sharp lenses may give much better AF than somewhat softer ones.

Andy said:

How do the newer Dual Pixel autofocus sensors compare to these methods? I would love to see an update!

Dirk S. said:

I just want to give my Cents to the discussion.
One problem is the focus shift.
Normally the AF sensors see only an aperture of 4.0 or 5,6. If the plane of best focus differs between 4.0 and 1.4, for say an 1.4 lens, the AF phase sensor never can detect the right focus, even if everything is perfectly mechanically adjusted. The AF sensor, limited to see f/5.6, never can determine the focus at 1.4!!!


Comment from Carl Zeiss on Focus Shift:
"Fast lenses of this optical design (without floating elements) shift the focus due to spherical aberration when the f-stop is changed. This phenomenon is especially visible on closer object distances and cannot be influenced. The AF system of most camera models does not respect those characteristics of a lens. The focus is measured and confirmed by the focus indicator as if the lens has been stopped down to f/5.6.
While using the lens wide open, the correct focus lies usually a little bit in front of the point that has been expected. Beside this, the focus confirmation function of any AF-camera is a relative improper tool for accurate focusing of an f/1.4 lens on closer distances. Also, a standard focusing screen and viewfinder of an SLR camera shows the limitations of accurate manual focusing with an 85mm lens at f/1.4. For instance, it is not possible in practical use to focus more accurately than about +/- 2 cm at an object distance of 1m without additional magnification systems. For accurate focusing at full aperture or stopped down a little bit, we strongly recommend:
-use a tripod if possible
-use zoom-in function in live-view mode while the lens has been stopped down to the designated f-stop.
-focus bracketing exposures (with small steps of rotating the focusing ring)"

So if I have lens with this problem, the contrast AF will work, but the phase AF will have problems with larger apertures. This can only be corrected by correction factors in the camera and/or lens firmware.



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