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The Effect of a Decentered Lens on Autofocus

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As some of you know, I've been testing a large number of Nikon cameras and lenses as part of the Autofocus Reality series. (Nikon full frame results will be up in a day or so.) I noticed an abnormality in one copy of a lens we were testing that some of you might find interesting.

We were using 28mm f/1.8 lenses (that had all previously passed resolution tests on our D3x test bodies) on D800 bodies. Each camera and lens combination was shot 10 times using Live View (contrast detection) autofocus, 10 times using standard phase detection autofocus. On the graph below, each color represents one run with one copy of the 28 f/1.8 mounted to one copy of the D800. Contrast detection are the diamonds, phase the crosses. If you don't understand what any of this is, there's a blurb about the units below, you can read the Autofocus Reality series, or just muddle along.

On the graph below, Runs 1 and 3 show about what we've come to expect. Contrast detection has a very tight pattern. Phase detection has a slightly looser pattern that's not quite as accurate, although still acceptable. Run 2, though, shows slightly lower resolution and very loose patterns for both phase and contrast detection. After some careful further testing we found that the lens used in this run was just a bit decentered.

Why did it show up now, and not before?

Several reasons. First, you can see that center resolution (the horizontal axis) is just as good as the other lenses. The average resolution is lower because the edges and corners are weaker. Unless a shooter was carefully evaluating the corners of his images it wouldn't be noticeable. In fact, even when we knew it was a problem we had to compare it to other copies to tell it was 'not as good'. If you just looked at an image from the lens, you would just think the corners were a bit soft.

Second, the higher resolution of the D800 exposes the weakness a bit more. It would still be a bit weaker lens in the corners of a D3X or D700 image, but less noticeably so.  Please don't start a D800 resolution-war argument - we all realize every lens resolves better on a D800 than on a D700, OK? That doesn't mean every lens wrings out all the resolution a D800 has to offer.

The final reason is testing methods. Like all lens testers and reviewers using Imatest or DxO our testing method involves careful manual focus, focus bracketing several shots on either side of that focus point, then taking the best shot. We have never tested "how accurately does the lens autofocus?" If I just checked the best image this lens made, it comes in at the lower end of passing numbers. If we hadn't happened to be testing multiple shots for AF accuracy we wouldn't have noticed this.

 So what did we learn today?

We learned a couple of things today. The most important part is that a decentered lens not only doesn't resolve as well as a good lens, it focuses less accurately. So, the best shot possible with the decentered lens has less resolution than the good lenses, but autofocus is less likely to get you the best shot possible with that decentered lens.

It seems logical when I think about it: autofocus, whether phase detection or contrast detection, requires pixels on a sensor evaluating an image. If the lens has some spherical aberration, decreased contrast, or (especially for phase detection) difference in side-to-side light paths, focus accuracy could well be affected.

The second thing is that even some lenses that seemed OK on a D3x (our standard test camera) may have flaws exposed on the higher resolution of a D800. This particular lens passed our standard resolution tests on a D3x, although it was at the lower end of 'pass'.

Finally, we'll be able to use this finding to improve our testing. Currently we do the same thing most lens testers do: Carefully manually focus, bracket on either side of that point, and use the best result for analysis. At least for borderline lenses, we'll start testing repeated autofocus accuracy to hopefully pick up these 'almost, but not quite OK' lenses more effectively.

Speculation

Every day I hear from one photographer or another that one of their lenses doesn't autofocus accurately on their camera, but their other lenses seem to. It's usually the usual: Poor technique or simply not microfocus adjusting the camera to the lens.

I wonder, though, if sometimes they're experiencing what we've seen here: A lens that seems reasonably sharp but has just enough decentering to interfere with AF. This particular lens had been used several times by experienced photographers and seemed acceptable to them. In the center (horizontal axis on the above graph) it's just as sharp as the other lenses. When evaluating it closely, it's just a bit soft in all 4 corners, and a bit softer on the right side.  But if I didn't have other copies to compare it to, I'd simply think "OK, it's a bit soft in the corners, and I probably wasn't lined up perfectly on the test target."

For us, and people like us, this will hopefully lead us to testing for autofocus accuracy in lenses just like we test for resolution. For regular photographers, if one of your lenses seems to miss focus a lot, it may be worth some careful evaluation to make certain there's no tilt or decentering present.

Roger Cicala

Lensrentals.com

September, 2012

Graph Explanation: These are the Imatest points for 3 runs of Nikon D800 with 28mm f/1.8 lenses, shot at f/2.8.  The vertical axis is average resolution at 13 points on the sensor (corners, edges, 4 midpoints, center) while the horizontal axis is resolution at the center point (all in line pairs / image height from unsharpened raw files).

13 Responses to “The Effect of a Decentered Lens on Autofocus”

Dirk said:

I have a 70-200VR that has AF problems - accurate and sharp in the center, but pretty significant backfocus when using an AF point on the sides, especially right side. The camera AF sensor alignment has been checked and adjusted, and the problem is isolated to this one lens. Is this something that could be caused by a decentered lens?

Thomas Alicoate said:

Roger,
Great, another thing to think about. I have a lens I wouldn't mind testing for this. Any idea how someone would best test for this at home?
Thanks for all of the info. Best on the web.

Tom

JN said:

"Every day I hear from one photographer or another that one of their lenses doesn’t autofocus accurately on their camera, but their other lenses seem to. "

Perhaps the above comment might also apply where the lens/body mount alignment may vary, between cameras?

LensRentals Employee

Roger Cicala said:

Dirk,

It's possible, but since it backfocuses consistently I'd think first of the AF motor or EPROM programming in the lens. Assuming AF microadjustment doesn't help, it sounds like a trip to Nikon Service is in order either way.

Roger

LensRentals Employee

Roger Cicala said:

Thomas,

I think the simplest test is using a Seimen's Star chart ($29 from B&H) for evaluation. If you have a friend with a copy of the lens and can compare corners and sides, that would certainly do it, too.

Roger

grubernd said:

stuff i have been trying to explain to people for years: even if you stop down your lenses all the time your AF will always use the lens wide open. hence if the lens wide open is soft as a pillow your AF speed and accuracy will suffer a lot. i have seen this effect with a lot of lenses, putting a great glass on a cheap camera will yield better pictures more often than a bad glass on the best camera.
good to see the scientific side matches the hands-on experience. ;)

Rory said:

Interesting findings Roger. You might have a typo - is it not run 2, rather than 3, that is showing the lower average resolution?

Joachim said:

And you checked as well, Roger, that your D800 has not as well the left focus issue? Mine has. Still, Nikon Switzerland is not able to provide a solution, but I checked it with 3 lenses at 1.4 and all pictures were worse on the left side.

Also, the 50/1.4G turned out to be tricky: at a distance of 1m it had backfocus. Correcting that by microcorrection, the infinite focus blurred - which was okay without microcorrection. Truly a case for service, too. My conclusion is, the D800 shows all weaknesses of lenses pretty obvioulsy. And also it's own. It must be hard to adjust the quality department on such a demanding cam.

LensRentals Employee

Roger Cicala said:

Joachim,

I very purposefully stayed away from anything other than center point in AF-S mode. I haven't seen any issues with center point AF on the D800, although until we find out what's causing the left side issue I can't say for certain it doesn't involve the center too.

The thing you describe with your 50mm is just reality: You can adjust at a distance for best focus, but not for every distance, with MF adjustment. Same with all the zooms: pick your used focal length on the zoom, or adjust it in the center, but you'll rarely get both ends the same.

If we ever find out for certain what the cause is. . . .

LensRentals Employee

Roger Cicala said:

Thank you, Rory. Corrected that one.

Francois said:

Nice to hear that. On my D800, my
- AFS 35 1.4
- AFD 105 2.8
focus accuratly on all the sensors. But my
- AFD 50 1.8
is way off with the left sensor on the D800. I was afraid it could be the "left sensor" issue, but I have the feeling that my lens has a problem.

Anyway, could you explain us how a lens can not focus accuratly ? I understand why a body could have some problems but I don't get why a lens can cause a problem. For instance, I imagine that a lens can't cause any problem to a contrast detect focus (live-view). So why a phase detect focus can have problem with some lenses and not with others ?

Joachim said:

It is difficult to tell which variable brings the issue to life:
Decentered lens?
Sensor not parallel to lens connection bayonet?
AF unit not parallel to sensor and / or lens connection?
Or a bit of each?
Since reading your blog I became more aware how many things can go wrong with camera and lens manufacturing. And how much a good combination must be appreciated. Meanwhile I count the sensitivity for tiny tolerances to one of the downsides of the D800. Even if one gets a "perfect" cam right out of the box, how long will it keep that perfection in normal or hard use? But still, each manufacturer would face the same difficulties once they come to this performance regions.

Thanks again for your profound, clear and not at least entertaining articles. I don't want to miss them.

LensRentals Employee

Roger Cicala said:

Francois,

If you consider that phase AF is comparing light paths from both sides of the lens, a decentered or tilted lens could interfere with how those light paths strike their respective sensor. That might end up confusing the AF sensors, giving conflicting information.

I'm not sure, other than simply reduced contrast or aberrations why it would effect contrast detection AF.

Roger

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