Comments on: The Effect of a Decentered Lens on Autofocus http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus Photo/video thoughts from the largest rental house Tue, 18 Jun 2013 13:47:01 +0000 hourly 1 http://wordpress.org/?v=3.3.2 By: Roger Cicala http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17552 Roger Cicala Tue, 11 Sep 2012 12:16:48 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17552 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 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

]]> By: Joachim http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17550 Joachim Tue, 11 Sep 2012 10:50:59 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17550 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. 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.

]]> By: Francois http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17542 Francois Tue, 11 Sep 2012 02:40:03 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17542 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 ? 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 ?

]]> By: Roger Cicala http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17538 Roger Cicala Mon, 10 Sep 2012 23:16:19 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17538 Thank you, Rory. Corrected that one. Thank you, Rory. Corrected that one.

]]> By: Roger Cicala http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17537 Roger Cicala Mon, 10 Sep 2012 21:07:35 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17537 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. . . . 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. . . .

]]> By: Joachim http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17534 Joachim Mon, 10 Sep 2012 18:31:35 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17534 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. 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.

]]> By: Rory http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17532 Rory Mon, 10 Sep 2012 17:22:39 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17532 Interesting findings Roger. You might have a typo - is it not run 2, rather than 3, that is showing the lower average resolution? Interesting findings Roger. You might have a typo – is it not run 2, rather than 3, that is showing the lower average resolution?

]]> By: grubernd http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17527 grubernd Mon, 10 Sep 2012 12:12:49 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17527 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. ;) 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. ;)

]]> By: Roger Cicala http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17526 Roger Cicala Mon, 10 Sep 2012 10:42:14 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17526 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 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

]]> By: Roger Cicala http://www.lensrentals.com/blog/2012/09/the-effect-of-a-decentered-lens-on-autofocus/comment-page-1#comment-17525 Roger Cicala Mon, 10 Sep 2012 10:40:30 +0000 http://www.lensrentals.com/blog/?p=9178#comment-17525 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 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

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