Interesting results from Left AF test

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So I finally got round to do some left side AF testing on my D800... My warranty is running out next month, so I figured I had to check...

I don't have many fast wide lenses, though. 24-70, 50/1.8 G, 35/1.8 DX - that's it. The results:

  • 24-70 - Tested at 24mm f/2.8 and 35mm f/2.8. No problems at all - left side and right side tack sharp, no obvious difference between live view and phase detect - nice!
  • 50-1.8 G - also no inconsistencies. A little soft wide open and at f/2, tack sharp at f/2.8 - but same results with live view and phase detect on both sides.
  • 35/1.8 DX - This one gave me problems… Center and right side sharp, even wide open, no difference between phase detect and live view… But the left side phase detect AF is off, live view is sharper! Tried many times, results seem consistent...
  • For good measure, I also tested my 105 micro - no issues at all.

So what gives??? I'm a bit stumped by the inconsistent results, and that only one lens gave me trouble! Also, the 24-70 at 35mm 2.8 is dead on where the 35/1.8 at 2.8 is off on the left side. Hmmm...

I have to say my test setup is not scientific - resolution chart on the wall, pretty close distances - so tiny errors in alignment can make a difference here. The trouble is, I don't have a better way to test...

I was going to sell my 35/1.8 anyway since i'm getting rid of all my DX gear. If I had already sold it I would have not seen this inconsistency and would be happy now :smile:

So what do you think - is it the lens or the camera???

Cheers

Mike
 
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Sounds like a decentered element in your 35 f1.8. With multiple lenses showing no focus errors I think you can feel confident your D800 is OK.
 
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Sounds like a decentered element in your 35 f1.8. With multiple lenses showing no focus errors I think you can feel confident your D800 is OK.

Thanks Luke - I was hoping for that answer :smile:

But if the 35/1.8 is decentered, why would it achieve perfect focus with live view on the left side...? Just looking for a possible explanation...

Thanks

Mike
 
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I’ve had my D800 for only a couple of days. Bought it used, and it had just come back from Nikon Service. Left focus issues fixed. I did some tests yesterday, and it looks tack sharp everywhere except on the right. I only used one lens. My old 28-70 f/2.8. Sounds like it’s a good idea to try this with another lens. I have been wondering about the 28-70 as I’ve used it hard for years, and I was thinking it was going soft. Now, it sounds like I need to redo this with another lens.

However, in my case anyway, this is something that would probably not show up in normal use, but still going to run another test and follow this thread.
 
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In my case I saw the left AF problem only at 35mm or wider and at wide apertures. My guess is that you have the problem (which is why you see it on your 35) but f/2.8 is masking the issue on your 24-70. I bet if you tried a 24 1.4 you would see it clearly.
 
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In my case I saw the left AF problem only at 35mm or wider and at wide apertures. My guess is that you have the problem (which is why you see it on your 35) but f/2.8 is masking the issue on your 24-70. I bet if you tried a 24 1.4 you would see it clearly.

Hmm - but I don't have a 24/1.4 (and don't plan to buy one)...

The thing is - at 35mm 2.8, the 24-70 is ok on the left side and the 35/1.8 is not! Shouldn't they both be off if the D800 had the problem?

Thanks

Mike
 
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Another factor is that since the 35 f1.8 is a DX lens the images it produces are cropped from the center of the D800's sensor. If the 24-70 @35mm is sharp to the edges and the cropped image from the 35 f1.8 is soft on the left side - it sure sound like something other than a autofocus issue.

You might try testing the 35 using the center focus point and see what the left side of the image looks like compared to the right. Make sure the target is perpendicular to the lens axis.
 
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Another factor is that since the 35 f1.8 is a DX lens the images it produces are cropped from the center of the D800's sensor. If the 24-70 @35mm is sharp to the edges and the cropped image from the 35 f1.8 is soft on the left side - it sure sound like something other than a autofocus issue.

You might try testing the 35 using the center focus point and see what the left side of the image looks like compared to the right. Make sure the target is perpendicular to the lens axis.

Luke,

I don't think the fact that the 35/1.8 is a DX lens makes a difference here - the AF sensors are in the same location either way, right? I'm just comparing the area under the left side AF sensors on the 24-70 and 35, and that is the same part of the frame (in fact, I set the camera to DX crop for all my tests since the outer area is of no significance here).

Another thing I tried - using AF fine tune to calibrate the 35/1.8 on the left side. I observed that with a setting of -15 the left side AF sensors would give me sharp images and approximately the same result as live view AF... Of course, with that setting the center & right side AF sensors were now off...

So it's possible that my D800 has the dreaded left side problem - or there is an issue with that particular lens.

I think I'll do some more testing tonight with other lenses that are important to me - the 70-200/2.8 VR II and 300/2.8 VR, for example. I'm not expecting problems with these because they're not wide angle lenses, but I want to make sure.

Assuming all my other lenses don't exhibit the problem, it seems my options are:

  1. Be happy with the D800 as is - the only lens affected is the 35/1.8 and I was going to sell it anyway :smile:
  2. Try to obtain a 24/1.4 from somewhere - probably would have to rent it - and see if the problem reappears. The downside of this is that I only have 3 weeks of warranty left and I would probably run out of time.
  3. Send in the D800 body only - it would likely come back as "within tolerances" since they typically test with the 24-70 and 50/1.8 G, from what I heard - and those are ok on my D800!
  4. Send in the D800 along with the 35/1.8 and hope that Nikon fixes the issue - this could be risky because they may actually mess up the AF on my other lenses - or it would require multiple round trips to get it all fixed...

Right now, I'm leaning towards option 1...

Cheers

Mike
 
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I just did another test with my 105 VR as that is probably my least used lens, and most likely in the best condition. It was sharp all across the sensors. The 28-70 might have a very slight issue probably not seen during normal use. Since the camera just came back from Nikon Service, I’m calling it good. Till I saw this thread, I didn’t realize I could get different results from different lenses. I feel better now. Sorry for the thread hijack.
 
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For what it’s worth, that would be my option as it seems like it is the lens and not the body. Sell the lens and enjoy the D800.

Sounds like a good plan, Gary :smile: And glad to hear your D800 looks ok now!

I also just re-read Thom Hogan's article on the Left AF issue. According to him, the left AF issue should not be correctible via AF Fine Tune:

Note: Some people report that an AF Fine Tune corrects the problem. No, it doesn't. This is a tricky area, and one reason why I suggested avoiding AF Fine Tune in the first place. It is not uncommon for a "good" camera and lens to have somewhat different AF Fine Tune values for left, center, and right sensors. A -10, 0, -5 wouldn't be surprising to me, and doesn't indicate the problem we're looking for. Generally, a misprogrammed left sensor response can't be brought back by AF Fine Tune. In other words, we're not looking for a modest difference in focus performance; we're looking for a gross and obvious difference in focus performance.

But I was able to get sharp left side AF on my 35/1.8 with AF Fine Tune set to -15... So that seems to indicate my D800 is ok! :biggrin:

Cheers

Mike
 
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The thing is - at 35mm 2.8, the 24-70 is ok on the left side and the 35/1.8 is not! Shouldn't they both be off if the D800 had the problem?

Perhaps not. I think at least part of the issue is that the D800 left AF sensor has trouble to perform focus at f/1.4 where the DOF is shallow. With your 24-70, AF is performed at f/2.8, while your 35 performs AF at f/1.8. I also mainly saw the issue with wide angle f/1.4 lenses.


But I was able to get sharp left side AF on my 35/1.8 with AF Fine Tune set to -15... So that seems to indicate my D800 is ok! :biggrin:

I'm afraid not. I tried several D800 with the left AF issue (large back focus on the left). On each one, the left point could be made to focus perfectly with AF fine tune. The problem is, that while correcting the back focus for the left with fine tune, the center and right would then front focus. I guess this is what Thom meant when he said that the problem cannot be fixed just by AF fine tune.

So your D800 sample may not be perfect, which one is, but I agree I would go with option 1.
 
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Perhaps not. I think at least part of the issue is that the D800 left AF sensor has trouble to perform focus at f/1.4 where the DOF is shallow. With your 24-70, AF is performed at f/2.8, while your 35 performs AF at f/1.8. I also mainly saw the issue with wide angle f/1.4 lenses.

That's definitely a plausible explanation, Thorsten - thanks! I was forgetting that the AF is performed with a wide open aperture, even if the lens is stopped down when the shot is taken.

Very intriguing... I wish I had another fast wide prime that I could test with. I'm almost inclined to rent a 28/1.8 G just to find out, but that seems a little masochistic to me :smile:

I tried several D800 with the left AF issue (large back focus on the left). On each one, the left point could be made to focus perfectly with AF fine tune. The problem is, that while correcting the back focus for the left with fine tune, the center and right would then front focus. I guess this is what Thom meant when he said that the problem cannot be fixed just by AF fine tune.

Hmm. Thom's statement is a little vague, but how about this sentence:

(Thom Hogan) It is not uncommon for a "good" camera and lens to have somewhat different AF Fine Tune values for left, center, and right sensors. A -10, 0, -5 wouldn't be surprising to me, and doesn't indicate the problem we're looking for.

That seems to be the case here - I can "correct" the issue by -15, 0, 0 for my 35/1.8. Thom seems to imply that no amount of AF fine tune would bring the left AF sensor into alignment on bodies that exhibit the issue, right?

So your D800 sample may not be perfect, which one is, but I agree I would go with option 1.

Glad you see it that way, too. I've read another set of horror stories over on dpreview where folks have sent their D800 in and received it back with the center and right side out of alignment, or other newly introduced focus issues :frown:

It really bugs me that Nikon USA can't seem to deal with this problem consistently, even after such a long time!!! :mad:

Cheers

Mike
 
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Hmm. Thom's statement is a little vague, but how about this sentence:

That seems to be the case here - I can "correct" the issue by -15, 0, 0 for my 35/1.8. Thom seems to imply that no amount of AF fine tune would bring the left AF sensor into alignment on bodies that exhibit the issue, right?

I think there may be different degrees of the left AF issue. Last year when I bought my D800, I went thru 3 bodies which were very bad, with the left point back focusing several feet behind the subject. The one I ended up with I considered relatively "good", but when I tested it thoroughly, it does have a left AF issue (just relatively mild). Yours might also have a small issue.



I've read another set of horror stories over on dpreview where folks have sent their D800 in and received it back with the center and right side out of alignment

That's what they did to me, too. I sent mine in last month, thinking to get that small issue fixed before the warranty is up, but all they did was introduce overall front focus. That's why I recommended option 1 for you.
 
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I think there may be different degrees of the left AF issue. Last year when I bought my D800, I went thru 3 bodies which were very bad, with the left point back focusing several feet behind the subject. The one I ended up with I considered relatively "good", but when I tested it thoroughly, it does have a left AF issue (just relatively mild). Yours might also have a small issue.

That's what they did to me, too. I sent mine in last month, thinking to get that small issue fixed before the warranty is up, but all they did was introduce overall front focus. That's why I recommended option 1 for you.

Sorry to hear about your ordeal, Thorsten :frown:

I'm definitely going with option 1 after hearing this! Don't want to risk it with Nikon.

But it totally sucks that they still don't have a handle on this issue :frown:

Thanks

Mike
 

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