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Nikon 12-24mm AF back focus?
#1
I just took the plunge and bought a second hand 12-24mm on eBay... seemed like too good an oppportunity as it wasn't up for auction but 'Buy it now' at £550 instead of £750-800 new... only way I was going to be able to afford this lens.

I thought I'd take the risk, check it very quickly for decentering and resolution and send it back straight away if any problems.



Lens arrived on Tuesday and I've been at work all week so no opportunity to get out with the lens, but I have tested for decentering (using your sanity check, Klaus - thanks again! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />) and it seems fine in that respect - I couldn't see any issues at all. Also looks good as new - not a mark on it.



A B&W F-Pro 010 filter was also incuded in the price... maybe I should have been more suspicious.. or on the other hand, maybe I'm being over-anxious about it now!



I think the lens is auto-focusing behind the intended focus point (aka "back-focusing"?)

I've spent several hours today trying to suss out whether this is really the case, and how much AF micro-adjust to apply.

I've now read several articles, and have focus tool by Tim Jackson, came across idea of lining up batteries at a 45 degree angle and making small changes to the MF adjust, looked at LensAlign (seems v. expensive) and have taken numerous pics, some in poor light... and am really no nearer to getting this sorted. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='Huh' />



Pics [url="http://iancds.smugmug.com/Tests/TEST-Nikon-12-24/19671189_b9tn3J#1543100152_JKXnWdS"]here[/url]. Camera is Nikon D7000, focal length used = 12mm @ f4. Clicking on the images should bring up option to view them at original size ('O').

In all cases, I used single-servo AF, with one focus point at the centre of the image. I appreciate some of these are much less useful than others for looking at this issue... some pointers to selected focus points:
  • Cat's eye was selected point of focus (not the dried blades of grass & ivy behind)
  • View looking up at the house: focus point is centre (in line with window centres), 3rd course of stones above patio doors. Focus seems to me to improve higher up the stone wall (again, farther away)
  • Teasel: front of cone was focus point
  • Dried out flowers in flower bed: front of foremost seedhead
I'd be very grateful for

  1. suggestions re the best way to find optimum AF micro-adjust setting - or if the lens focus might be too far out of spec so that it needs Nikon to adjust (and I say bye bye to the £ saving..!)
  2. whether getting micro-adjust right at 12mm will work through zoom range - if not, is it possible to save different AF m-a settings to use at different focal lengths on the same lens - I don't think it is?
  3. whether the overall image quality is in line with what I should expect from this lens... or should I send it back fast on Monday..?! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='Sad' />
Ian
#2
hmmm, this suggestion may not be the correct way to do a check but it seems to work ok for me -



i have a fine even pile carpet - i point the camera on tripod at it at about a 45 degree angle or less - i place a large enough white card on the carpet so that the camera can't find focus with the focus cross point pointing at the centre of the card, but so that you can see carpet on both sides - i then place a black line on the white card, the camera then must focus on that line and, looking to either side, i note if the carpet is in focus in line with that or behind or in front of it, then it should be easy to make a correction - this didn't cost me anything, and seems to work well enough - but you might be lucky to pick it at 12mm, although you should still see the spread - and i suppose it might rely on good lens centering - but you've done that -



if you find this works check at 12 & 24 - if it's not the same either end, bummer - and sorry but i can't tell accurately from your shots -



and, having said all the above, i'm generally manually focusing at 16mm (the max wide on my not-nikon gear) and i wish the corners were better, but i accepted the second copy i tried of my lens from the shop back when i was young and innocent <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='Sad' />
#3
Honestly, I find AF-problems with UWA lenses basically a non issue.



That is because in most of the cases you can safely focus manually

with excellent results. Ultra wide is not easy for the AF system, since

for a certain change on the focus ring, there is not much change at all

on the AF sensors (on the tele side, the situation is just the opposite).



That's why I usually do not rely on AF at all when using an UWA lens.

Between two UWAs, one with correct AF and slight decentering, and

one with AF slightly off and no decentering, I would always prefer the

one without decentering. But what works for me might not work for you.



Just my toughts ... Rainer
#4
Thanks both... more later
#5
Hi,

I would suggest to compare results of PDAF vs CDAF (LV). If they are on par, lens AF is fine. If you try "batteries" test (I use books), don't stack them too close as you won't see the difference (check DOF for your working distance and separate objects according to it).



A.
#6
I don't find the viewfinder enables me to manual focus accurately enough on close objects wide open. Although I could potentially use Live view (magnified), I'd need to use my glasses as my eyes won’t focus without them (I have cataract replacements in both eyes: bionic lenses, but fixed focus..!! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' /><img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />).

So using Live View is a bit of a hassle. Hence trying to get autofocus adjusted as well as possible.



I used a pebble path in the garden today, focusing on a single brightly coloured leaf as focus point and checking the focus of pebbles nearer and farther away.



Results:
  • I've gradually increased the micro-adjust to -20 (as far as it will move toward the camera) but in every picture the pebbles in front of the leaf are less sharp than those behind.
  • This is not the case when I manually focus in (maginified) Live View: almost equal focus in front and behind selected focus point, marginally sharper closer
  • I also compared results using my 16-85mm. Focus with that seems as sharp in front as behind the focus plane. Which makes me think it should be possible for the autofocus to be adjusted to work more accurately, in spite of what you’ve said, Rainer, about the difficulty UWA lenses present to autofocus systems – although I guess 12mm might be more difficult than 16mm.
3 new pics [url="http://iancds.smugmug.com/Tests/TEST-Nikon-12-24/19671189_b9tn3J#1543102218_3Vz5vdp"]here[/url], illustrating above (same link as before).



I’m satisfied enough to keep the lens, but am wondering if I need to get it calibrated / adjusted (with my camera) by Nikon..?



Arv: Thanks: is that the right way round? I thought Live View uses Phase detect and 'usual' is Contrast detect?
#7
No. phase difference detection is "normal" for DSLRs. Contrast detection is "normal" for compact digitals and live view with contrast detection.



The lens indeed does not seem to focus very accurately, although to me it sounds as if you are using a weird AF mode? Not sure, as I am not at home with the different Nikon nomenclature (the "servo" part makes me think of AF for shooting something that moves?).



The edges look rather crappy, but that may well be normal with this lens. You are right in thinking that adjusting AF with a micro adjust feature may be ok for one focal length but not another, that depends on the lens. It should more or less function fine without, and it seems as if it does not.
#8
[quote name='Brightcolours' timestamp='1319398903' post='12439']

No. phase difference detection is "normal" for DSLRs. Contrast detection is "normal" for compact digitals and live view with contrast detection.

[/quote]

Hi BC,

Ok, thanks.



[quote name='Brightcolours' timestamp='1319398903' post='12439']

The lens indeed does not seem to focus very accurately, although to me it sounds as if you are using a weird AF mode? Not sure, as I am not at home with the different Nikon nomenclature (the "servo" part makes me think of AF for shooting something that moves?).

[/quote]

Just 'normal' AF. Nikon literature for the D7000: “AF-S. Single-servo AF: for stationary subjects.” “AF-C. Continuous-servo AF: for moving subjects.”



[quote name='Brightcolours' timestamp='1319398903' post='12439']

The edges look rather crappy, but that may well be normal with this lens.

[/quote]

!!!

This lens model or this particular copy? Which pics / edges are you referring to?

I hadn’t seen that – I thought it seemed ok when borders in same focus plane or within DoF?

(lens test here doesn’t suggest border res. should be ‘crappy', even wide open)



[quote name='Brightcolours' timestamp='1319398903' post='12439']

You are right in thinking that adjusting AF with a micro adjust feature may be ok for one focal length but not another, that depends on the lens. [/quote]

Ok, thanks.



[quote name='Brightcolours' timestamp='1319398903' post='12439']

It should more or less function fine without, and it seems as if it does not.

[/quote]

What I’ve read (e.g. Robert Ciala’s articles, post [url="http://forum.photozone.de/index.php?/topic/1388-article-really-worth-reading/"]here[/url]) suggests that it depends on the particular lens-body combination: if one is at the limits of specs in one direction, and the other is at the opposite limit, fine-tuning AF is going to be needed.



That said, I am wondering if this lens is indeed out of spec.

Ian
#9
[quote name='IanCD' timestamp='1319317170' post='12425']

I just took the plunge and bought a second hand 12-24mm on eBay... seemed like too good an oppportunity as it wasn't up for auction but 'Buy it now' at £550 instead of £750-800 new... only way I was going to be able to afford this lens.

I thought I'd take the risk, check it very quickly for decentering and resolution and send it back straight away if any problems.



Lens arrived on Tuesday and I've been at work all week so no opportunity to get out with the lens, but I have tested for decentering (using your sanity check, Klaus - thanks again! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />) and it seems fine in that respect - I couldn't see any issues at all. Also looks good as new - not a mark on it.



A B&W F-Pro 010 filter was also incuded in the price... maybe I should have been more suspicious.. or on the other hand, maybe I'm being over-anxious about it now!



I think the lens is auto-focusing behind the intended focus point (aka "back-focusing"?)

I've spent several hours today trying to suss out whether this is really the case, and how much AF micro-adjust to apply.

I've now read several articles, and have focus tool by Tim Jackson, came across idea of lining up batteries at a 45 degree angle and making small changes to the MF adjust, looked at LensAlign (seems v. expensive) and have taken numerous pics, some in poor light... and am really no nearer to getting this sorted. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/huh.gif' class='bbc_emoticon' alt='Huh' />



Pics [url="http://iancds.smugmug.com/Tests/TEST-Nikon-12-24/19671189_b9tn3J#1543100152_JKXnWdS"]here[/url]. Camera is Nikon D7000, focal length used = 12mm @ f4. Clicking on the images should bring up option to view them at original size ('O').

In all cases, I used single-servo AF, with one focus point at the centre of the image. I appreciate some of these are much less useful than others for looking at this issue... some pointers to selected focus points:
  • Cat's eye was selected point of focus (not the dried blades of grass & ivy behind)
  • View looking up at the house: focus point is centre (in line with window centres), 3rd course of stones above patio doors. Focus seems to me to improve higher up the stone wall (again, farther away)
  • Teasel: front of cone was focus point
  • Dried out flowers in flower bed: front of foremost seedhead
I'd be very grateful for

  1. suggestions re the best way to find optimum AF micro-adjust setting - or if the lens focus might be too far out of spec so that it needs Nikon to adjust (and I say bye bye to the £ saving..!)
  2. whether getting micro-adjust right at 12mm will work through zoom range - if not, is it possible to save different AF m-a settings to use at different focal lengths on the same lens - I don't think it is?
  3. whether the overall image quality is in line with what I should expect from this lens... or should I send it back fast on Monday..?! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/sad.gif' class='bbc_emoticon' alt='Sad' />
Ian

[/quote]



I have this lens and would not expect small errors to show themselves. It was not until I acquired an F/2.8 telezoom that I discovered both bodies were backfocusing - I do know what it's like when you suspect a problem and start to panic a bit.



Try taking pics of people's faces, not square-on, but so that one eye is closer. The focus point is then not on quite a such 'rounded' shape as the thistle, and it gives more chance to reduce focus errors. We are used to looking at eyes, so any error will be apparent. As a further check, focus on a small, detailed area which is flat. Not so small, however,that the focus area marked in the viewfinder can 'spill' over and pick up some of the background and cause an error. Maybe a greetings card on a table at the other end of the room. Use flash, but also remember that a low ambient light level can cause autofocus inaccuracies. Borrow a similar body to try, if you can. Or lens even, if at all possible.



If all else fails (I assume you are in the UK) I suggest you call 'Fixation' in London for advice. They often have a 2-day turnaround and the quality of their work ranks with Nikon's own. They do Nikon guarantee work, for example.



Good luck.



I assume you are using the central focus sensor only, and yes, the 'S' (servo?) focus mode. Use no other.
#10
Phoned Nikon today, they said to post them some images...



So, I've taken a series of battery shots this evening, adjusting AF finetuning -3 at a time from 0 to -18, then -20



Taken with remote, mirror lock up, AE-L/AF-L button used to focus. Camera resting on table.



Fairly convinced now that the optimal adjustment is -8 (or maybe -7)



I'll post some of them to Smug Mug tomorrow, but here's one with AF fine tuning at -8.

Focus is on the centre battery (labelled LR03)



[ATTACHMENT NOT FOUND]
  


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