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12-12-2013, 07:19 AM   #1
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K3 - AF consistency using corner focus points

I'm curious to know how other K3 owners are finding this. I find that the K3 seems to miss focus quite a bit when using the outer most focus points in the square of 25 points (not the 2 non cross type points on the far left and right). Admittedly, I'm using my FA31 at f2.8 indoors in lighting that isn't perfect but the camera will nail the focus using the centre point or any of the next ring of 8 points surrounding the centre. Once I shift to the outer ring the focus is very hit and miss at f2.8 or lower and seems to improve after f3.2 when using the FA31. It's not softness, at f2.8 it just seems to miss focus. I know that the centre point and the ones directly above and below it are the only ones rated for f2.8. I just don't recall my K30 having this issue. I shoot with the outer focus points quite often for live music shots.

Is this common with the k3? Is it a lens thing where contrast isn't as strong in the FA31 the further away from the centre you get or something? I find it interesting that only the outer ring is hit and miss. I would have expected all points other than the 3 f2.8 ones to behave the same way.

Just to cover bases, I'm using AF-S, NR is off.

Interested to hear opinions and experiences.

12-12-2013, 07:39 AM   #2
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I think it's common for DSLRs to have less accurate outer AF points. This also seems to occur with a D800: LumoLabs -- Nikon D800/E outer AF sensor accuracy -- Technical study / Whitepaper.
12-12-2013, 08:01 AM   #3
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QuoteOriginally posted by MBT74 Quote
Is it a lens thing where contrast isn't as strong in the FA31 the further away from the centre you get or something?
Yes. The lens mediates the effectiveness of the AF due to the amount of contrast it can deliver to the camera body and sensor across the frame. This is natural and to be expected. Usually lenses are sharpest in the centre and softest at the extreme edges. However there are a few lenses (eg the DA40) which perform very consistently across the frame. Test your ideas about K-3 AF against the DA 40, perhaps.
12-12-2013, 08:01 AM   #4
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QuoteOriginally posted by starbase218 Quote
I think it's common for DSLRs to have less accurate outer AF points. This also seems to occur with a D800: LumoLabs -- Nikon D800/E outer AF sensor accuracy -- Technical study / Whitepaper.
Interesting article. Gave it a quick skim. From the edit in the conclusion it seems that Nikon fixed theirs via calibration at a service centre. It said that the wider the lens the more pronounced the AF miss rate. I briefly tested my DA55 last night with the outer points and it was spot on. Might try it again. Might also have to put the K3 in for calibration in the New Year.

12-12-2013, 08:05 AM   #5
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QuoteOriginally posted by rawr Quote
Yes. The lens mediates the effectiveness of the AF due to the amount of contrast it can deliver to the camera body and sensor. This is natural and to be expected. Usually lenses are sharpest in the centre and softest at the extreme edges. However there are a few lenses (eg the DA40) which perform very consistently across the frame. Test your ideas about K-3 AF against the DA 40, perhaps.
I don't own the DA40 but from my testing it seemed that the FA31 is worst, the FA43 was better and the DA 35 f2.4 and the DA55 were pretty good using the wider points. I can't remember how the FA77 went.

The new AF unit must play some part. If the K30 did this I would have noticed at some point I would have thought.
12-12-2013, 08:29 AM   #6
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QuoteOriginally posted by MBT74 Quote
I don't own the DA40 but from my testing it seemed that the FA31 is worst, the FA43 was better and the DA 35 f2.4 and the DA55 were pretty good using the wider points. I can't remember how the FA77 went.
Just look at the photozone.de or lenstip.com resolution charts. Or the dxo lens test field maps (if they have them for your lens). They will give you a good picture of what to expect of contast and sharpness across the frame for your lenses.

If a lens is soft across the frame, it is kind of unreasonable to expect the camera AF to be uniformly sharp and effective. The camera AF can only work with what your lens gives it.
12-12-2013, 08:40 AM   #7
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I haven't noticed a problem and the FA31 is pretty much the only lens I've had on my K-3 since I am doing the single in December challenge. I too have come from a K-30 and find the extra focus points a godsend for getting better focus.
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