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02-07-2014, 08:13 AM   #1
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K-5 IIs, AF, and fast primes

I understand that only the center point of the K-5 IIs is sensitive to f/2.8, so when focusing such a lens, it is best to set to center spot and recompose as necessary to ensure accurate AF. But what about when lenses are faster than that? I've been using a K 50mm set to f/2.0 and when I can focus to a beep (AF assist), the result is great. Since it's an old lens, it only works in center focus mode. My questions are

1) Is AF assist accurate below 2.8? I assume the AF assist and AF engine are the same, but maybe not?
2) What happens with fast AF primes below f/2.8? Does the camera know this, stop them down to 2.8, and focus them, or does it leave them wide-open and try to focus, even though it knows the result may be inaccurate?

02-07-2014, 10:06 AM   #2
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Regarding 1):

More light on the AF sensor from a faster lens and the shallower depth of field should only help the AF system one would think, but I don't know of any tests addressing this.

But for accuracy: AF is only so accurate. The inaccuracies are masked by depth of field, so shooting with a fast lens fully open may bring the inaccuracies out in the open.

It is the same circuitry that performs autofocus and focus confirmation.

I have used the FA 31/1.8, FA 50/1.4, and FA 77/1.8 with without any focusing issues on the K-5, K-5IIs and K-3.

Regarding 2):
The camera leaves the lens wide open during the autofocus operation.
02-07-2014, 10:08 AM   #3
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It's accurate, but it's really tricky to focus manually that wide open. The focus point is relatively large, though it depends how close you are to your subject as well.

Accuracy and precision are two different things.

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