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04-13-2012, 06:49 AM   #1
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Does lens aperture affect AF performance?

I have a feeling it does. I've been shooting a few family gatherings using the K-5, bounce flash and the kit lens at f/5.6. I was hit by the infamous low tungsten light front focus a few times, and lost some shots on that account. The light level was about EV 4-5 (i think).

So it struck me, the kit lens is f/3.5-5.6. if I got a faster lens, say a constant f/2.8, would the front focus then first appear at say EV3 and lower?

I may be able to buy a used 16-50 from the local classifieds. I've been looking for a excuse reason to buy it, you see. I've been satisfied with the kit lens otherwise.


04-13-2012, 07:36 AM   #2
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Yes, the larger the max aperture (smaller the number), the more light that reaches the PDAF sensors allowing them to "see" better. Only the max aperture matters though, if you were shooting say the DA* 16-50 at say f/11, it will remain open at 2.8 until you press the shutter at which time it stops down to take the picture.

For variable aperture zooms it's the max aperture at a given focal length that counts. To figure out what that is at any given focal length, switch to Av mode and dial the aperture down as low as it will go, then operate the zoom ring. It's good to make a mental note of where the transitions happen. "Ok at 28mm it switches to f/4."

Last edited by maxfield_photo; 04-13-2012 at 10:57 AM.
04-13-2012, 07:51 AM   #3
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Max aperture definitely affects PDAF. The two regular type points on the extreme left and right are only good to f./5.6 and the 9 cross type points in the center are only good to about f./8. As mentioned, this is the max aperture of the lens we're talking about, unless it's a stopped down manual lens (m42 and such). It becomes a concern with long telephotos, mirror lenses, and such. The amount of available light plays a role, too, so your mileage may vary from the numbers above, but they're all that can rightfully be expected from the AF sensors.

As to yours front focus question, I can't really say. A larger max aperture does make it easier for the AF to lock, generally speaking.
04-13-2012, 08:38 AM   #4
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If you were shooting at f/5.6, there is very little chance you were affected by any FF issues - the couple of millimeters that you might be off would be more than compensated for the DOF at f/5.6. It's only at larger apertures like f/2.8 where there would be the possibility of seeing FF. So I'd say it's more likely you simply saw a case of the camera *choosing* to focus somewhere other than you wanted, but that's not FF. FF is when it chooses the right target but then misses.


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