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12-07-2014, 10:07 AM   #1
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How Does autofocusing work?

Greetings

Just got myself a da* 50-135 and was hoping to get decent shots of my daughter skating practice yesterday but autofocus locked on anything but my daughter. I use centre point focusing. Even when she stood still autofocus was not looking at the right place. I was surprise because so far in all other situations af is right on the spot. Is there something special about a hockey rink that confuses af?!?

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12-07-2014, 10:34 AM   #2
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No, there's nothing about a a hockey rink that will fluster the autofocus. But it will confuse the heck out of the metering system. You'll just need to dial in some significant EV compensation to fix that, though.

The autofocus points are considerable larger than you see. Are you looking through the viewfinder? CDAF (Live view) is going to have major issues, especially if she's wearing dark colors, because there's a huge jump in contrast between the white background and whatever she's wearing; if she's wearing a light pink, it would be better, but the white ice is basically the maximum level of brightness that any image can have, so the contrast is maximized at the edge of clothing. If you can aim at her face, then it will detect that and it should work. I'm not sure what PDAF would find.

SDM lenses, unfortunately, have issues like this. They really like to take a ride from time to time.
12-07-2014, 11:46 AM   #3
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Shot street hockey for years using the same lens. DOF is razor thin @ f2.8 at the typical shooting distances. Try stopping down to f5.8-8.0 even if you have to raise the iso.
12-07-2014, 12:28 PM   #4
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Different camera body, but the same problem. You have two different selections - 1) the light metering setting (spot, center averaging, and uniform across the frame) - that should not be confused with AF points and 2) you have the focus point selection (which differs across the various bodies).

I was shooting some birds yesterday, and I needed to switch to the center point focusing to get it to focus on the bird, and not the various branches around the bird. Here is the manual for the K30. Check out page 102Just remember you have "spot" for the light metering mode, and you can also have "spot" for the AF mode. It can be one in the same for both functions, but they are set separately.



12-07-2014, 02:11 PM   #5
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Could the camera be trying to focus on a diffuse background that always looks blurry because it lacks any detail to begin with? In or out of focus, white ice could generally look the same to the AF system. The same effect happens when trying to focus on a blue sky. You can't.
12-07-2014, 04:56 PM   #6
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Thanks everyone, very informative answers. Metering is not a problem: manual settings 1/400s f/2.8 Iso 1600 there is one dark corner where that doesn't work.
@MadMathMind My daughter gens pretty small at 100', maybe af is finding the background boards, which have everything an af need (lots of details), more interesting that my daughter. Looking at the shots again I noticed on many that af did lock on them even if I had placed my DD right in the center.
@mdave13 that could be why some close shot (30') are missed even though my DD is a lot bigger in the viewfinder. Dof calculator returns 16" of dof at that distance! I guess I could bump f - stop a bit.
Thanks interested_observer I checked and af is on center point
@6BQ5 lots of white all around when she's in the Middleton of the rink. I also had a chance to check af in a similar situation: a White barn with a snow covered field in front and white clouds in the back: af problems again. Had to redo focus a few time to get it right.

Would the problem be the same with screw drive?
@MadMathMind would liveview be better than looking through the viewfinder?

Thanks again
12-07-2014, 05:04 PM   #7
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QuoteOriginally posted by SunValley Quote
Would the problem be the same with screw drive?
It may be better because SDM lenses can fumble for locks. I've had trouble with my DA*300 taking a ride from time to time. It occasionally will just get totally lost for no apparent reason. Focusing 90% manually and then AFing the rest seems to help.


QuoteQuote:
@MadMathMind would liveview be better than looking through the viewfinder?
No, it's likely to be a lot worse. For one, CDAF is much slower. Secondly, the bright background will likey throw off the algorithms that maximize the contrast for focus.
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