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Old Mar 10, 2007, 8:27 AM   #1
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I took some pictures yesterday of some frozen waterfalls with my K100D, but after reviewing them on my computer I see thatsome of them that where taken using auto-focus aren't in focus (they're not terribly out of focus, just very slightly off). The one's that I took using manual focus came out fine, so it couldn't have been anything on the lens.

Could it be that since the waterfalls where relatively far away that maybe theywhere out of range of the AF sensor?I suspect the fact that they where covered in snow and ice wasn't helping? And, it was getting close to sunset too, I'm sure that can factor in too.

In any case, it didn't seem to be 'hunting' for the correct focus. Also, the AF point is set on auto. Is this a normal limitation of the auto-focus system? I'm sure there's got to be certain situations that will trip up any AF. Either way I'm finding that I like manual focus better anyway I'm just trying to understand whats going on here and if its 'normal'.
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Old Mar 10, 2007, 9:33 AM   #2
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The autofocus should be able to focus the lens to infinity. There are certainly things that can fool autofocus. The most common is when it locks onto something other than your intended target. If you turn on "Superimpose AF point", you can see what the camera is focusing on. Posting one of the pictures would be helpful to figure out what happened.

You can set the autofocus to auto, selected point, or center. Use the "Swtch dst msr pt" setting in the Rec. menu. I've never used auto, so I don't know how it decides what to lock onto. The manual may have a description.

How far away were the waterfalls (check your EXIF data)? Should the lens have been focused to infinity?

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Old Mar 10, 2007, 9:58 AM   #3
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It could be other factors as well, exempli gratia shooting wide open (low F-number) and a long focal length lens. This would result in shallow DOF and any imperfection in focusing could render your main subject out of focus.

Can you give more details of what you used?
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Old Mar 10, 2007, 10:15 AM   #4
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I got reading about how the AF works, since I have a K10 and sometimes have a bit of backfocus (usually operator caused). If I understand the article I read correctly (I'm no expert, just read a couple of articles to try to figure out what's going on), the AF focus system needs contrast to work (true for any digital camera). Often if you have the camera set to choose it's own point, it will choose what has the greatest contrast, and that could be a foreground fence in a snow scene, not the waterfall in the center of the lens. I find that I get along better if I just set the AF point to center for the most part - my old film SLR was a split screen and center exposure, so I'm used to focusing then recomposing. I've used select point on occasion, but find it's an extra thing to think about, so I use it for specific things, not as a usual setting.

The other thing is that an individual sensor can only see contrast on one plane. Some sensors will "see" vertical lines, while others "see" horizontal lines. There are two sensors in the center - one for vertical and one for horizontal, making it the most versatile and accurate point.

All this means that AF isn't perfect. I find a lens might hunt when taking a picture of a subtly colored sunset - not enough contrast to lock focus. For your far away frozen waterfalls, either try center focus point or do what you did - use manual.
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