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09-24-2011, 01:40 PM   #1
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Is it normal for K-x not to be able to AF on a white wall ?

I noticed that my K-x doesn't AF at all when a wall or something of the same color fills the frame. Is this normal behavior ?

09-24-2011, 01:46 PM - 1 Like   #2
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Yes, there needs to be contrast for the AF system to work, on any camera for that matter.

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09-24-2011, 02:01 PM - 1 Like   #3
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K-x manual, page 65: "Subjects that are difficult to focus on" has a list, including "(a) Extremely low-contrast subjects such as a white wall..."

Completely normal. Luckily, nobody wants to take photos of white walls.

For the record, all digital cameras do this. The Nikon D7000 manual says "Autofocus does not perform well... [when] the subject is the same color as the background" and they show a person in clothing that is the same color as a solid wall. Their manual labels this a "little or no contrast" subject.

Last edited by Aegon; 09-24-2011 at 02:07 PM.
09-24-2011, 03:57 PM   #4

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It's normal for any camera that uses contrast detection for auto focus.

09-24-2011, 05:05 PM   #5
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my k-x doesnt focus on white walls either. So its normal
09-24-2011, 05:11 PM   #6
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This is the same reason a grey rainy/foggy day can raise havoc with autofocus as well. The camera can't make out any details to lock on to.
09-25-2011, 12:55 PM   #7
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Our TTL AF systems need to see differences, to know just what to focus upon. Blank expanses haven't enough differences. Now, back in the day, was a revolutionary SLR called the Polaroid SX-70. One of its revolutionary technologies was ultrasonic focusing (USF). And it was such a hit that Polaroid sold the USF kit as a separate package for experimenters. Many robot-makers had fun with it. Now, besides the GPS clip-on, suppose Pentax made a USF add-on? Kewl idea, eh? Then we could shoot blank expanses all day, and they'd be in focus! Huzzah!
09-26-2011, 08:26 PM   #8
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Think of it this way. You have a page that is black on half and white on half. The transition from black to white is a sharp, distinct line. When the image is out of focus, that sharp distinct line is a grey blur. When it is in focus, it is a sharp distinct line with absolute contrast (black to white). Your camera looks for that blur to turn into a line. That is how it tells when the image is in focus. If you don't have a darker and lighter color with at least a semi distinct separation at a point, the camera has noting to determine focus on. A white page with sharp black lines is ideal to auto focus on (sharp contrast and lots of points to auto focus on). A solid color and or gradual color change is the worst.

09-28-2011, 05:17 PM   #9
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Most things my K100D can't focus on (White Wall, Empty Blue Sky, reflective plastic) are things I have no desire to photograph.

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