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07-04-2010, 12:13 PM   #1
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ytterbium's Avatar

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F 50/1.7 random focus with distant subjects

When shooting close subjects most of the time focus is spot on.

When shooting subjects (landscapes) at considerable distance the focus often jumps around 3..10 meters (with each half press). Even with no visible foreground objects it is often hard to make it lock at infinity.

Most of the time i'm using f8..f16 for such shots which results in soft images (if im shooting 20m..infinity landscape) when focus locks somewhere at 10m.

Is such behavior common for this lens. If not i'd give 80% it's the cheap UV filter, but haven't actually done extensive testing.
If it is a known issue i will simply deal with it using MF/hyperfocal, if not i'd like to find the cause and resolve the issue.

Thank you

07-04-2010, 12:43 PM   #2
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More info would help - like, are you selecting a focus point, is there a clear object that takes up the entire region around that point, etc. And are you saying ti won't go to infinity at all, or just that it often tends to choose something closer to focus on?
07-04-2010, 12:55 PM   #3
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Nop. No simple mistakes. That's the first think i check. Single AF point, manual select - center. As i said, either no foreground stuff AT ALL or nowhere near AF point.

Clear object.. with distant subjects this is quite impossible. A wall of trees 100..200m away. Line of horizon 5..10km away. Some gray/yellow walkway in foilage ~50m away. More like clean lines at desired distance.

Unfortunately i have no pictures i could post to illustrate, imagine something similar to this:



But with less foreground objects.
There are clear, contrasty lines far away in the center of frame, but the focus is on something at the very edges of frame or can not be found at all in the image taken.

I have a bad suspicion that this might be issue with k200d, because i experienced similar misbehavior with 30/1.4. In addition with 30/1.4 i could not get infinity lock AT ALL, so i thought the lens was defective and sold it ridiculously cheap as MF.

Another thing that comes into my mind is repeating lines. If you know what TTL Phase Difference AF is and how it works, you know it can get confused with repeating patterns. If the AF system for example chooses to use horizontal line sensor instead of vertical one, it does not measure the phase of horizon line. It takes reading from trees/foliage that can produce very similar repeating lines. Such way the system might match two different lines.

Last edited by ytterbium; 07-04-2010 at 01:02 PM.
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