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08-05-2011, 03:24 PM   #1
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Landscape Autofocus Problems...

I recently took a trip to Italy, and used both a 50-200mm and 18-55mm - and both had trouble autofocusing at a distance when fully zoomed - I had to start reeling the lens in until it would focus...

I would say the conditions where hot (high twenty's to mid thirties) and humid on days, and with a white haze at a far distance.

In addition to the lenses, I was using only a ciruclar polarising filter - no UV filter.

Is this a problem with the conditions or the camera - could it have been fixed by using a filter?

Any help or experience would be appreciated.

Many thanks.

08-05-2011, 03:58 PM   #2
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It's possible that because the aperture of both your lenses at the longest focal length and your polarizing filter that the camera couldn't autofocus properly.

In that case, even trying to focus on something close wouldn't work.
08-05-2011, 04:05 PM   #3
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Autofocus and focus-confirmation depend on the focus point(s) having some contrast or pattern. For instance, trying to autofocus on a blank wall or sky is difficult. Infinity may also be hard to autofocus. I would not use AF to shoot distant mountains etc -- I would manually focus to infinity (or just before) and set the aperture for enough DOF. But daytime shots of distant features with a long lens may be affected by atmospheric turbulence and particulates, which reduce contrast and clarity. Trying to AF on distant mountains at 200mm in daytime is an exercise in frustration.
08-06-2011, 12:26 AM   #4
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Thanks for the replies - I opened this thread as I've never experienced this problem before.

It seems odd, (thinking back...) even if I took the CP filter off, it would still not focus properly... even moving the focus point around from a building to a tree to something else... trying to hunt a more contrasting focal point. The only thing that would help was to bring the zoom back in.

...this was using my K-r. I've had no problems taking distance landscape shots in the UK (Lake district, Dales, etc...) before, but perhaps I have and never thought it an issue! Saying that - those locations are pretty contrasting in the summer months.

My wife on the otherhand had no problems using only a 18-55mm with her K100D...

Just can't believe I've never experienced it! - I've just tried focusing at a distance on white sky (can't always focus) and distant roof tops (can't always focus, but better than focusing on sky!)...

08-08-2011, 12:11 PM   #5
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As riorico has mentioned, K-r's AF works on Contrast detection. White wall, Clear Sky, Landscape with 50% and more sky has AF hunting. Its usually the case if AF points are selected to Auto, 5 or 11 points. There are two workarounds that I use

1) Switch to landscape mode, it drastically reduces AF hunting
2) Manual AF to infinity

Also, you can set the AF point to some high contrast point in the view, that will ensure that you are not focusing some low contrast area.
08-10-2011, 01:02 AM   #6
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Was this on multi segment? Did you try spot focus. I have had the same problem in aviation photography and switched to spot and that worked fine. Skydiving photos with along lens at 500mm and blue sky is a real challenge to get both exposure and focus spot on, I normally use spot for both and bracket a little.
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