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09-03-2011, 12:53 PM   #1
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Join Date: Jul 2011
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Sigma focusing issue


I'm new to the forum and photography. Pretty inexperienced with trouble shooting photo equipments.

I recently purchased a brand new sigma 17-70 f2.8-4.0 HSM to pair up with my K-x. The lens is from authorized US dealer.

When I mounted the lens for some test shots, I found that it will often focus hunt, especially with single colored flat wall, even under well lit conditions.

I have tried to focus on the same object in all AF point settings. It would often jump between two points not too far apart, one behind, one in front of the desired spot each time I half press the shutter.

In the view finer the octagon focus symbol would flash and I will have to keep trying half pressing the shutter until it finds the focus. (2-5 tries). The result is not too sharp either.

The HSM focusing system is a lot quieter but not as fast as I expected. For close shots, the focus would adjusted in little steps couple times, each time making a little motor noise.

I have a feeling that my copy might be defective. With my little experience, I can't say for sure if this is the lens or a body calibration issue.

I appreciate any comment from the helpful users here and thank ahead for any suggestion !

09-03-2011, 01:49 PM   #2
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Join Date: Sep 2006
Location: Copenhagen, Denmark
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The object you focus on must have some contrast. You cannot expect to auto focus on a single color wall.

Start your test with a well-lit object with some contrast - here focusing should be snappy with no hunting. Then you can proceed to lower light levels where you may experience slower autofocus, perhaps occasionally even with the stepwise corrections you have noticed.
09-03-2011, 04:16 PM   #3
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Make sure to set focus point to center point for your tests.

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