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04-12-2012, 07:20 AM   #1
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Issues with the da 35mm 2.4?

I purchased this lens a couple weeks ago. I seem to be having trouble locking in the correct auto focus for my 2 1/2 yr old son. At times the camera will think it has the focus locked in but he will be out of focus and the background in focus. I notice this most when the lighting is at all tricky. Like a backlit or dark lighting situation. It also seems to hunt a bit to find a lock. More so than the kit lens. Is this a bad copy that I should get replaced or normal for this lens?

Edit: I should point out that I put a Marumi uv filter and a hood on it. Could those be effecting autofocus?

Thanks


Last edited by sun_shard; 04-12-2012 at 07:38 AM.
04-12-2012, 08:14 AM   #2
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Which camera do you have,
what autofocus setting are you using,
and is face-detection on?
Also, don't forget that each autofocus "point" covers quite an area.
04-12-2012, 10:24 AM   #3
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QuoteOriginally posted by lytrytyr Quote
Which camera do you have,
what autofocus setting are you using,
and is face-detection on?
Also, don't forget that each autofocus "point" covers quite an area.
The camera is the k-x. Auto focus is primarily af-s. Using the center point. I don't believe I have face detection on. Is it recommended for people shooting? Where is that setting?
04-12-2012, 11:48 AM   #4
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Almost certain this will turn out to be a simple case of the AF sensor being largerthan you realize, thus putting both your son and area behind him on range, and the cmaera not having any way of knowing which of the two you wanted to focus on.

04-12-2012, 11:58 AM   #5
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QuoteOriginally posted by sun_shard Quote
I don't believe I have face detection on. Is it recommended for people shooting? Where is that setting?
It works in live view contrast-detect focusing (which is often slow, though).
See page 145 of the K-x manual.
Point-and-shooters love it, but it may not be very effective on the K-x.
04-12-2012, 12:53 PM   #6
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My only issue with the lens is slight but considerable back focusing in low light. Apart from that, it's a remarkable lens for the price.
04-12-2012, 12:56 PM   #7
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There are two possibilities: user error, in which case you need to be sure what you're focusing on, or a back-focusing lens. You need to rule out the former first. If the lens focuses ok in good light it should focus ok in poor light (though may fail more often).
04-12-2012, 01:14 PM   #8
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QuoteOriginally posted by Jonathan Mac Quote
There are two possibilities: user error, in which case you need to be sure what you're focusing on, or a back-focusing lens. You need to rule out the former first. If the lens focuses ok in good light it should focus ok in poor light (though may fail more often).
Well, I was able to take nice, sharp portraits of my wife when she posed. Does that rule out 'back focussing?'

04-12-2012, 01:38 PM   #9
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If you got a sharp image focusing automatically even just once it sounds like the lens itself is fine. Ensure you're pointing your camera over a large enough area for the camera to lock onto the right focus point for your portraits.
04-12-2012, 03:40 PM   #10
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Ok, guys. I am hoping that it was user error as you pointed out can happen. Will do more testing this weekend and post again with the results. Thanks for the input!
04-12-2012, 04:00 PM   #11
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What kind of lights are you using? Tungsten light can generate focusing errors especially in closer focusing distances.
04-12-2012, 05:44 PM   #12
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Most of the focussing errors occurred on a bright sunny day. I did snap off a couple pictures of my son under weak yellowish household cfl lightbulb and no flash. Those didn't work out to well!
04-12-2012, 11:10 PM   #13
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QuoteOriginally posted by sun_shard Quote
Most of the focussing errors occurred on a bright sunny day. I did snap off a couple pictures of my son under weak yellowish household cfl lightbulb and no flash. Those didn't work out to well!
Put a hood on your lens. Flare might be confusing the camera. Manually specify the white balance under tungsten, which confuses AWB.
04-13-2012, 01:55 PM   #14
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I do have a hood and filter on it. Could those have an effect on the autofocusing?
04-13-2012, 02:05 PM   #15
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QuoteOriginally posted by sun_shard Quote
I do have a hood and filter on it. Could those have an effect on the autofocusing?
Assuming that it is a UV filter, then remove it. It is useless and can cause problems under certain conditions. It will generate ghosts and flares when the lens is pointed toward a bright light source and may interfere with focusing if the artifacts are positioned within the AF area.

Definitely keep the hood.
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