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04-08-2010, 06:17 AM   #1
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why did my sigma have trouble focusing?

This weekend I was taking pictures of my kids, standing at the entrance of an old covered bridge. They were just in the shadow area and you chould see the "light at the end of the tunnel" or bridge as the case was. So I had it on Auto Focus and it kept "searching" but had trouble actually focusing. I thought maybe the light coming from the end of the bridge could have done it because it had never happened to me before. It freaked me out a bit because I love this lens. I've only had it a couple months, but I've never had any trouble with it.

Anyone have any ideas what would have made it not want to focus? Would sharing a picture of the scene help?

04-08-2010, 09:01 AM   #2
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Yes post a picture to help determine this. I suspect the kids were too deep in shadow and being mostly black there was no contrast for the system to focus on. It happens and there's nothing wrong with the lens I bet.
04-08-2010, 09:11 AM   #3
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I'm so glad to hear you think the lens is fine. It's become my favorite since I've bought it and only my 50 mm has been on the camera once since the Sigma came to live with me. I kind of figured it had something to do wtih the lighting since it had never happened before and only happened in that one location on that day. Here is one of the pictures in the location where it was happening. I forget what I did to get it to focus, but it took me quite a few tries to get it.

04-08-2010, 09:15 AM   #4
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QuoteOriginally posted by Peter Zack Quote
Yes post a picture to help determine this. I suspect the kids were too deep in shadow and being mostly black there was no contrast for the system to focus on. It happens and there's nothing wrong with the lens I bet.
^^^^^^

What he said.. Shadows on shadows in the dark aren't very autofocus friendly. I'll bet if you had aimed at a location where there was light vs dark in this situation, the lens would have locked right on. Besides that, it isn't really the lens that drives AF, but the camera's AF system. The lens is just focusing the light. Now, if you cannot AF in ANY situation, try a different lens.

In a situation like your photo (edit), I would have the camera set for Center Spot focusing (my cameras live on that setting), and aim for the area where the little girl's sweater is against her brother's shirt. Unless they are standing in total darkness, that should get you there every time.

Cute kids btw





04-08-2010, 09:19 AM   #5
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Did you use auto AF point selection or do you use cente-point focus? With auto AF point, the camera looks for the highest contrast point (which could be behind the kids). It would have been probably easier if you had selected a focus point on the T-shirt of the bigger (or any one of the) kids.
04-08-2010, 09:21 AM   #6
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I used center point focus.
04-08-2010, 09:28 AM   #7
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Assuming this isn't cropped, the center point is here:
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That surprises me a little but the scene is fairly bright all around the spot. There just may not have been enough contrast for the AF to nail the shot as you like. Now looking at this image, it seems fairly sharp. Was this a good one but others were not so good?

Cute kids Btw.
04-08-2010, 09:31 AM   #8
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If you have a bad shot, post that. I bet the focus point is between the boy's heads and it focused on the backbround.

04-08-2010, 09:33 AM   #9
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Thanks...I think they are kind of cute too...when they aren't being naughty LOL

I think I did crop this one a bit. I'll have to get a sooc...and I'll go find a blurry one. This was the best of the bunch.

Thanks for taking the time with helping me on this.

also, normally I use center focus, focus on eyes, lock it and then recompose...Is that not the best method when shooting a large number of people?
04-08-2010, 09:35 AM   #10
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I am equally puzzled as it seems to be relatively straight forward - unless it is the lens.

I also have the Sigma lens (18-50mm 2.8); it does hiccup sometimes. I think (again not quite sure) sometimes when the battery in the camera runs a bit low - the auto focus mechanism may act little slow.. If it was me, I would probably change another lens and try it again.

Great kids...
04-08-2010, 09:36 AM   #11
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Just post an uncropped version of an out of focus shot. Then we'll have a better chance to figure this out.
04-08-2010, 09:44 AM   #12
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04-08-2010, 09:50 AM   #13
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To me it looks more like the contrast situation. My eyes tend to look straight pastt the children and to the light behind.
If you tried focusing on your daughters white dress it may have worked better.
04-08-2010, 09:58 AM   #14
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good looks run in the family

it looks like your camera was focused right on that plant you see at the end of the tunnel
04-08-2010, 10:05 AM   #15
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I am in aggrement with the above posts. Keep in mind that the actual focus sensor spots are a bit larger than the focus indicators would lead you to believe. Looking at the out-of-focus shot it seems that the sensor found more contrast in the background than that of the area of the eyes of the boy in the middle and locked into that.

For shots like these I tend to lock focus on a high contrast area at the relative distance I wish to be in focus and recompose. Then adjust your DOF to make sure you have everything you want is in focus.
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