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12-21-2010, 02:31 PM   #1
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Please help, lens focus problem

Hello all, I just bought a fast AF lens for my camera and I have some issues with front-focus. Can you please tell me your opinion about this. http://dl.dropbox.com/u/649244/ob%20defect/SG207389%20-%20front%20focus.jpg As you can see from this focusing chart there is a 2mm left front focus and a more severe 6mm in the right. The seller said that he haven't any problems with this lens. Can you confirm that is a lens problem (he suggested that maybe my camera has issues). I prefer not to reveal for now what lens we are talking about, I'll satisfy this curiosity later.

12-21-2010, 02:52 PM   #2
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There's not going to be any argument from anyone that the text on one side is more in focus than the other. I would be cautious about making the judgment from this image alone, however. How are you sure that your sensor is precisely parallel to a plane passing through both lines of text.

Also, what was your light source for this shot? Apparently tungsten sources can lead to front-focus (I've only just learned this, and cannot reproduce it myself, but several people seem to be having the issue with the K-5, and it's said to extend to most cameras, to some extent.
12-22-2010, 12:40 AM   #3
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Thank you for your answer. I made several shots this is just one of them. I'll shoot some more in daylight and also try to find another body to test the lens on.
12-22-2010, 03:37 AM   #4
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New observation: The problem appears only at small focusing distances (under 1m). At infinity and larger distances to the subject the lens is as sharp as it should.

12-24-2010, 06:48 AM   #5
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Latest update: I tested the Sigma 28mm f:1.8 (this is the one I'm talking about) on a k10 body and it worked perfect. So my camera is defective. Fortunately it is still under warranty.

Below is the result on the k10.

http://dl.dropbox.com/u/649244/ob%20defect/IMGP9442.JPG
12-24-2010, 07:09 AM   #6
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QuoteOriginally posted by cristimm Quote
New observation: The problem appears only at small focusing distances (under 1m). At infinity and larger distances to the subject the lens is as sharp as it should.
Are you using a tripod? Also I would not overly concern myself with an apparent issue at minimum focus distance. Unless this is aacro lens you may be expecting more than the lens can vive
12-26-2010, 07:25 AM   #7
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See my post above. Problem is identified and will be solved.
12-26-2010, 08:00 AM   #8
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QuoteOriginally posted by cristimm Quote
Latest update: I tested the Sigma 28mm f:1.8 (this is the one I'm talking about) on a k10 body and it worked perfect. So my camera is defective. Fortunately it is still under warranty.


It's also possible that the lens firmware needs to be updated. Camera makers do change things from time to time.

12-26-2010, 09:56 AM   #9
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There's likely nothing wrong with your lens or your camera. In the first picture the paper was not perpendicular to the the sensor plane. The left side focuses closer because it is closer. In the second picture, the paper is not flat, so it might not be useful either.
Attached Images
 

Last edited by kxr4trids; 12-26-2010 at 10:19 AM.
12-26-2010, 10:24 AM   #10
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When I got my K-7, I noticed that it front-focused with a few lenses that were spot-on with my K20D, so I concluded it must have come from the body. I sent it for warranty service and it came back perfect.
At the time, I was told by the people who did the repairs (here in France) that there had been quite a few K-7s sent back for the same issue (they also said that front/back focusing problems affected other camera brand - they also service Nikon, Canon etc... - so it wasn't Pentax-specific).
They also explained that it wasn't a linear problem, i.e. it wouldn't affect different lenses in the same way, which is why some lenses that were perfectly adjusted on my K20D were still as good on my K-7 (like my Tammy 90mm macro).
If I remember correctly, they were three factors that played a part in this: the sensor plane (it needs to be perfectly adjusted down to a small fraction of a millimetre), the angle of the mirror (which was sometimes off by a few degrees!), and the angle of the surface where the AF sensor sits.
12-27-2010, 04:18 AM   #11
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QuoteOriginally posted by kxr4trids Quote
There's likely nothing wrong with your lens or your camera. In the first picture the paper was not perpendicular to the the sensor plane. The left side focuses closer because it is closer. In the second picture, the paper is not flat, so it might not be useful either.
I see what you mean but believe me I repeated the shot over and over with the camera perfectly perpendicular and the results are the same.
12-27-2010, 04:20 AM   #12
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QuoteOriginally posted by nicolas1970i Quote
When I got my K-7, I noticed that it front-focused with a few lenses that were spot-on with my K20D, so I concluded it must have come from the body. I sent it for warranty service and it came back perfect.
At the time, I was told by the people who did the repairs (here in France) that there had been quite a few K-7s sent back for the same issue (they also said that front/back focusing problems affected other camera brand - they also service Nikon, Canon etc... - so it wasn't Pentax-specific).
They also explained that it wasn't a linear problem, i.e. it wouldn't affect different lenses in the same way, which is why some lenses that were perfectly adjusted on my K20D were still as good on my K-7 (like my Tammy 90mm macro).
If I remember correctly, they were three factors that played a part in this: the sensor plane (it needs to be perfectly adjusted down to a small fraction of a millimetre), the angle of the mirror (which was sometimes off by a few degrees!), and the angle of the surface where the AF sensor sits.
Ok so this is a common problem, that's good because they'll know how to fix it. I just hope that the service in Romania does this without having the camera shipped to another country which is time consuming.
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