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10-14-2010, 08:57 AM   #1
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K-7 focusing problems

I just sent my camera for service, because it tends to focus behind the object. Did some focus tests at home and came up with that the camera seems to focus OK on flat surfaces like in front of a wall. But when there's depth in the picture the camera often focuses on objects 5-10cm behind the one I aim for at 50mm or less. When I go above 100mm it seems to get corrected. Compared with a few lenses, same shit. Maybe gets corrected because the object grow? But I've tried against small things in above 100mm and it's fine. The service person said the camera gets problems with AF in wider angles, but in 50mm?

The objects I tried on are about as big as the circle around the center af point or bigger. It should be able to lock on an object that big on every pic? - the Pentax service guy said the Pentax AF isn't inferior to other brands. My gf's nikon d5000 does better.

What do you think is the problem? Anyone heard about this before please share.

Attaching an example, this is rather extreme, but it did happen. Maybe not the best example, but I think it should nail this situation every time and not focus on the object behind. Not OK for a camera in this class imo. Sigma 50-150 f2.8 at 55mm 2.8 iso 200 1/400

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10-14-2010, 09:07 AM   #2
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It's not unusual for DSLRs to have front or back focus issues. This is not restricted to Pentax.

All DSLRs and lenses are built to a '+ or -' tolerance. In some instances camera + lens tolerance = front focus; another lens might give back focus.

Pentax cater for this by providing a user adjustment in-camera.

Look for AF adjustment in your manual and in Custom Setting in-camera.

Use a focus target and each of your lenses in turn, making adjustments as required. The camera will store these adjustments per lens in Apply One mode, so each time you fit a particular lens the camera will apply the associated focus adjustment.
10-14-2010, 09:08 AM   #3
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Sigma lenses in general are trouble makers. They always had problem with Canon and now they are the same with Pentax. You should perhaps visit Sigma service instead of Pentax one...

FF/BF correction in camera menu didn't help at all?
10-14-2010, 09:33 AM   #4
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Same problem with 4 lenses.

Over 100mm the problems is gone in the same tests as when 50mm fails.

No problems on flat surfaces like straight on a wall (quick test only)


Last edited by fikkser; 10-14-2010 at 09:43 AM.
10-14-2010, 09:35 AM   #5
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The Sigma 50-150mm is known to have issues when focusing at or near the min focusing distance. Send it for service.
10-14-2010, 09:36 AM   #6
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QuoteOriginally posted by fikkser Quote
Same problem with 4 lenses.

Over 100mm the problems is gone in the same tests as when 50mm fails.

No problems on flat surfaces like straight on a wall (quick test only), Problems with objects a few centimeters away.
What are your 4 other lenses? Sigma lenses are well known to either back focus or front focus.
10-14-2010, 09:39 AM   #7
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pentax 18-55 wr, 50-200 wr and 16-45
10-14-2010, 11:43 AM   #8
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Hi fikkser,

In your example, I'm assuming you used center point focusing, and the camera focused on the highest contrast border within the AF sensor area, the back edge of the lens cap. It will also bias towards a contrast border that is closer to perpendicular to the direction to either of the cross lines of the sensor. You are correct in assuming that the center focus area is approximately the size of the central ( ) in the VF.

From your example, I'd call this user error. Eliminate the lens cap from the sensor area by actuating AF with the camera pointed a touch higher, lock focus on the bottle, recompose, then shoot. As the photographer, it's your responsibility to make sure the camera is doing what you want it to, not leaving it to chance.



This one took multiple tries at AF before I could take the shot.

This is not to say that your lens/camera combo might not be correctly adjusted to each other -- but if the gear works properly with flat surfaces as you mentioned, then it points to a problem in your technique. . .

Scott

10-14-2010, 11:45 AM   #9
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The front/back issue can vary through a zoom lenses range. Either (a) correct for front/back focus at the most used focal length, or (b) check it at min, max and middle, then average the result.

For a much better explanation, try this site>PENTAX DSLRs: PART-1. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35
10-16-2010, 01:08 AM   #10
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Got the body back from Pentax service with the comment "back focus corrected". I get more pictures in focus now.

Thanks for answers.

And, yea snostorm, it could have been an user error, but I think the back focus makes it happen more often.
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