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12-19-2009, 03:36 AM   #1
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FA 50 1.4 front focus issue

hy pentaxians

i have a little concern with my fa50m 1.4
i just realized it miss focus, focusing a few cm front of focus point
it is even worse with higher distance to the subject

i did bump that lens accidentally on a wall while walking in the streets a week back, although it hit the rubber focus ring and did not leave any visible marks.

after some tweaking, i find on my k20d i have to adjust the focus to -10 to get to the focus right
it seems the km doesn't have AF adjustment or i didn't find it in the menu.

is there anything to do about that lenses? would it need some repair/adjustments
was it always that way and i didn't notice because i wasn't shooting much at very wide aperture (i do remember having problems focusing before but thought i was the source of the problem,... or maybe not)

any advise welcome

thanks

12-19-2009, 04:08 AM   #2
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You can enter the debug mode of K-m to do a global setting but then all your other lenses will be affected as well. -10um is really not a lot btw. You might wish to redo the tests just to make sure because AF errors exist and might not be BF/FF issue.

Debug mode
12-19-2009, 04:18 AM   #3
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thanks for the tip

actually -10 is the smallest value i can set on the k20d
i get ok results with that. but would it be possible to set lower values?

i've done quite a thorough test i think
repeated it, tried different targets, and it is obvious without that adjustement the focus target is blurred.

thanks
12-19-2009, 04:21 AM   #4
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+/-10um is the minimum value you can set. If you are unable to achieve sharp image with this increment, perhaps the problem lies somewhere else. It could be the method was flawed or the lens was optically impair. How did you do the test btw?

12-19-2009, 05:20 AM   #5
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tripod, center metering f2 and a textured target
central focus point, the target is the right end of the lip
distance is about 1m
it's not that bad at this distance, and is very well corrected with -10 adjustment
but with subjects further away it gets worst


100% crop

no adjust:



-5 adjust



-10 adjust

12-19-2009, 06:01 AM   #6
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Can you show a resized uncropped shot? Just by looking at the cropped pics, one thing I have noticed is the target choice is not very good because they aren't flat. The camera can pickup any point within the wide arear sensor to AF on. A good way to test AF is to focus on some distance high contrast target, like building; or something flat if for 1-3 metres. I personally prefer doing AF calibration with distant target because it allows less margin for error.
12-19-2009, 03:56 PM   #7
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Right, you need to use a good focus test chart or an *extremely* well controlled focus test of your own devising. What usually happens - an clearly seems to be the case here - is that there are too many things in the frame the camera could decide to focus on, even when choosing a specific focus "point" ("large oblong" would be a more accurate term). In these crops, it seems pretty clear the camera simply chose to focus on the brighter, higher-contrast fabric in front of the stuffed smiley face you presumably intended for it to focus on.
12-24-2009, 06:46 AM   #8
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hi everyone and thanks for your answers
they reassured me

i've been doing other test and found it seemed to work much better
although it seems sometimes, it just focus perfectly
and some other times, it just doesnt work
i'm not sure why, perhaps lighting conditions.

anyhow, its Xmas and no time for testing,
of course this evening i'll try again doing some portrait in low light and see how it behaves.
thanks again for your precious advises
and merry Xmas

P

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