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12-14-2007, 06:13 PM   #1
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Different focus with non-center sensors?

I have been playing with my K10D and FA 50mm/1.4, and it seems to me that I get a different focus setting depending on which of the focus sensor points I select. Specifically, I was trying to do a 'quick and dirty' sharpness test using a sheet of newspaper taped to a wall, and my K10D on a tripod with shake reduction turned off. I started with the center sensor selected and the lens set to f/1.4 about 1 meter from the wall. ASA set to 100. First shot was blurry all over! Switched to manual focus and was able to get a nice sharp imaged capture... switched to 'SEL' for the AF point, and found that the upper and lower AF points gave me sharp images (I passed my hand in front of the lens to pull the focus off, then let it re-lock on the newsprint). In fact all eight of the points surrounding the center point gave sharp images. Only the two 'outside' points had trouble providing a sharp image. The misbehavior of the central point was very repeatable, as was the correct focus from the other points.

Interestingly, yesterday I performed Tim Jackson's front/back focus test and found no front/back focus issues with this same lens - though overall the image at f/1.4 seemed pretty soft. I also did Tim's test with a Pentax 100mm f/3.5 macro, once again with no Back/front focusing issues. I did all of those tests using the center AF point!

So, has anyone had a similar experience? Any suggestions as to what might be going on? Thanks for any enlightenment or suggestions for other things to try...

Curt

12-15-2007, 01:10 AM   #2
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QuoteOriginally posted by MNCurt-K10D Quote
I have been playing with my K10D and FA 50mm/1.4, and it seems to me that I get a different focus setting depending on which of the focus sensor points I select. Specifically, I was trying to do a 'quick and dirty' sharpness test using a sheet of newspaper taped to a wall, and my K10D on a tripod with shake reduction turned off. I started with the center sensor selected and the lens set to f/1.4 about 1 meter from the wall. ASA set to 100. First shot was blurry all over! Switched to manual focus and was able to get a nice sharp imaged capture... switched to 'SEL' for the AF point, and found that the upper and lower AF points gave me sharp images (I passed my hand in front of the lens to pull the focus off, then let it re-lock on the newsprint). In fact all eight of the points surrounding the center point gave sharp images. Only the two 'outside' points had trouble providing a sharp image. The misbehavior of the central point was very repeatable, as was the correct focus from the other points.

Interestingly, yesterday I performed Tim Jackson's front/back focus test and found no front/back focus issues with this same lens - though overall the image at f/1.4 seemed pretty soft. I also did Tim's test with a Pentax 100mm f/3.5 macro, once again with no Back/front focusing issues. I did all of those tests using the center AF point!

So, has anyone had a similar experience? Any suggestions as to what might be going on? Thanks for any enlightenment or suggestions for other things to try...

Curt
Try the test again, but make sure the center point actually has at least one printed letter to see. I suspect you might have been between words with the center AF detector.
12-15-2007, 01:41 AM   #3
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QuoteOriginally posted by Canada_Rockies Quote
Try the test again, but make sure the center point actually has at least one printed letter to see. I suspect you might have been between words with the center AF detector.
Sounds quite reasonable, so I would double check it...
12-15-2007, 09:17 AM   #4
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Thanks for the replies. I will repeat my tests again today, but correct me if I'm wrong, but isn't the center AF detector actually two crossed linear arrays (one vertical, the other horizontal) extending roughly the diameter of the central "spot" circle in the viewfinder? Hence the probability of there being a "+" shaped blank where I pointed the camera is small... however I suppose the average contrast in the center area could be could be limited. Perhaps I should make up a test target of just parallel lines (vertical and/or horizontal), or perhaps a square grid. By the way, I notice that my 50/1.4 is not a very contrasty lens, particularly wide open... this could be a further contributor, I suppose.

12-16-2007, 08:42 AM   #5
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RiceHigh's Pentax Blog: Focus Calibrations for (Pentax) (D)SLR Bodies and Lenses

QuoteOriginally posted by MNCurt-K10D Quote
I have been playing with my K10D and FA 50mm/1.4, and it seems to me that I get a different focus setting depending on which of the focus sensor points I select. Specifically, I was trying to do a 'quick and dirty' sharpness test using a sheet of newspaper taped to a wall, and my K10D on a tripod with shake reduction turned off. I started with the center sensor selected and the lens set to f/1.4 about 1 meter from the wall. ASA set to 100. First shot was blurry all over! Switched to manual focus and was able to get a nice sharp imaged capture... switched to 'SEL' for the AF point, and found that the upper and lower AF points gave me sharp images (I passed my hand in front of the lens to pull the focus off, then let it re-lock on the newsprint). In fact all eight of the points surrounding the center point gave sharp images. Only the two 'outside' points had trouble providing a sharp image. The misbehavior of the central point was very repeatable, as was the correct focus from the other points.

Interestingly, yesterday I performed Tim Jackson's front/back focus test and found no front/back focus issues with this same lens - though overall the image at f/1.4 seemed pretty soft. I also did Tim's test with a Pentax 100mm f/3.5 macro, once again with no Back/front focusing issues. I did all of those tests using the center AF point!

So, has anyone had a similar experience? Any suggestions as to what might be going on? Thanks for any enlightenment or suggestions for other things to try...

Curt
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