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02-08-2010, 08:57 PM   #1
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K20D and F50mm f1.7 will not consistantly focus under f2.8

I tried out a F50mm f1.7 recently on my K20D to supplement the A50mm f1.4. Although all shots focused fine at f2.8 and smaller, I had about a 50% hit rate during testing for all shots at f2.4, 2.0 and 1.7. I tried it both with test charts and normal shooting where I could see the fore and backgrounds, both indoors and outdoors. All my other Pentax AF lenses focus fine with the camera but no lenses are faster than f3.5. The F50mm focuses fine on two other Pentax cameras, although none were a K20D.

So my obvious problem is where is the fault? The hit rate under f2.8 was randam and at times VERY off, like missing focus by 50cm on a 3m distance, or 10cm on a 2m shot, always in the back as if it hunted then did not come back in far enough.

Anybody else with a K20D run into this with fast AF lenses? Is there a fix besides learning to use my A50mm f1.4 better?

thanks,
lee

02-08-2010, 10:54 PM   #2
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QuoteOriginally posted by imtheguy Quote
All my other Pentax AF lenses focus fine with the camera but no lenses are faster than f3.5. The F50mm focuses fine on two other Pentax cameras, although none were a K20D.
Under tungsten light the K20D will consistently front focus and this will only show up with faster lenses.

QuoteQuote:
So my obvious problem is where is the fault?
Did you do your tests with a tripod? If you shake the camera the AF system can get a bit confused. Have you tried the AF adjustment in the camera? (Different adjustment number will be required for different ambient light temperature.)
02-08-2010, 11:02 PM   #3
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QuoteOriginally posted by asdf Quote
Under tungsten light the K20D will consistently front focus and this will only show up with faster lenses.


Did you do your tests with a tripod? If you shake the camera the AF system can get a bit confused. Have you tried the AF adjustment in the camera? (Different adjustment number will be required for different ambient light temperature.)
yes, both inside and outside, natural light, tungsten and flash.

Absolutely. Tripod used.

I used the AF adjusment for one lens but the others test out fine. Besides, the misses are so far off the AF adjusment would be no good even if they were consistant. But thanks for asking. I would love for it to be that easy.
02-08-2010, 11:11 PM   #4
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QuoteOriginally posted by imtheguy Quote
I used the AF adjusment for one lens but the others test out fine. Besides, the misses are so far off the AF adjusment would be no good even if they were consistant. But thanks for asking. I would love for it to be that easy.
If you find another relatively fast lens (you said your other lenses are f/3.5 or slower?), try it out. My guess is you'll get similar (bad) results.

02-08-2010, 11:26 PM   #5
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Post some samples. My standing guess is that the camera simly chose to focus somewhere other thn you wanted something else was in range of the focus sensor, and it legitimately chose that instead. I'd need to see sample images to prove this *wasn't* the case here.
02-09-2010, 04:28 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
Post some samples. My standing guess is that the camera simly chose to focus somewhere other thn you wanted something else was in range of the focus sensor, and it legitimately chose that instead. I'd need to see sample images to prove this *wasn't* the case here.
I can't show the hundred or so test shots to give you the feel of the randomness but I here are some notated samples. Just remember these were pulled from over a hundred shots taken over multiple days using every control I had for light, tripod and subject. Could the camera have chosen a different subject? Outside, maybe but then it lied about the focus point clearly shown with the red square (center only). Inside where I used a white wall or white paper there was no other point the camera could find. Thats why I used them and I tested before hand, like the wall, where the lens just hunted before I placed a target up there.

Sample of bad miss outside. This shot follows a spot-on focus on the gum ball. I then simply tipped the tripod back, refocused to infinity, and reset the focus point on the gum ball and using multiple half-presses, got the symbol indicating focus. Shooting in AF-S mode only. F50mm f/1.7 @f1.7


White wall at f2 from 2m away at about a 45 deg angle. Lens only hunted until I placed the spot, then with the red square on the spot and the in focus symble in the viewer, it still missed by about 10cm. I can distinguish the are in focus from a crop where the bumps in the wall are clear.


Shot from about 1.5m to white paper. Lens only hunted until flash chip placed. I marked the center of the focus and the chip is out of focus by only a few cm. The previous shot was front focused and the chip just made it inside.


Since this is the next thing you would like to see, but I only have it for this one sample at hand, the PhotoME designation of the focus point on the uncropped shot of the above sample.



Whatever suggestions on better technique or testing are fine but unless I can walk around and get better than 50% in focus at f/2.4, something is wrong.

I am just plain stumped.

Thanks for any suggestions or alternate interpretations. For now, I am back to the A50mm f1.4 for manual focus.
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