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05-24-2011, 07:35 AM   #1
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A-lens front focus but kit lens ok - Why and solution?

I have been investigating whether my K-r suffers from the FF issue and have found out that it is ok. It has a slight front focus on every lens I test, but that is adjustable with the AF fins tuning (the 2 mm mark has a tendency to be more in focus than the 0 on the internet focus chart I used). The body and kit lens had been to the repair people here in Sweden to calibrate the AF.

It shows the same behavior with my other lenses. All but one - my SMC Pentax-A 50/1.7. Of course its not an AF lens, but when the green hexagon shows it is in focus it is actually not, it focuses 12-16 mm in front of the target (more in flourecent and tungsten, less in daylight). With the correction set to -10 it is still 6-10 in front. So I guess it is something wrong with this lens.

Is there something that can be done or do I just have to live with it? Since it is manual focus I of course have to depend on my eyes anyway, but that focus confirmation can be quite handy.

05-24-2011, 08:22 AM   #2
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Just a thought, is this the only lens you are manually focusing?

If so then you might want to check the diopter settings and the accuracy of the focusing screen. Neither of which are used by the auto focus system.

To set the diopter: take off the lens, point the camera at a bright neutral background and move the diopter slide until the focusing screen is in sharp focus.

To check the focusing screen: compare the results focusing manually using Live View and using the focusing screen. They should be identical but if the focusing screen is off then the Live View will be more accurate.

You will need to use a tripod, remote or timer and a good target. But I'm sure you already knew that.
05-24-2011, 09:32 AM   #3
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QuoteQuote:
Just a thought, is this the only lens you are manually focusing?
Good point and some great tips! I did try out another manual lens though, a Sigma Mini Wide II 28/2.8, and it behaved like the kit zoom with just a hint of front focus. Should have mentioned it my first post. So I guess the 50/1.7 must be the culprit
05-24-2011, 05:41 PM   #4
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QuoteOriginally posted by LIJ Quote
Good point and some great tips! I did try out another manual lens though, a Sigma Mini Wide II 28/2.8, and it behaved like the kit zoom with just a hint of front focus. Should have mentioned it my first post. So I guess the 50/1.7 must be the culprit
Modern focusing screens just don't have the resolution for accurate manual focus of faster lenses.
With your f/2.8 lens, there is enough depth of field inherent in the lens to mask the fact, but in faster lenses, this isn't the case.
The solution is, unfortunately, fairly expensive, in that a split image screen should be installed and calibrated for accurate manual focus.

05-25-2011, 01:34 AM   #5
LIJ
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Modern focusing screens just don't have the resolution for accurate manual focus of faster lenses
But is it really the focus screen that is the problem? When I focus using only my eyes I get the focus more or less spot on, which indicates that the screen does its job ok, but when I adjust the focus so the green hexagon appears the lens front focuses (even though the green hexagon indicates correct focus on other manual and AF lenses). I was under the impression that the focus sensor had nothing to do with the quality of the focus screen. And the sensors works ok with other lenses. Therefore my conclusion that its something with the lens. Can an lens be adjusted/calibrated in some way?
05-25-2011, 01:36 AM   #6
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AF confirmation for manual lenses is an estimate at best- as others have said, a split screen would be the only ideal solution.
05-25-2011, 06:07 AM   #7
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I read in the other forum about a member that adjusted the focus of a manual lens. Seems to be very tricky, requires disassembling the lens. He admitted that he didn't get it perfect, but got it closer than it was before. I wouldn't try it.
05-25-2011, 06:25 AM   #8
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Perhaps you are approaching focus from the same direction each time and overshooting a bit. Purposefully overshoot then slowly correct so you approach focus from the other direction....see if that makes a difference.

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