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01-05-2013, 11:15 AM   #1
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Acceptable or unacceptable fount-focus

My K5 is about a month old now. I've had my suspicions, but with the kit lens it was hard to tell. Today, when re-shimming my focus screen and doing careful measurements and using the catch-in focus with a 50mm f1.7 lens and a tripod, I can confirm that a) the original foucs-screen shim was twice as thick as it should have been and b) the camera's AF system is front-focusing. Even with the maximum (-10) set in AF fine adjust the best I can currently achieve is about 2-3cm FF at about 70cm. While this is never going to be noticeable with the kit lens, for a brand new K5, surely this is completely outside Pentax's tolerances? Time to contact Pentax I guess :-(

01-05-2013, 11:23 AM   #2
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QuoteOriginally posted by frstep Quote
My K5 is about a month old now. I've had my suspicions, but with the kit lens it was hard to tell. Today, when re-shimming my focus screen and doing careful measurements and using the catch-in focus with a 50mm f1.7 lens and a tripod, I can confirm that a) the original foucs-screen shim was twice as thick as it should have been and b) the camera's AF system is front-focusing. Even with the maximum (-10) set in AF fine adjust the best I can currently achieve is about 2-3cm FF at about 70cm. While this is never going to be noticeable with the kit lens, for a brand new K5, surely this is completely outside Pentax's tolerances? Time to contact Pentax I guess :-(
Two issues there. The focus screen has no effect whatsoever on AF, so would have nothing to do with catch-in-focus. And with a 1.7 or faster lens using the stock screen, it is probably impossible to MANUALLY focus super-accurately by eye no matter if the shims are perfect or not -- you need a better screen for accurate manual focus with fast lenses, period. Again, the screen does not affect AF or focus confirmation at all.

As far as the AF, how does the 50/1.7 do? Also front focuses?
01-05-2013, 12:22 PM   #3
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What he said, though I would add: If you are serious about manual focus...
  • Get a screen with split image, the stock screen offers a best-case focus precision of about f/4...not good if you are using an f/2 lens.
  • Shim only if needed and calibrate against live-view using a magnifying loupe. The AF system is much less precise than live-view or a split image screen.
  • When doing calibration, use a flat, high-contrast target, camera on tripod, sensor parallel to target

As for your new camera...if you have evidence that the focus is off, send it to Pentax. Don't mess with trying to shim it yourself. You may void your warranty. 2-3cm at 70cm distance is much more severe than what is normally the topic of discussion among the pixel-peepers on this forum.


Steve

Last edited by stevebrot; 01-05-2013 at 01:07 PM.
01-05-2013, 12:55 PM   #4
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QuoteOriginally posted by stevebrot Quote
What he said, though I would add:
  • Get a screen with split image
  • Shim only if needed and calibrate against live-view using a magnifying loupe. The AF system is much less precise than live-view or a split image screen.
  • When doing calibration, use a flat, high-contrast target, camera on tripod, sensor parallel to target
Ah, people replying here might not realise that:

a) I have a split screen, hence playing around with shims
b) I fully understand that AF has nothing to do with focusing screens.

Id was due to playing around with the above that it became apparent that the AF confirmation was way off the actual focus (as in what appears in digital preview or the final shot). My issue is with AF is that even with max fine adjust, it's still 2cm off at 70cm with a 50mm lens. At f1.7 the dof is less than this. I confirmed my tests by using catch-in focus, first starting a infinity, then starting at min: The AF is quite consistent about when it considers focusing is reached.

To be sure I was focusing on a point on the same plane, I repeated the test with a target directly at right angles to the camera. After getting the optimum focus according to AF, I can move the target 2cm towards the camera and take another shot... which is pin sharp compared to where the camera's idea as to where focus should be.

If I had a fast AF prime it would be very obvious that the camera was FF - 2cm is huge when dof is so shallow.

F.

01-05-2013, 02:18 PM   #5
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I think Pentax had a problem with this on the K-5. My K-5 - one of their first shipments - showed similar issues. My K-5 IIs was well calibrated out of the box. I'd send it in.
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