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05-18-2009, 04:42 PM   #1
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K20D focus adjust

was this feature designed for prime lenses ony, cause with the zoom lenses i get different readings for different focal lengths on the same lens

Dave

05-19-2009, 12:34 AM   #2
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That normally happens when the AF sensor in the camera is misaligned.
But minimal variation is allowed with zoom lenses.
05-20-2009, 08:46 PM   #3
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i never knew i had a front or back focus problem until i down loaded a test chart and tried it out
should i take this on it even gives fluctuating results if distance from test chart to lens changes

Dave
05-21-2009, 06:18 AM   #4
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The test chart also gives me different results with different shooting distance. What I did instead was just shoot around town at the widest aperture for 30-40 shots and use the results for AF adjustment. Ends up with about 90% accuracy with the correct AF adjust setting.

05-21-2009, 07:35 PM   #5
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well i took some time off today and adjusted the autofocus to suit the test chart,
and wow!!!!!!!!!!!!!!!!!
99.99999% of all the shot taken after were wayyyyyyyyyyyyyyyyyyyyyyyyyyyyyy out of focus

Dave
05-22-2009, 01:47 AM   #6
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QuoteOriginally posted by dafiryde Quote
well i took some time off today and adjusted the autofocus to suit the test chart,
and wow!!!!!!!!!!!!!!!!!
99.99999% of all the shot taken after were wayyyyyyyyyyyyyyyyyyyyyyyyyyyyyy out of focus

Dave
The test charts are unusable from my point of view. Why?

Because they are angled. So, an out of focus image might(!) be due to a real AF problem (Back/front focusing) - but it could also simply be caused, by the typicall slight misalignment of the real AF sensor point and the red viewfinder indicator - which is very common. Also the real AF sensor points seem to be bigger than the indicators, so you never really are able to know, at which part of the angled scale your camera focuses!

The only reliable way to measure for front or back focus is, to take test shots of flat surfaces, parallel to the camera.

Ben
05-22-2009, 04:29 AM   #7
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QuoteOriginally posted by blende8 Quote
That normally happens when the AF sensor in the camera is misaligned.
But minimal variation is allowed with zoom lenses.
AFAIK, it is completely normal for most zoom lenses to require different focus adjustments for different focal lengths settings.

Micro adjustment also depends on the distance to the target, independently of whether or not the lens is a prime.

With any Pentax DSLR AF before the K-7 make sure you don't do the test chart alignment under Tungsten light. This will yield backfocus under daylight conditions.
05-22-2009, 10:56 AM   #8
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After I had my K10 AF sensor recalibrated, my DA* zooms worked just fine over the full range.
Before, they were different for various focal length.
I have tested this extensively.

Other fellows reported the same.

It should be noted though that most people do the focus tests wrongly.
All those 45 degrees targets are just BS.
Test against a target parallel to the sensor.

05-22-2009, 12:24 PM   #9
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QuoteOriginally posted by blende8 Quote
After I had my K10 AF sensor recalibrated, my DA* zooms worked just fine over the full range.
Before, they were different for various focal length.
I have tested this extensively.

Other fellows reported the same.

It should be noted though that most people do the focus tests wrongly.
All those 45 degrees targets are just BS.
Test against a target parallel to the sensor.
i did a test with playing cards stuck into a long sheet of styrotex at 1" apart and my lenses showed no front or back focus results.
so your right, those 45 degree targets are BS

Dave
05-22-2009, 09:50 PM   #10
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I find it easy to make focus adjustments with my Split Prism. Between that and the KPS 1.35x, I can clearly see precise focus and, of course, I can clearly see when front & back focus conditions exist.

If I have a front or back focus problem with a lens, I simply calibrate it to my Split Prism--it is quick and accurate. I think the margin for error is much better doing it this way than using a focus chart.

Obviously, you need to determine the Split Prism provides spot-on focus first, but that is easy to confirm.
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