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02-24-2011, 05:04 PM   #1
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Focus test with split screen-Do I need to calibrate?

Hi all.

Recently I replaced the OEM screen in my K-100D with a $15 Chinese split screen from eBay. I laid out a yardstick at an angle (with the 1 inch mark toward the camera, 36 inches farthest away) and manually focused on the 15 inch mark & took shots on a tripod with the lens wide open in all shots. Here are the lenses I used:

Pentax 18-55mm 3.5-5.6
Cosina 28-210mm 5.6-6.3
Samyang 100-500mm 5.6-7.1

With all lenses, focus was dead-on at the shortest end of all zooms, but focused about an inch to the rear (focused on the "16", rather than the reference mark at "15") at the longest end of the zoom range with all 3 lenses.

Do I need to calibrate the screen, or are these results within normal tolerances?

Thanks,
Bob :-)

02-24-2011, 06:08 PM   #2
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QuoteOriginally posted by GibbyTheMole Quote

With all lenses, focus was dead-on at the shortest end of all zooms, but focused about an inch to the rear (focused on the "16", rather than the reference mark at "15") at the longest end of the zoom range with all 3 lenses.

Do I need to calibrate the screen, or are these results within normal tolerances?
Good to see you being methodical. As for calibration, I don't think there'd be much you can do, shimming the screen could only yield a compromise result because it's really a global effect AFAICT.

Interesting. Can you try the same test on AF with your 18-55? (I presume the other two are legacy MF-only lenses.)
That could be helpful to know if that 1 inch variance stayed the same at the respective ends of its zoom.

I suppose if you pinpoint targeted the 15" mark using MF while stable on the tripod, then spun the focus ring to make it off, then switched the camera onto AF and let it makes its own focus decision, does the same 1" difference stay?

.R.
02-24-2011, 07:10 PM   #3
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Interesting thought. I may give that a go & see what the result is. The Samyang is strictly a MF lens, but the other two are AF.
02-25-2011, 06:33 PM   #4
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Try to do the inverse. Let AF focus your lens and then check for: 1) if the focus is off, 2) if the split screen says it's off.

02-25-2011, 06:56 PM   #5
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For myself, I'd prefer it to be more accurate at the long end, but if you calibrate the AF to the long end you can kind of have the best of both.
I'm not sure if I'd bother reshimming the screen for that much error.
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