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08-04-2010, 01:03 AM   #1
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Focus screen adjustment

In my K10d I have some version of an eBay Chinese focus screen. When I am working on close up subjects there is a slight degree of error to what the screen is reporting. I'm not 100% sure on which is front focus and which is back focus - heck, it took me years to sort out near & far sighted, and I wear glasses! I will do my best to describe the problem When the split image shows something 2 feet away as in focus, it is really focused to 2.05 feet. (measurements are a guesstimate)

So what is the correct term for this? More importantly, is the problem:

a. The screen is too thin, and will need to be shimmed to be 100% accurate, or
b. The screen is too thick, and the error can only be cured by replacing the screen

If it is too thick, or if it is just too close to "fix" then it is not a big deal, as I can work with it as is. It's not even noticeable from f3.5 on (at 50mm), and is rarely noticeable from f2.8. On the other hand, every once in a while it will throw off a f1.7-2.4 shot, or a f2.8-4 (5.6?) shot from my 135mm. Therefore if it is a simple fix then I would like to take a stab at it.

08-04-2010, 01:09 AM   #2
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Wouldn't replacing the screen with a genuine one resolve both problems? You would likely have to remove it to examine it anyway.

Adam
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08-04-2010, 06:33 AM   #3
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You will need to shim the screen in. I expect a set of K20 shims will work, since the screens are the same.
08-04-2010, 10:30 PM   #4
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Seconding the shim statement. This is especially true if you've managed to get the AF dialed right and now can't trust when the camera says something is in focus and your eye disagrees.

The process is a little more delicate that changing the focus screen as the shims are held in place by a small catch that sits above the focus screen.

Pentax sells the shims in sizes A-E. FWIW, my cameras both came with C and needed E.

08-05-2010, 06:19 AM   #5
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This is the Pentax part numbers for the K20 shims:

76830.M02200A - 0.15mm
76830.M02200B - 0.20mm
76830.M02200C - 0.25mm
76830.M02200D - 0.30mm
76830.M02200E - 0.35mm
76830.M02200F - 0.40mm
08-05-2010, 12:02 PM   #6
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QuoteOriginally posted by Adam Quote
Wouldn't replacing the screen with a genuine one resolve both problems? You would likely have to remove it to examine it anyway.
But the one I put in has a split image.
08-05-2010, 01:05 PM   #7
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QuoteQuote:
When the split image shows something 2 feet away as in focus, it is really focused to 2.05 feet. (measurements are a guesstimate)
What you described is back focus.

To correct this, I think you have to remove the shim or replace with a thinner shim. I mean, you would want the focus screen to move a bit further back to match the distance to the sensor.

To quantitatively measure the amount of back/front focus, google "focus test chart" and follow the instructions.
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