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02-28-2011, 02:29 PM   #1
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Split focusing screen & macro

Hey all.

Awhile back, I outfitted my K100D with a split screen from eBay. After shooting with different zooms, I noticed shots taken at the short end were uniformly in focus, while shots taken at the long end were slightly out of focus, with the focus point about 1/2" to the rear of where it should have been.

Today, I screwed a 4x close-up filter on my Pentax 18-55 zoom and tried a couple of closeups of a piece of wood, one shot at 18mm and the other at 55mm. The one taken at 55mm is noticeably softer than the one taken at 18mm, which is in perfect focus. This is demonstrated in the shots attached. Both were cropped to about the same size. When viewed at actual size, the 18mm shot is tack sharp, but the 55mm is very noticeably out of focus. There's also a mark to show where the focus point was on the screen.

If I were to calibrate the screen with a shim, would I want to add thickness to bring the image more in focus at the long range of the zoom, or would I want to use a thinner shim than the one that's in there now? With macro in particular, I usually shoot at the long range of the zoom.

Thanks,
Bobbo :-)

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02-28-2011, 06:57 PM   #2
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I'm not too sure but if i had to guess, the softness you're seeing at the 55mm range is just a consequence of the quality of the lens. This should be particularly true if part of the photo is sharp/in focus and the other parts are not.

Plus, IIRC, the sweet spot on the 18-55 is around 38mm or something.
02-28-2011, 08:27 PM   #3
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Back Focus or Front Focus?

QuoteOriginally posted by GibbyTheMole Quote
Hey all.

Awhile back, I outfitted my K100D with a split screen from eBay. After shooting with different zooms, I noticed shots taken at the short end were uniformly in focus, while shots taken at the long end were slightly out of focus, with the focus point about 1/2" to the rear of where it should have been.

Today, I screwed a 4x close-up filter on my Pentax 18-55 zoom and tried a couple of closeups of a piece of wood, one shot at 18mm and the other at 55mm. The one taken at 55mm is noticeably softer than the one taken at 18mm, which is in perfect focus. This is demonstrated in the shots attached. Both were cropped to about the same size. When viewed at actual size, the 18mm shot is tack sharp, but the 55mm is very noticeably out of focus. There's also a mark to show where the focus point was on the screen.

If I were to calibrate the screen with a shim, would I want to add thickness to bring the image more in focus at the long range of the zoom, or would I want to use a thinner shim than the one that's in there now? With macro in particular, I usually shoot at the long range of the zoom.

Thanks,
Bobbo :-)

From what I have read on the other forum by folks who have made a shim adjustment, a thinner shim in place moves manual focus backwards.

The shim is located above the screen or between the screen and the prism/viewfinder area, so one could say a thinner shim above the screen (as you look into the camera, bottom of the camera is down) will move the focus plane backwards.

So, you need to know if the mis-focus is to the front or to the back of the desired focus point.

Good luck and I hope this helps.

Ray
03-01-2011, 05:43 AM   #4
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Original Poster
QuoteQuote:
I'm not too sure but if i had to guess, the softness you're seeing at the 55mm range is just a consequence of the quality of the lens. This should be particularly true if part of the photo is sharp/in focus and the other parts are not.
Tests I've ran with 3 different zooms all resulted in a "backfocus" of about an inch at the long end of all 3 zooms.

QuoteQuote:
From what I have read on the other forum by folks who have made a shim adjustment, a thinner shim in place moves manual focus backwards.

The shim is located above the screen or between the screen and the prism/viewfinder area, so one could say a thinner shim above the screen (as you look into the camera, bottom of the camera is down) will move the focus plane backwards.
Yep, the mis-focus is to the rear of the desired point. So I'll try a thinner shim. Thanks! - Bobbo :-)

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