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10-29-2009, 11:29 AM   #1
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Prism Blackout. Why?

My camera has a split image focusing screen which although I love, whenever I use a telephoto lens, a tele-converter, or in certain lighting conditions the top portion of the prism "blacks out" and its hard if not impossible to really compose, or for that matter focus properly on the subject.

I have read that this occurs when the focusing screen doesn't "see" a sharp enough line on the subject to work and the manual for the camera says to use the outer microprism collar in this case which makes perfect sense. The only problem I have is that if my prism is blacked out how am I supposed to really see what Im doing outside that microprism.



The interesting thing is my newer P30 uses a diagonally split focusing screen which doesn't black out as bad as the horizontal one on my ME.


I would really appreciate anyone who could explain this to me technically.


Thanks MUCH!

10-29-2009, 02:31 PM   #2
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It has to do with the amount of light passing through the split prism - slower lenses (the usual consumer tele zoom) and tele converters (rob light) transmit less light than fast ones.


Try moving your eye around. You'll see you can switch which half of the split goes dark... and right in between is the sweet spot where you may be able to see both.

This btw is one reason why everyone doesn't love the split prism, but perfers a microprism instead - or even a plain ground glass.
10-29-2009, 06:17 PM   #3
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The wikipedia article on autofocus systems has a link to a document that explains prism blackout. See figures 11 and 12 in

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

(cited in Autofocus - Wikipedia, the free encyclopedia)

As Nesster suggests, try moving the viewfinder or your head up/down to find the sweet spot where both halves of the prism just barely work.

(BTW, welcome to the forum!)

Last edited by troyz; 10-29-2009 at 06:21 PM. Reason: BTW
10-30-2009, 02:04 AM   #4
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QuoteOriginally posted by Nesster Quote
It has to do with the amount of light passing through the split prism - slower lenses (the usual consumer tele zoom) and tele converters (rob light) transmit less light than fast ones.


Try moving your eye around. You'll see you can switch which half of the split goes dark... and right in between is the sweet spot where you may be able to see both.

This btw is one reason why everyone doesn't love the split prism, but perfers a microprism instead - or even a plain ground glass.
I wonder if we did a survey of those who love them and those who don't care for them we would find a clear difference between those of us who mainly use fast(er) primes and those of us who use slow(er) zooms. I suspect we would.

10-30-2009, 03:46 AM   #5
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Or are into macros for that matter, whether with extensions or the old kind which were f/4 or 3.5...

I grew up with microprisms, and I have to say I still prefer them.
10-30-2009, 04:29 AM   #6
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QuoteOriginally posted by Mike Cash Quote
I wonder if we did a survey of those who love them and those who don't care for them we would find a clear difference between those of us who mainly use fast(er) primes and those of us who use slow(er) zooms. I suspect we would.
Most of my manual focus lenses tend to be of the slower variety (35mm f:3.5, 28mm f:3.5, 135mm f:3.5, 50mm macro f:4, 24mm f:3.5, 20mm f:4, etc.). I use the Katz-Eye split focus screen. The blackout on this type of screen used to begin at 3.5 on film cameras. The Katz-Eye engineers seem to have extended this cutoff point a bit, making it more feasible for slower lenses like mine, especially the auto-aperture early K & M varieties. Besides this, when I have trouble as I stop down my M42s, I defer to the surrounding microprism collar, & the Matte viewing screen as a last resort. If all these options fail, I close my eyes & hope for the best.

Last edited by raymeedc; 10-30-2009 at 04:34 AM.
10-30-2009, 12:08 PM   #7
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Kerr's excellent article (http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf) explains why small apertures cause split prisms to go black. Somepeople are able to use the surrounding microprisms when this happens, but I haven't much success at that.

However, I find the K100D's AF detection system works well at smaller apertures so rely on the green hexagon when the split prism fails.
10-30-2009, 12:25 PM   #8
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I've only shot with split prisms with microprism collars around them. I find, however, that the microprisms also begin to fail with smaller apertures. Do microprism-only screens behave any differently from the collars?

10-30-2009, 01:01 PM   #9
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Strictly speaking, no, the mircroprisms start to show the half darkening too... but you don't have to look at the image with a microprism, by simply minimizing the shimmer.
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