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02-20-2011, 07:52 PM   #1
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film noob ME viewfinder question

I recently decided to give film shooting a try & picked up a cheap old Pentax ME body (from a forum member). Been fun playing with it this weekend. I mostly was using a couple M50 lenses, but then, just for grins, tried putting my AF Quantaray 28-90 zoom on it. What I noticed was while using the AF zoom, the top half of the center circle in the viewfinder goes black, leaving the bottom half to adjust focus. With my manual lenses, it used the split-prism (is this correct?) which you adjust by aligning the two halves of the center circle. Anyway, just curious why the viewfinder would change like that. Thanks.

Paul

02-20-2011, 07:59 PM   #2
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That's a common phenomenon when using a split-prism finder with a relatively small-aperture lens. Split-prism viewfinders are rare on modern cameras for two reasons:

1. "slow" zooms are much more common now than a few decades back, when "fast" primes were the norm;

2. most people use autofocus and don't care about having focusing aids in the viewfinder.

Someone else might be able to supply the technical reasons. I can just tell you there is nothing wrong with the camera.
02-20-2011, 08:07 PM   #3
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...what he said...

+ 1 more...

3. Focus aids readily show deficiencies in the AF system.


Steve


(Does not trust either AF or Focus confirm for fine work...)
02-20-2011, 08:22 PM   #4
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There are others here that are far more articulate than me,
split prism is 'splitting' light,if there isnt enough light available to meet design criteria
of split prism and subject is out of focus, result is what you see.Can also happen
in-focus.Open apeture up to minimum(wide open) and point at bright lamp or
even bright picture on telly,everything should be fine,both halves seen.Stop down
to F11 and one prism half or both sometimes are blacked out.
Why all those super fast len's cost super big money.
Usally begins around F5.6,say outdoors in dark shade,for example
Try addressing viewfinder to eye differntly (up/down) may help somewhat
Might indicate slight need for (C)leaning,(L)ubrication,(A)djustment on an older body
like ME,no big deal.mostly cleaning is all.Sure some other folks here "in the know"
can point right direction.

02-20-2011, 08:48 PM   #5
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Original Poster
OK, the Quantaray 28-90 is 'slow', max aperture f/3.5, and I was indoors, pointing at the window. I don't plan on using it with the ME, I have some decent manual lenses for that. Thanks everyone for your input!

Paul
02-21-2011, 02:16 PM   #6
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Just to add another two bits in. I have had similar issues when using my Vivitar 28-210 with my program plus. But usually it is not insurmountable (it used to be "permanently" fixed to the front of my camera). As Bill said, under high light situations I had no issues, as the light level dropped I had more problems.

One of the things that you may want to keep in mind if you want to use the lens is that it is only f3.5 at 28. Fully extended the aperture effectively closes reducing the amount of light leading to your issue. So this leaves you to either futz around with your eye level or only use the matte background or micro-prism ring to act as focusing aids (another workable solution if you want to use your lens).

Now I hardly use my Vivitar on my MF cameras I typically only use 3 fast(ish?) primes which solve my problem (they also fit better in my satchel).
02-21-2011, 02:16 PM   #7
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Split-prism physics

Split-prism viewfinder black out physics are discussed with links in this thread:

https://www.pentaxforums.com/forums/pentax-film-slr-discussion/78057-prism-blackout-why.html

http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

My understanding is that split-viewfinders and prism cut focusing screens gather light from wider fields of view. Since longer lenses and smaller apertures decrease the field of view, the prisms end up looking at the inside of the lens.

Moving your eye around in relation to the viewfinder may help to some degree prior to reaching the hard black-out point with any particular lens. But physics is the law and there's not really any way around it.

If you have a good zoom lens you can focus at it's minimum zoom and then ramp it up to to frame size desired for that particular shot's composition.
02-21-2011, 02:27 PM   #8
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You can still focus with the ground glass and perhaps even with the microspism collar surrounding the split screen, when the split image goes black. There is no reason to not use your zoom.

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