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04-05-2011, 03:55 PM   #1
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Replaced focus screen, some questions

I replaced my focusing screen with the split prism from jinfinance yesterday. I re-used the original shim, it was very easy to do. It seems to be accurate, except for macro.

I have a Sears 135mm f2.8 macro lens (Ricoh pin removed). It's an A lens, and it has the close focusing rotating front element. Well, the focus screen seems to work okay when the lens is normal... focusing distance a yard and a half or so. But when I rotate the front element all the way, the focusing screen isn't even CLOSE. Not a little off, I mean, way way way off. We're talking a good 15 degrees more rotation of the focusing ring.

The camera finds focus fine, it beeps at me when the image in the focusing screen is still a long way from focused, and live view confirms, the camera knows what its doing.

Now, is this because of some strangeness with the rotating front element, or because I don't have any other close-focusing lenses and any shim discrepancy is amplified up close?

Thanks.

Charles.

04-05-2011, 06:16 PM   #2
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I expect you will need to reshim the screen. Focus errors tend to be magnified at close distances.
04-05-2011, 07:16 PM   #3
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Great, I bought this one because it wasn't supposed to need shims. Note that the focus problem doesn't exist on my 50mm f1.7 lens, which can focus around a foot (closer than the 135mm), but is not 1:5 like the Sears.

Charles.
04-05-2011, 07:58 PM   #4
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Well, that changes things doesn't it? I have a hard time blaming the lens, since the camera seems able to find focus with it, but do you have access to a different camera body that you can try it on?
Failing that, try reseating the screen on the offchance that makes a difference.
Also, does the screen have a focusing aid? How does it agree with the rest of the screen?
It's a curious problem, that's for sure.

04-05-2011, 08:41 PM   #5
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focusing aid? The screen is a dual split prism.

The problem seems significantly better stopped down, but wide open the split prism is quite clearly not focusing at the same place the camera's focus indicator is.

Charles.
04-05-2011, 08:58 PM   #6
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The jinfinance screens have a microprism collar around the split-image centre, if that's what's meant by "focussing aid". I have one fitted to my K-5, and haven't noticed any issues with a difference between it and the auto-focus confirmation - now you've raised the question, I'll have another look at it.
04-05-2011, 10:53 PM   #7
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it also overexposes by a full stop, even with matrix metering after installing this screen. At least, at f2.8 in both the Sears 135mm lens and my Tamron 28-75mm. But that's what exposure compensation is for.

I've always had a very hard time focusing with microprism, so I can't really tell. That's the biggest reason I got rid of my K1000 body in favor of the Dakota/Ritz camera body for shooting film. Well, that and the light meter on the Dakota is much, much better.

Charles.

Charles.
04-05-2011, 11:20 PM   #8
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Hrm, okay. So, I mounted a 50mm f1.7 manual lens on the front of the 135mm with a poor man's reversing ring (aka: tape). With the front element of the 135mm set normal, focus lock in the camera and focus shown on the focusing screen are identical. RAZOR thin depth of field at just about 1cm from the end of the lens. So... it's got to be that strange rotating front element that's the cause, right?

Charles.

04-06-2011, 06:25 AM   #9
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QuoteOriginally posted by ChopperCharles Quote
Hrm, okay. So, I mounted a 50mm f1.7 manual lens on the front of the 135mm with a poor man's reversing ring (aka: tape). With the front element of the 135mm set normal, focus lock in the camera and focus shown on the focusing screen are identical. RAZOR thin depth of field at just about 1cm from the end of the lens. So... it's got to be that strange rotating front element that's the cause, right?

Charles.
Must be, that's really funky...

Just curious, with the 135mm under macro, does the normal area outside the split/microprism look out-of-focus, or is it just the prisms that are wack?
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