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06-24-2009, 02:16 PM   #1
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Focus Screen thickness ??

Hi,

I have often seen posts referring to the thickness of a focusing screen affecting it's ability to focus accurately. I'm confused as to why the thickness of a replacement (Split Focus) screen plays any part in the screens focusing accuracy

My understanding is that the image is focused/created on the frosted surface of the screen which is fitted so that the the frosting is on top and registers against the bottom of the penta prism/mirror box, The screens (frosted surface) distance from the lens can be adjusted with the shims (U shaped fitted between screen and mirror box) to adjust the lens to frosted surface distance so that it is exactly the same as the lens to sensor distance.

I can not see how the thickness of the screen plays any part, as the distance from the frosted surface to the lens is not affected by the thickness of the screen, the light just passes through it.

Will someone please put me right.
Greg
K200


Last edited by gregmoll; 06-24-2009 at 09:01 PM.
06-24-2009, 09:00 PM   #2
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Hi,

Could the Moderator please move this thread to the more appropriate Forum, namley
Pentax DSLR Discussion


Thanks
Greg
06-26-2009, 01:37 PM   #3
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I hate being ignored :>) it make me feel that I asked a stupid question but I would really like to know if the thickness of the focusing screen affects it's accuracy.
Greg
06-26-2009, 02:09 PM   #4
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Seems like it shouldn't, but the general consensus is that it does matter.

06-26-2009, 05:28 PM   #5
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I guess I thought it had to do with the focus screen aligning with the camera's "true" focus. The split screen may say you're in focus, when really you're a hair off. Thus, the use of shims?
06-26-2009, 06:45 PM   #6
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The focusing screen is held into position by the frame, which supports it from underneath. The focus screen itself is the upper surface. The thickness of the screen can possibly affect the position of the matte surface, and cause the apparent focus to be ever so slightly off. The distance from the lens to the sensor/film is not precisely identical to the distance from the lens to the matte surface of the focusing screen.
06-27-2009, 10:04 AM   #7
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Thanks for the replies, I wish I could understand exactly how it works.
Greg
06-27-2009, 02:40 PM   #8
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This is how I think it works:

Old type screen have a frosted side and would work as you said. Modern screens are different, they are frosted straight through to provide a brighter viewfinder. This means that the screen does not only capture the sharp picture, but also unsharp parts. I do not know, but I assume they therefore put the sharp image in the middle of the screen, therefore with different thickness the focus area moves (half the difference in thickness).

I also assume (have not yet tried this) when you put an old type screen in you will need to chim it a bit, so that the focus will end up where the middle of the old (plastic) screen were....

06-27-2009, 05:57 PM   #9
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That is roughly my experience, but this only seems to affect the K-series

I bought a cheapie from Hong Kong and it worked great on the DL2, which already had a shim, but on my new K20D the focus is off.

Get the split screen spot on and take the shot - check the picture and the focus is off.

However, without knowing how thick the shim should be, someone used a post-it note..., or what side it should be, I went back to the original screen.

Too much trial and error and the frame could break!!!


Dal
06-27-2009, 09:56 PM   #10
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Thanks for the ideas.

I wonder if anyone has actually had the experience of fitting a split focus screen and after properly calibrating it for perfect focus with a specific thickness shim then changed to another make of split screen (different thickness) using the same shim and then found that they had to recalibrate the focus with a different thickness shim.

I'm starting to think that this question of screen thickness affecting focus is a self perpetuating myth based on confusion about screen thickness with shim thickness at some in the past and which is now quoted as fact.

Greg
06-27-2009, 11:52 PM   #11
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QuoteOriginally posted by gregmoll Quote
I'm starting to think that this question of screen thickness affecting focus is a self perpetuating myth based on confusion about screen thickness with shim thickness at some in the past and which is now quoted as fact.
The image is being projected onto the upper side of the focus screen. That side is firmly sit on the metal frame which is fixed and must be calibrated from the other side (usually 3 spring-loaded screws). Now if you push it hard, you can actually move that fixed metal frame and alter the preset focus. But I personally cannot see how the centre focus could be affected by the screen thickness. The outer areas however, might be affected by its thickness as the light ray path will be longer as the thickness increases (I think). However, I don't think any end users actually did any tests to confirm the suggested theory. People just take repeated myths as facts sometimes. I did use MZ-M screen which is smaller and thinner in MX with 100% accurate centre focus (I am not equiped to test outer area focus).
07-14-2009, 09:56 AM - 1 Like   #12
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Some feed back.

I've now made two additional focus screens, i.e. I now have four screens.

Original K200D:- Thickness 1.33mm all over.
Jinfinance diagonal:- 1.11mm on the flats and 1.20mm on the split centre lens.
Pentax ME Super horizontal:- 1.19mm on the flats and 1.36 on the split centre lens.
Unknown K mount body, micro prism only:- 0.99mm all over. This is the brightest of all the screens.

All of these screens focus perfectly using the same shim, therefore I have not found that thickness affects the focusing, it's the shim thickness, which I had to adjust for the first split screen, which is important.

Shoot me down :<)
Greg
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