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Focus screen calibration Canon-P with Canon 50mm 1.4 calibration??
Old 02-18-2019   #1
imaje
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Focus screen calibration Canon-P with Canon 50mm 1.4 calibration??

Hi all

I need to pick your brains once again.. after having disastrous roll go through my Canon-P after what I thought was a successful calibration.

I first calibrated the lens' infinity focus by way of making my own Shim to replace the single (I assume not original) one that was in there already. All was well after testing it.

Using the 'Focus screen' on the film rails method I calibrated the rangefinder to what I was seeing whilst looking at the focus screen with a Loupe.

All good!

But, all my shots from my test roll.. are complete mess! Thinking maybe I knocked the camera at some-point, I re-checked the calibration and it still looks absolutely fine.

However.. it appears that the focus screen maybe be the problem.. when I use a piece of tape instead, the focus is indeed off.. which means I may have wasted many hours of calibration.

Does this sound right? Are focus screens the best DIY to calibrate?
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Old 02-18-2019   #2
Ko.Fe.
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Matted, ground glass. And understanding what is lens coarse and fine calibration is.
Canon, shmannon is irrelevant.
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Old 02-18-2019   #3
View Range
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There are two sets of rails. Did the focus screen rest correctly only on the inner rails with the ground glass side against the rails? It should work.
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Old 02-18-2019   #4
Peter Jennings
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You can make your own ground glass by cutting a piece of clear acrylic (Lexan, Perspex, etc.) to the necessary size and rubbing one side on some very fine abrasive. This way you make sure it fits perfectly on the inner rails.
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Old 02-18-2019   #5
johnf04
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Make sure the matted side is facing the lens. For the material, you can use a piece cut from a CD case.
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Old 02-18-2019   #6
Sarcophilus Harrisii
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Focus screens sound good in theory, but not all will sit at the correct height relevant to the film rails. Or may bridge the pressure point rails if the camera features these. As has been said a plain ground glass is the way to go but a loupe is essential for best accuracy.
Cheers,
Brett
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Old 02-18-2019   #7
Beemermark
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Quote:
Originally Posted by Peter Jennings View Post
You can make your own ground glass by cutting a piece of clear acrylic (Lexan, Perspex, etc.) to the necessary size and rubbing one side on some very fine abrasive. This way you make sure it fits perfectly on the inner rails.
Good old wax paper works fine and is very simple to use.
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Old 02-18-2019   #8
retinax
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So you fiddled with both the rangefinder and the lens? That's obviously not ideal, as long as you're not certain which one is out, because a) it makes things harder and b) you could end up with a body and lens that only focus correctly with each other. I hope you didn't (have to) change the gain of the rangefinder (the relationship between close and infinity focus). If you only had to adjust infinity, no big deal. I see two ways to go about this:
a) get actual infinity focus correct, regardless of the RF. I.e. shim the lens if needed. I've never found a ground glass and loupe to be quite accurate enough for that, closer distances are much easier, so you will need to shoot a test roll and scan/enlarge. Then you can adjust the RF at infinity.
b) adjust the RF at a close distance to match the focus in ground glass, this works a lot better than infinity because you can see the target coming into focus and going out of focus on the ground glass. Then focus the lens to infinity. If the RF is now also at infinity, you are good. If not, infinity focus of the lens needs to be adjusted. This way is preferably to me, because it works without repeated test shots ( I'd still take some to be sure).
Bot of these methods only work if RF gain is correct from the outset.
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