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11-08-2013, 02:49 AM   #1
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Problem with focussing screen

Hi!
I have a problem that I cannot find an answer for...
I bought a (Chinese) split prism focussing screen, because most of my lenses are MF and for the rest, AF on my old K10D sucks. But I found out, that it is more precise with some lenses and less with other. It is possible? I wanted it mostly for two lenses - a F1.2 Revuenon anf F1.4 Samyang. While the Revuenon is OK - the focussing screen (I use mostly the microprism collar) shows sharpness and I know the image will be correctly focussed. But when using the Samyang, I found out, that the best sharpness is obtained when the microprism collar is slightly defocussed in front of the subject. Isn't that a weird behavior, or is it normal? Or it's all just in my eyes? I would not be very happy if I would have to change focussing screens with changing the lenses, moreover, my two most used ones...
Thanks!

11-08-2013, 01:07 PM   #2
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Hi

It might be the screen needs shimming.


Jeff
11-08-2013, 01:59 PM   #3
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Thee was a thread running about a year ago about focus shift of fast lenses when stopped down. You might want to try some tests to see if it happens, although I have not seen it on my K10 with my 85/1.4
11-08-2013, 02:23 PM   #4
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Most likely answer #1. It is not just a matter of swapping screens, but calibrating the exact position of the screen with shims (or bits of scotch tape) so that the light path from the mirror to the screen is exactly the same as the mirror-up light path to the sensor. If they don't match, you'll focus on the screen and get front or back focus on the sensor. So you must do precise tests, and adjust the shims until you are happy.

11-08-2013, 05:46 PM   #5
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How about the lenses? If adjust the shims for only one lens then the other lenses will not have terrible BF/FF problem?
11-08-2013, 07:08 PM   #6
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QuoteOriginally posted by zany225 Quote
How about the lenses? If adjust the shims for only one lens then the other lenses will not have terrible BF/FF problem?
No. Not lens dependent. Well, you should use the fastest lens you've got to calibrate, as it will have the smallest sliver of in-focus area wide-open and you'll be able to get it spot on. Some screens (like the stock screen) anything faster than a particular aperture be very hard to tell the difference. In any case, if you can get it so you can nail focus manually with your fastest lens, it will also nail focus on all your other lenses...
11-08-2013, 10:34 PM   #7
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But in his case it seems his camera is fine with the f/1.2 and has BF for the f/1.4 lens.
I'm confused now.
11-08-2013, 10:45 PM - 1 Like   #8
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QuoteOriginally posted by zany225 Quote
But in his case it seems his camera is fine with the f/1.2 and has BF for the f/1.4 lens.
I'm confused now.
Simply put, I don't think he's actually right about that. He needs to do controlled tests. Basically the concept is simple -- throw a ruler on a table in front of the camera so it is going away from you. Now from in front of the table put the camera on a tripod or fixed somehow just a bit above table height. Now focus on a point on the ruler -- because of the extreme angle it should only be possible to focus sharply on a very small area -- like a single hash mark on the ruler. Make sure everything is locked down, note what the mark is, take the shot (at the widest aperture), now compare the focus of the actual photo and see if your chosen hash mark is in-focus or if the real focus point is in front or behind. Then adjust shims on screen to move it forward or back by a tiny amount, replace screen, repeat until dead on.

Now, it is true some (older, likely) lenses will have a focus shift when they are stopped down from wide-open. That's not the focusing screen's fault. If that is the case, you just have to mentally compensate or stop down before you focus, and hopefully there will still be enough light showing. If you do these tests while focusing at the widest aperture (which is what you normally do) and also shooting at that same aperture, then focus shift won't be an issue.

Oh, and before you start, set the diopter on your eyepiece properly for your eyesight and leave it there forever -- adjusting it after shimming the screen can throw things off for you a bit. If two different diopter settings seem about right, just pick one and stick with it. And if while doing manual focusing tests you find your eye getting tired and it is getting a bit harder to focus, stop and take a break. Do it with fresh eyes.

11-08-2013, 11:24 PM   #9
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Thanks for make me more clear.
Another question, does changing screen or adjusting shim make any effect on BF/FF when using auto-focus?

I think I read that thread, which talks about s-type split screen from Canon, but the thread was too way long and too many comments. I was too tired to read and to think along with those massive information. But I think they talked only for manual focusing case, right?

I have some auto-focus lenses that show BF, some lenses are fine, and some lenses show FF. So, I'm not sure how to deal with them.
11-08-2013, 11:49 PM - 1 Like   #10
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QuoteOriginally posted by zany225 Quote
Thanks for make me more clear.
Another question, does changing screen or adjusting shim make any effect on BF/FF when using auto-focus?

I think I read that thread, which talks about s-type split screen from Canon, but the thread was too way long and too many comments. I was too tired to read and to think along with those massive information. But I think they talked only for manual focusing case, right?

I have some auto-focus lenses that show BF, some lenses are fine, and some lenses show FF. So, I'm not sure how to deal with them.
Focusing screen has zero effect on AF. It does effect metering somewhat though.

AF adjustment is a whole other thing. The K10D doesn't have it (without entering "debug" mode), but all the newer models have AF adjustment you can set for individual lenses.
11-09-2013, 10:58 AM   #11
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Thank you for your answers! I was very surprised by what i saw, because I was also thinking that the front/back focus should not be lens dependant. You are probably right that it is due to focussing screen position and that more 'scientific' test are needed. I will update if I found out something new. Thank you all for your time again!
11-10-2013, 08:37 AM   #12
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Tried shimming the focussing screen with piecs of tape until both lenses seemed OK. Tested on a 2D target with fine pattern. The inconsistency I wittnesed earlier was probably only in my head Now it seems both are focussed well using the microprism collar (I found out the split center is to coarse for precise focussing on extreme apertures. You have to look at it directly, not in an angle and still it is not precise enaugh - may that have caused my confusion about difference in focussing both lenses?)
So this thread was really helpful for me! Thanks
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