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01-09-2013, 09:57 PM   #1
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Will a ground glass/split focusing screen affect autofocus on a K-5?

I've acquired a few large aperture primes I want to use with my K-5 (like a 50 1.2). It's damn near impossible
to accurately focus with the standard focusing screen supplied from the factory. Will changing out the focusing screen
cause af problems? If not, where can I get a good one, and any recommendations on changing it out without damaging the mirror,
prism or sensor?

01-09-2013, 10:15 PM   #2
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The auto focus sensor is beneath the mirror, so changing a focusing screen will not affect the auto focus in any way. It can, however, have an affect the metering because the light meter is above the screen. The split prism ones will severely affect the spot metering, but the plain ground glass ones will not.

If you're wanting a plain ground glass screen, similar to the original, but is a lot more accurate the Canon S type screen is one of the best. You can get them from Focusing Screen already cut to fit the K-5 with shims needed for calibrating it. These don't affect the metering in a negative way, some people notice improved metering when using the green button for stop down metering. They also provide instructions for installing and calibrating the screen.
01-09-2013, 11:14 PM   #3
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I think that ground glass screens don't work well with smaller aperture lenses.

I don't have firsthand experience in this regard, but you might want to look into it. It may not even be a problem to you.
01-09-2013, 11:24 PM   #4
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QuoteOriginally posted by Aegon Quote
I think that ground glass screens don't work well with smaller aperture lenses.

I don't have firsthand experience in this regard, but you might want to look into it. It may not even be a problem to you.
I would disagree with this. In fact, years ago, when I was shooting film, my second body was deliberately purchased with a ground glass focusing screen (it was a Pentax KX) because when I used my 400/5.6 or my celestron C90 (1000mm F11) the split image in my Ricoh X-R-2s would black out, same for the micro prism collar around the split image, so I lost full use of the center of the viewfinder. A ground glass screen is the only way to focus small aperture lenses.

As for metering, you should do a test with your camera, and its present focusing screen, with your lenses and check the linearity of exposure vs aperture. I use a uniformly lit block wall and let the camera meter at each aperture, then check linearity by measuring the median greyscale value in the histogram. It should be about 115-120 for correct exposure, and if it moves by +or-45 then you have had a one stop change in metering. Two stops is 90 greyscale but after that the exposure becomes non linear. All shots should be taken using JPEG and neutral settings, I.e. no contrast boost, bright mode or other settings used.

You may find for manual aperture lenses this changes quite a bit as a function of aperture, and changing the focusing screen can , due to differences in light transmission and scatter off the focusing screen , lead to very interesting results

See this thread, and the chart I attached and you will understand what an impact a focusing screen can have on metering performance

Do not hesitate to reply or PM if you want to discuss further

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/124627-k-5-met...-lenses-2.html


Last edited by Lowell Goudge; 01-09-2013 at 11:29 PM.
01-10-2013, 06:44 AM   #5
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I think I read about it from Ken Rockwell. Take with a grain of salt.
Canon 5D Focus Screens
01-10-2013, 07:06 AM   #6
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Lowell, thanks for the info. What you're saying makes sense to me, and I've experienced split image blackout with film cameras myself...makes things more difficult, to be certain. Again, to state my reason for doing this is being able to use large aperture manual lenses. I've tried with a K-5 before, and anything more open than a 2.8 is a roll of the dice, especially in low light. Pretty easy for me in my twenties, not so easy now. If I'm understanding correctly, a different screen will only be a major issue in smaller aperture lenses and maintaining metering consistency vs. aperture? I tend to avoid anything smaller than a 2.8, in any case. I need speed for what I like to shoot, and if I can't get the size I need for my budget, I'll just wait until I do. That said, do you think it would be worthwhile to change out my screen? I shoot a good portion of my work in manual, and can keep the exposure compensation stored in my brain--assuming linearity from stop to stop is consistent. (I'm not big on using in-camera analysis for my shots. I learned on a Canon QL17 quite a while ago, and what it provided still works for me today.)
01-10-2013, 07:42 AM - 1 Like   #7
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I'm using a double split screen on my K10D and its been very helpful when using manual lenses with f1.4 - f1.7.
It helps me focus more accurately.

Now how it affected the camera's metering ... honestly I am not sure since I never really trusted the camera's metering system.
I pretty much do everything manually, the old school way .
01-10-2013, 01:25 PM   #8
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QuoteOriginally posted by jbennett1971 Quote
Lowell, thanks for the info. What you're saying makes sense to me, and I've experienced split image blackout with film cameras myself...makes things more difficult, to be certain. Again, to state my reason for doing this is being able to use large aperture manual lenses. I've tried with a K-5 before, and anything more open than a 2.8 is a roll of the dice, especially in low light. Pretty easy for me in my twenties, not so easy now. If I'm understanding correctly, a different screen will only be a major issue in smaller aperture lenses and maintaining metering consistency vs. aperture? I tend to avoid anything smaller than a 2.8, in any case. I need speed for what I like to shoot, and if I can't get the size I need for my budget, I'll just wait until I do. That said, do you think it would be worthwhile to change out my screen? I shoot a good portion of my work in manual, and can keep the exposure compensation stored in my brain--assuming linearity from stop to stop is consistent. (I'm not big on using in-camera analysis for my shots. I learned on a Canon QL17 quite a while ago, and what it provided still works for me today.)
I think what you are experiencing with large aperture, F1.4 and F1.2 perhaps is that there are two differing issues, first the five wing screen seems to have a depth of field that makes focusing under F2.8 difficult to nail down because the viewfinder does not show the true image depth of field. Second some fast lenses display as a function of element geometry focus shift when you stop down, this is impossible to counter for, unless you work it out in advance.

01-10-2013, 01:27 PM   #9
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QuoteOriginally posted by mrNewt Quote
I'm using a double split screen on my K10D and its been very helpful when using manual lenses with f1.4 - f1.7.
It helps me focus more accurately.

Now how it affected the camera's metering ... honestly I am not sure since I never really trusted the camera's metering system.
I pretty much do everything manually, the old school way .
I use a single split on the K 10D and a dual split on the *istD I like the dual split better too.

See my graph for the K10D metering, it makes every shot an adventure until you learn the graph by memory and only change aperture if the light changes
01-10-2013, 03:17 PM   #10
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JBennett, if you are truly doing a lot of manual focus using old lenses, you should really get an EE-S screen... It shows true DoF right down to f/1.6, and it's so nearly perfectly linear wrt stop-down metering that it's pure joy for legacy lenses!
We had a pretty lengthy thread going on about it in the K5 section...
01-16-2013, 07:38 AM   #11
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Thanks for all the advice! Still a little confused as to which supplier could give me the best product, though. After some research, it looks like it'll be a snap to do...so not really an issue there. Thoughts?
01-16-2013, 08:09 AM   #12
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QuoteOriginally posted by jbennett1971 Quote
Thanks for all the advice! Still a little confused as to which supplier could give me the best product, though. After some research, it looks like it'll be a snap to do...so not really an issue there. Thoughts?
Not sure if this has been said already ... but when switching your screen, depending on how your camera is set-up, you might need to re-adjust the focus points.

If you are using AF, then this will not be a big problem. Just trust the AF.

If you are using MF, what you see in the view finder might seem in focus but when looking at the picture you might notice that image is either front or back focused. And that's why you might need to adjust ... if your camera allows you to do so ... and K-5 allows you some fine adjustment.

Other than that, I think that any screen might do the job.
I got the $20 - $30 screens from ebay (ordered one for the K-5 as well) and it works for me. Can't complain.
01-16-2013, 11:06 AM   #13
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QuoteOriginally posted by mrNewt Quote
Not sure if this has been said already ... but when switching your screen, depending on how your camera is set-up, you might need to re-adjust the focus points.

If you are using AF, then this will not be a big problem. Just trust the AF.

If you are using MF, what you see in the view finder might seem in focus but when looking at the picture you might notice that image is either front or back focused. And that's why you might need to adjust ... if your camera allows you to do so ... and K-5 allows you some fine adjustment.

Other than that, I think that any screen might do the job.
I got the $20 - $30 screens from ebay (ordered one for the K-5 as well) and it works for me. Can't complain.
While what you suggest can work, I would suggest that it may be better to correctly shim the focusing screen to fix this issue
01-16-2013, 11:25 AM   #14
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QuoteOriginally posted by Lowell Goudge Quote
While what you suggest can work, I would suggest that it may be better to correctly shim the focusing screen to fix this issue
Shimming will be the way to go for for older cameras, where adjusting the focus point will be more difficult if not impossible; however, the K-5 as far as I know has a very nice and easy way to modify the focus point.

For some, shimming might be a little difficult and more challenging. Additional materials, keep on removing until you get the right stuff ... etc
But yes, shimming will be the other way of doing things, completely agree.
01-16-2013, 12:22 PM   #15
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QuoteOriginally posted by mrNewt Quote
Shimming will be the way to go for for older cameras, where adjusting the focus point will be more difficult if not impossible; however, the K-5 as far as I know has a very nice and easy way to modify the focus point.

For some, shimming might be a little difficult and more challenging. Additional materials, keep on removing until you get the right stuff ... etc
But yes, shimming will be the other way of doing things, completely agree.
Shimming the screen is the only way to do it, there are no other adjustments for the OVF.
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