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05-05-2014, 03:09 PM   #1
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Pentax MF Back Focus

Hi, I'm new to the Forums, but have been viewing many of the different posts. Especially those related to Manual Focus issues. I have downloaded and printed a focus test chart and completed some very preliminary tests today. I have a Pentax K10D and 3 Pentax lenses. Kit 18 -55mm. Manual 50mm f1.7 and Manual 200mm f4. The issue I have experienced, is that I feel I have focused on the subject, but the results are out of focus. This only seems to happen with manual focus. The AF seems to deliver reasonable results. From the tests of the lens, it seems to show Back Focus with the manual lenses. I think from the other posts I have been reading, that this could be related to the position of the focus screen. I am interested in the K10D (Canon derivative) S screen, that focusingscreen.com sell. I was wondering if anyone from the UK has purchased from this company and what the import duty charges might be? I cant tell from the website. Any help and advice would be appreciated. Thanks. Phil

05-05-2014, 03:40 PM   #2
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Hey, welcome.
The camera doesn't "back focus" with MF... unless you mean that the focus confirmation (little hexagon in viewfinder) is wrong, but that would be equally wrong whether you switch to AF or MF. Some specific lenses might cause the camera to think something is in focus, when it isn't. This is why you use focus adjust (micro adjust, fine adjustment, not sure what its called). It happens when the lens and camera are at other ends of tolerance (these things are very precise, so some error is to be expected which is why most modern DSLRs have a focus adjust option). Anyway, if the AF is correct, then the only problem with MF is the person doing the manual focusing. I think you just need practice to better understand how MF works.
The only problems with the camera that could affect MF are:
a) focus confirm, if you rely on it, but it seems like you don't. The confirmation would also be wrong with the same lens if it was set to AF
b) focus shift, which is a lens problem, but can make the user think its the camera
c) maybe the OVF diopter being set wrong

I wouldn't go spending a lot of money on a focus screen just yet. How many MF lenses do you have? Is it really the focus screen that is causing you problems, or do you just need some more experience? focusing is a big part of photography and it takes some time to learn it with each lens that you get. Take your time, no need to quickly jump to conclusions and buy things that are expensive or might be difficult to install.
On the other hand, lots of people like a special focus screen. KatzyEye and the website you listed are very popular. Then there is the JinFinance screen and the cheaper knockoffs of ebays. You can get matte or split prism screens. Usually the vendor will list exactly which cameras the focus screen fits, and will ship it along with some tools to help with installation (tweezers, finger sleeves)

Last edited by Na Horuk; 05-05-2014 at 03:45 PM.
05-05-2014, 04:47 PM   #3
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Welcome to the Pentax forums!

As noted above, there is no "back focus" when using manual focus lenses, though there are possible difficulties with using them on most dSLR cameras. The problems can be summarized as follows:
  • Problems determining focus point due to focus screen design (poor precision)*
  • Poor calibration of focusing screen to focal plane
The first point is a fact of life with the stock focusing screen and any lens faster than about f/4. The second is hard to detect and correct if the first has not been rectified.

You can fall back on the AF system for focus confirm, but the precision (actually finding the true focus point twice in a row) is not much better with faster lenses than the stock focus screen. Sorry An aftermarket focus screen is often a good solution. I have been using the KatzEye screen with Optibrite on my K10D since 2008 and can strongly recommend that product. I am currently trying out the Type-S screen from focusingscreen.com on my new K-3. I am happy so far, but sort of miss the solid confirmation of the split image. Time will tell.

There are some negatives to an aftermarket screen however. They include:
  • Poor quality/service from some vendors (I have good experience with both KatzEye and focusingscreen.com)
  • May be significantly less bright than stock screen
  • Loss of spot metering with screens having center split-image focus aide
  • Focus aides generally only work well with faster lenses
My opinion is that the negatives are worth dealing with if a person is going to do much manual focus at all. This would include any subjects where critical focus is essential.


Steve

* The problem is present across brands and not limited to Pentax.
05-05-2014, 05:01 PM   #4
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With my first DSLR (a K10D) I had the same problem you're describing -- when focusing manually, the actual result would be focused farther away than what I saw in the viewfinder. I was unable to source proper Pentax focusing screen shims so I added thin strips of Post-It to the stock shim. 1 thickness of Post-It turned out to be just about perfect in my case. Since then I've started using aftermarket focusing screens, which I am completely happy with -- the one from focusingscreen.com comes with extra shims in case you need to adjust.

AF is not based on the focusing screen, which is why it is possible for AF to be correct even if the focusing screen is improperly adjusted.

05-05-2014, 07:19 PM   #5
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There are two issues here. One if you use the viewfinder and the focusing screen (with properly adjust diopter) to confirm focus rather than using the camera's AF system to confirm, then it is possible that you just need shims (along with the focusing screen). Second if you use the viewfinder and stock focusing screen but you also rely on the camera's AF system to confirm (that is until you see the green hexagon in the viewfinder), then you are subject to the FF/BF error of the camera's AF system. In the first case, you have to use release priority for both AF.S and AF.C otherwise, you can not release the shutter. In the second case, you can use focus priority so you can trip the shutter when you get the green hexagon. With the first method, BF or FF is irrelevant since you decide when it is in focus based what you see on the focusing screen. However, if you use the second method, the camera's AF system erroneously provides focus confirmation when in fact it is either BF or FF. When that happens, you can change the camera's AF adjustment menu setting and apply a value (up to -10 for FF, and up to +10 for BF) to correct that. You can only adjust the value that "applies to all" as there is no lens specific information for the camera. If you don't want to mess the AF adjustment with your other FA, DA or DA* lenses, you can set up your own user-mode just for handling manual focus lenses only. If you need to go beyond -10 or +10 then perhaps, the camera needs to be tuned; hopefully you don't need to do that.
05-07-2014, 01:13 PM   #6
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Guys, Thanks for all the replies. Much appreciated. More food for thought and some more tests.????

I am pretty sure the focus is working ok as far as the DA lens and AF is concerned. This is the original kit lens that came with the camera, when I bought it 6 - 7 years ago. We are planning a trip to Canada, later in the year, so this is what rekindled my interest. Initially I was looking for a longer zoom lens to take with me on the trip. However, before investing in what could be a substantial new purchase and having to seek the accountants approval...... ;>), I thought it better to test the water with a couple of the older prime lenses to see if the quality would suffice for my needs.

I have used a test target downloaded from Jeffrey Freidl's Blog for this and had the camera on a tripod, whilst utilising the 2 second delay self timer. The target is for checking AF focus, but I think it should provide some basics to use the test in a manual mode.

Initially I have some confusing results, so I need to try a few more tests, before posting back.

Thanks again for your comments and suggestions.
05-07-2014, 02:08 PM   #7
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QuoteOriginally posted by philbrown1 Quote
Initially I have some confusing results, so I need to try a few more tests, before posting back.
The stock focusing screen shows an image that is more or less equivalent to f/3.5. You might try some tests at that aperture to make it easier to compare what you see in the VF against the captured image.

05-10-2014, 03:01 PM   #8
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QuoteOriginally posted by aleonx3 Quote
There are two issues here. One if you use the viewfinder and the focusing screen (with properly adjust diopter) to confirm focus rather than using the camera's AF system to confirm, then it is possible that you just need shims (along with the focusing screen). Second if you use the viewfinder and stock focusing screen but you also rely on the camera's AF system to confirm (that is until you see the green hexagon in the viewfinder), then you are subject to the FF/BF error of the camera's AF system. In the first case, you have to use release priority for both AF.S and AF.C otherwise, you can not release the shutter. In the second case, you can use focus priority so you can trip the shutter when you get the green hexagon. With the first method, BF or FF is irrelevant since you decide when it is in focus based what you see on the focusing screen. However, if you use the second method, the camera's AF system erroneously provides focus confirmation when in fact it is either BF or FF. When that happens, you can change the camera's AF adjustment menu setting and apply a value (up to -10 for FF, and up to +10 for BF) to correct that. You can only adjust the value that "applies to all" as there is no lens specific information for the camera. If you don't want to mess the AF adjustment with your other FA, DA or DA* lenses, you can set up your own user-mode just for handling manual focus lenses only. If you need to go beyond -10 or +10 then perhaps, the camera needs to be tuned; hopefully you don't need to do that.
QuoteOriginally posted by aleonx3 Quote
However, if you use the second method, the camera's AF system erroneously provides focus confirmation when in fact it is either BF or FF. When that happens, you can change the camera's AF adjustment menu setting and apply a value (up to -10 for FF, and up to +10 for BF) to correct that. You can only adjust the value that "applies to all" as there is no lens specific information for the camera. If you don't want to mess the AF adjustment with your other FA, DA or DA* lenses, you can set up your own user-mode just for handling manual focus lenses only. If you need to go beyond -10 or +10 then perhaps, the camera needs to be tuned; hopefully you don't need to do that.

Aleonx3, How do I get into the menu to make the fine adjustments of the AF? I cant see it mentioned in the K10D manual.

Thanks
05-10-2014, 05:10 PM   #9
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K10D does not offer that feature.
05-12-2014, 03:05 PM   #10
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QuoteOriginally posted by philbrown1 Quote
Aleonx3, How do I get into the menu to make the fine adjustments of the AF? I cant see it mentioned in the K10D manual.

Thanks
Sorry, it has been so long since I had my k10D (sold it after I bought my k-5). Unfortunately, as others have mentioned, the only way to correct FF/BF on the K10D is to use shims and focus screens to allow you to confirm focus instead of relying on the camera's AF system.
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