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06-24-2012, 11:21 PM   #1
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Weird issue after new focusing screen

So, I got my S type focusing screen, and am generally happy with it. But after a while, I noticed something new.

I adjusted and shimmed using a K 50 f1.4 lens, using the focusing screen as the primary focus, and when I focus using the screen, the results are quite good.

Sometimes, though, when I use the indicator to focus (sometimes in the dark, and when I need to focus quickly, I find the indicator more convenient) I am now finding that the subjects are clearly out of focus. I have shot with the old screen using the indicator alone (with a f1.4 lens) since I got the camera 6 months ago, and my keeper rate was reasonable. Now, they are more often than not, off focus.

As far as I can understand how the focus system works, the new screen should not have made a difference on how the indicator works, so I am wondering what changed.

Doing some tests in a controlled environment and various lenses, I find that the focus quality when I use the screen is 9/10, but when I use the indicator is 7-8/10.
Now, when I use AF lenses which are calibrated, the focus is 10/10, so I don't think it is a general back/front focusing issue. I suppose the manual lenses can use some calibration as well, but of course there is no way to calibrate those.

So, what do you think happened, is there a way to break the indicator accuracy with a new focusing screen?
Would you think it would be better to align the screen to match the indicator, and then use a camera-wide fine adjustment (does that even work with manual lenses?)?

06-25-2012, 04:37 AM   #2
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The focus plane of the focusing screen has to coincide with the camera's AF sensor in order for manual focus to be accurate. Most likely the thickness of the new focusing screen is different and this has to be compensated with the appropriate thickness shim.
06-25-2012, 04:46 AM   #3
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Shim it with the thinnest shim that is supplied with the S-type screen.
Then check by focusing at something that's 10-15 ft. away @ f/1.4 (preferably in daylight). this worked great for me.
06-25-2012, 08:21 AM   #4
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Creampuff an Paulusje, you give valid comments (although the focusing screen needs to be aligned with the sensor for manual focusing to be accurate, if one wants to be picky, which will of course mean that it is also aligned with the AF sensor if the AF sensor is properly adjusted). However, I think that you have misunderstood the question. The OP stated that manual focusing works very well with the screen, but that he has problems using the focus indicator of the camera (when the red focus square flashes when something is in focus, and the green hexagon lights up). In other words, the OP has most likely succeeded in adjusting the focusing screen correctly.

The AF system (which is what triggers the indicator) should be independent of the focusing screen, just like Creampuff mentioned. My guess is that what you are experiencing, Atekant, is seeing a pattern where there is none. Humans are extremely good at this -- our brains seem to love finding correlations and patterns even when they do not exist. If that is not the case (in other words, something really did change), then something has happened to the AF system. Possible changes: you shoot different subjects, have different lighting, have changed your focusing technique (likely without thinking about it) -- e.g., maybe you tend to turn the focus ever so little more or less now before you stop compared to before when you see the indicator. There is a tiny interval of the indicator, and focusing with it can be effected somewhat by the user. It is also possible that you have just seen statistical variation, and that the difference would disappear with a larger number of tests. There is also the possibility of the placebo effect (seeing what you want or expect to see). This effect should not be underestimated.

I may very well be wrong, and I do not know how stringent your tests were, but I have seen all of the above effects many times -- both in myself and others -- and it amazes me how powerful they can be.

EDIT: it seems that I, too, was a bit quick in reading the question, and I missed the last part. The focusing screen should only be adjusted to the sensor. If you would re-align the screen (re-shim it) to match the AF indicator, you would set the screen off-focus. There is no way to adjust for that with the camera focus adjustment. If the screen focus properly as it is, it is at its correct setting and should not be changed. From your description, I would make a system-wide AF adjustment for the camera (leaving the screen as it is), to have the focus indicator match the screen. After that, you would have to adjust all of your individual settings for different lenses, IF those two settings are cumulative (combined). If they are not cumulative, you would not need to change the individual settings. I do not know which way it is, so just try or maybe someone can enlighten us. I would guess that if they are cumulative, you could just subtract from the individual lenses' adjustments what you add to the global adjustment (or vice versa).


Last edited by hjb981; 06-25-2012 at 08:35 AM.
06-25-2012, 10:18 AM   #5
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I think that I might be saying what has already been said, but here goes.

I used a good AF focusing lens to set mine up. The best one that I have is the 35mm f2.4. I took two shots of everything. One using AF and one using MF, and one right after the other. When I got the shimming correct the AF appeared focussed in the viewfinder, and both the AF and MF images appeared identical and sharply focussed when viewing them on my computer. Also, when the shimming was correct, after I took the AF shot, I didn't need to adjust the focus when taking the MF shot. It was all very simple logic.

As far as the hexagon goes, I just see that as a general indication. I really don't follow it closely. I almost wish that I could turn it off.

The OP's problem may be that his/hers AF lens needed calibration. It would probably be best to use the procedure that I described above with an AF lens that is spot on at 0 for the fine tuning (or have the fine tuning turned off.) Also, I don't think that you can fine tune a manual lens per his/hers last statement.

Last edited by lammie200; 06-25-2012 at 11:11 AM.
06-25-2012, 10:44 AM   #6
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Wait a minute: people are still missing what the OP reports: manual focus by the screen is fine, so the screen is shimmed properly. Auto-focus lenses also work fine, so the A-F sensor seems fine. BUT focus indicator when doing manual focus gives poor results.
That doesn't surprise me, as I can never trust the focus indicator with fast lenses. It seems to have a pretty broad band where it thinks things are in focus, and my eye can tell better, even with the poor viewfinder of the K-5.
On the other hand, the OP said he uses AF lenses that are CALIBRATED and they work fine. Thus it is likely that the basic setting of AF may be off, and it is covered up by the individual lens adjustments. So I'd agree with hjb981: start by removing the individual lens calibrations; then adjust the general autofocus to give best results matching the screen and test shots. That would make the indicator agree as closely to the screen as possible. Only after that tweak the adjustment for each AF lens.
06-25-2012, 11:53 AM   #7
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Thanks for the replies, but one thing is still questionable, are the camera and lens adjustments cumulative or does one override the other?
Having read through many threads of fine adjustment when I first got the camera, my understanding was either/or.
Does the debug mode adjustment different than the regular one, as in it can be used cumulatively (if the regular one isn't)?

Although, I would like to give hjb98's theory some thought before I start mucking around with the FA settings which took many many hours to get right.
06-25-2012, 12:41 PM   #8
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QuoteOriginally posted by atekant Quote
...
Although, I would like to give hjb98's theory some thought before I start mucking around with the FA settings which took many many hours to get right.
I would write down all the settings, so that everything could be restored if needed.

06-26-2012, 08:58 AM   #9
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QuoteOriginally posted by atekant Quote
Thanks for the replies, but one thing is still questionable, are the camera and lens adjustments cumulative or does one override the other?
Having read through many threads of fine adjustment when I first got the camera, my understanding was either/or.
Does the debug mode adjustment different than the regular one, as in it can be used cumulatively (if the regular one isn't)?

Although, I would like to give hjb98's theory some thought before I start mucking around with the FA settings which took many many hours to get right.
Do you have an AF lens that doesn't need adjustment? That is where I would start. You may need to put the stock screen and shim back in to determine if you do. Otherwise it seems like you will be chasing your tail. In other words, I am not sure if the hexagon changes with AF fine tuning adjustments. If it doesn't, then the fact that you fined tuned for your lenses might be the reason for the discrepancy.

P.S. I know that I am repeating myself and parts of what others have written. However, I don't want to complicate things for you. It sounds like you are working without a control. Hope this helps.
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