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02-28-2012, 09:47 AM   #1
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Viewfinder not accurate with fast lens?

Hi all,

I've been trying to think what the issue is when using wide apertures with the viewfinder. I only noticed it recently, but it's annoying the hell out of me. With any of my manual lenses - 50 1.7, 55 2, 135 3.5, my focus seems to be out when using the viewfinder, but it's fine when using live view.

For example, when using my 50mm at 1.7, everytime it appears correctly focused in the viewfinder, and the focus confirm comes up, it always ends up being out.

First thing I thought was wrong was my eye sight, been getting dizy lately and it seems I do need new glasses, and to add to that the diopter adjustment is already all the way to the right even with my glasses on. Which is fine, but why would the focus confirm light be on if that were the case?

Second thing i'm thinking is that the either the stock focusing screen is just inaccurate or could it have been moved/knocked out of place? If so, would that affect the focus confirmation? Is the green hexagon linked to the focusing screen?

I'm getting an eye test this week, so I think that will definatley help one way or the other - I just want to be able to rely on one thing or the other, either my eyes or the focus confirmation.

edit: sorry forgot to say this is on the K-r.

02-28-2012, 10:03 AM   #2
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I think the FF/BF issue with manual lenses. Does it happen to the AF lenses? In the k-5 (also apply to k-7) there is AF fine adjustment option in the menu to correct some minor FF/BF, but I am not sure if this option is there in the k-r. Perhaps, other folks on the forum can help.

Also, if you stop down on the lens, does the focus accuracy seem to get better? Some lenses are soft wide open while some remain sharp even at the largest aperture.
02-28-2012, 10:18 AM   #3
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The focusing screen doesn't really accurately show the depth of field at anything less than f/2.8 or f/3.5. This is true of all DSLRs. Additionally the DoF is quite thin with those lenses at apertures of less than 5.6. The focus points are large enough that the AF system will think something is in focus.

For example. This was taken with an A50/1.4 @ f/2.8

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I was using the center point, but the AF system decided the cupcake was what I wanted to focus on. If I had used f/5.6 the DoF would have been enough to get her face in focus. In this case turning off Catch-in-Focus would have given me a better chance at 2.8.
02-28-2012, 10:22 AM   #4
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To see how much you need correction : first, focus with LV, do a little mark with a pencil (to know what is the "perfect focusing value in the distance scale)on the lens when the focus is perfect. Then do the same with the AF confirm, if you have different value, then there is some adjustement to do. The pencil mark will help you to know if it's BF or FF, and how much you need adjustement.

02-28-2012, 10:52 AM   #5
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there is a relatively wide area where the focus confirmation will light up, usually the real focus center will be closer to one side of the spectrum. to my experience it varies depending on the specific lens you are using. you can try taking the same shot focused inside the confirmed zone closer or further away and that way find out where is the real center of focus for your lens in your camera.
02-28-2012, 10:57 AM   #6
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QuoteOriginally posted by aurele Quote
To see how much you need correction : first, focus with LV, do a little mark with a pencil (to know what is the "perfect focusing value in the distance scale)on the lens when the focus is perfect. Then do the same with the AF confirm, if you have different value, then there is some adjustement to do. The pencil mark will help you to know if it's BF or FF, and how much you need adjustement.
Ah, just gave that a go, and it seems there is a bit of front/back focus (I still don't know which :P). Adjusting it to -10 got it working better, could do with a bit more than that actually, but thats the max. I hadn't noticed anything wrong with autofocus to be honest, but that could be because the only autofocus lens I have is the kit 18-55, which I never used below f5.6 really, so it would'nt have been obvious.

Is there anyway to adjust it further than +/-10?
02-28-2012, 11:09 AM   #7
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QuoteOriginally posted by Pablom Quote
there is a relatively wide area where the focus confirmation will light up, usually the real focus center will be closer to one side of the spectrum. to my experience it varies depending on the specific lens you are using. you can try taking the same shot focused inside the confirmed zone closer or further away and that way find out where is the real center of focus for your lens in your camera.
Indeed, even after adjusting it, just seems like one second it can be bang on then the next oof, also depending on the direction I focus from as you said.

I'll see how it is next week also, if I need new glasses then that should definitely help - then I'll be able to judge it better. Might have to grab a better focusing screen if I can't get used to it too.
02-28-2012, 09:05 PM   #8
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QuoteOriginally posted by Tom Woj Quote
Is there anyway to adjust it further than +/-10?
You can get into the debug menu of the K-r using an application called PKTether. This will let you set a second corrective adjustment, which AFAIK is cumulative with the normal one. So, if you reset the one you normally have access to to 0, and change the debug mode adjustment to compensate for the degree of ff/bf you encounter with ALL your lenses, then you should still be able to use the normal in-camera setting to add to it or reduce it, if needed for one particular lens.

I haven't done this myself, so there may be other factors I'm unaware of. It's just what I've gleaned from other posts.

None of this should be necessary though. It's very likely the fault of the large AF "points" locking onto unwanted elements of the scene. A better focusing screen should help quite a bit though, provided it's shimmed correctly (if needed).

02-29-2012, 01:53 PM   #9
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QuoteOriginally posted by boriscleto Quote
The focusing screen doesn't really accurately show the depth of field at anything less than f/2.8 or f/3.5. This is true of all DSLRs. Additionally the DoF is quite thin with those lenses at apertures of less than 5.6. The focus points are large enough that the AF system will think something is in focus.
What he said, except that my experience is that the apparent DOF with the stock focus screens is closer to that of f/4. Bottom line is that you can't get reasonable precision with a fast lens and the stock screen. As for using the AF system for focus confirm.... The problem is not one of accuracy (BF/FF), but rather the issue again is precision. That and the AF system not "knowing" what the intended focus point is supposed to be.

All that being said, it is possible that your focus screen was not properly shimmed at the factory. If that is the case, there is no use with additional troubleshooting since the precision of the screen is inadequate to calibrate against live view.

If you intend to include a mix of fast manual focus lenses in your shooting, I would suggest an aftermarket screen designed for manual focus. I use one with microprism/split image focus aid, but much can be said for a high resolution matte surface for some subjects. If needed, the new screen can then be calibrated against live view.


Steve
02-29-2012, 02:28 PM   #10
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QuoteOriginally posted by stevebrot Quote
All that being said, it is possible that your focus screen was not properly shimmed at the factory. If that is the case, there is no use with additional troubleshooting since the precision of the screen is inadequate to calibrate against live view.
Well, perhaps it was a dumb/risky of me, but I decided to take out the focusing screen in the hope of re-shimming/altering it. Took out the shim that was in there and placed the focusing screen back without it, just to see which way it affected focus - and hey presto, look's perfect through the viewfinder and with -10 adjustment on the AF confirmation, I'm getting perfect focus every time.

My brain tells me I probably shouldn't have done that/or shouldn't have had to, but what the hell it works so I'm happy!!
02-29-2012, 08:13 PM   #11
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QuoteOriginally posted by Tom Woj Quote
and with -10 adjustment on the AF confirmation, I'm getting perfect focus every time.
Glad your focus screen is more accurate at this point and doubly glad that you are happy. Still, though, this does not sound right. The shims on your focus screen have nothing to do with AF confirmation or the lens focus adjustment. The AF sensor is in the bottom of the mirror box and uses a different optical path than the viewfinder.

Manual-focus lenses don't front/back focus. They did not on manual focus bodies and they do not on AF bodies. They simply focus. Focus accuracy is dependent totally on finding a means to visualize the image at the plane of focus.


Steve
02-29-2012, 08:31 PM   #12
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QuoteOriginally posted by stevebrot Quote
The shims on your focus screen have nothing to do with AF confirmation or the lens focus adjustment.
Oh, indeed I know they don't relate at all, but I wanted both the viewfinder and the green hexagon as best as I could get - it did seem odd that i'd need to do this, but, hmmmm oh well.

The AF adjustment setting does affect the green hexagon ... right? Because if not, my mind is playing some evil tricks on me :/
02-29-2012, 10:04 PM   #13
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QuoteOriginally posted by Tom Woj Quote
The AF adjustment setting does affect the green hexagon ... right? Because if not, my mind is playing some evil tricks on me :/
Yep...
03-01-2012, 05:15 AM   #14
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QuoteOriginally posted by Tom Woj Quote
it did seem odd that i'd need to do this
I had to adjust my focus screen also. In my case I needed more shim instead of less. If you shoot a lot with MF lenses at apertures faster than f/3.5 or so, you might consider replacing your focus screen.
03-02-2012, 07:01 PM   #15
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Are you aware that virtually every lens gets soft wide open? That is something you need to factor in when focusing. Don't focus with the lens wide open, but stopped down 1/2 -1 stop. You'll notice a difference.
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