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10-02-2008, 01:54 PM   #1
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Is it common to have AF discrepency of this kind??

Using center spot for AF, sometimes I aim at something and I can tell thru my Chinese Split Screen that the AF has not fully closed on the subject.

Whenever this happens, I use the quickshift to manually adjust the focus.

I figure this was normal and thats what the quickshift is there for (ironically available for Pentax!)...


I havent actually tried to compare the AF the camera gives me versus my manual adjustments, to see who is actually correct.


Question is, if the splitscreen shows me the subject is not focused but the AF indicates it is, whats the deal?


Thanks!

10-02-2008, 02:59 PM   #2
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QuoteOriginally posted by BBear Quote
I havent actually tried to compare the AF the camera gives me versus my manual adjustments, to see who is actually correct.
That would be the first test. If the AF system is right and the screen is wrong, then you need to re-install the screen and try to get it closer or further from the mirror using shims (many third party screen comes with some for exactly this purpose). if the screen is right and the AF system is wrong, then it's probably time for an AF calibration - either by sending the camera and/or lens in for service, or by using the AF adjustment menu if you've got a K20D.

note: first you want to make sure it's not just case of the camera not focusing on the thing you think it is. That center focus point isn't just a point; it's a pretty broad area. You might think you're focusing on someone's eye, but the tip of the nose might be just close enough to center that it's really focusing on that.
10-03-2008, 12:46 AM   #3
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you might also want to try a few lenses if you can before you send your camera off for calibration

As far as I can tell my K10 focus is spot on with AF using DA35 Macro and with trap focus manual focussing using Voigtlander Nokton 58mm f1.4. But when I tried a DA70 recently, it most definitely front focussing all the time - some individual lenses just seem to do that

there is always the hacked firmware self calibration route for the K10 if you feel brave . . .
10-03-2008, 02:55 AM   #4
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I have shot about 500 phots this weekend, in very well lit outdoors and a porch under a shade.. All were shot using AF and 1 out of 3 images were OOF.. I usually shot pretty fast with nearly a second between two scenes..

I find the K100D AF system unsatisfactory. I haven't had experience with Nikon or Canon dSLR's but i feel even my old Canon compact was better at AF..

10-03-2008, 05:13 AM   #5
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I wouldn't get too down on it yet. I am making the presumption that your camera is fairly new and still under warranty. Let the Pentax folks make it right for you. It's inevitable that an occassional dud gets out of the factory but from all I've heard Pentax service usually does a pretty good and fair job to make things right, either by repair or replacement. Good luck with everything.
10-03-2008, 05:46 AM   #6
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QuoteOriginally posted by throndor Quote
I have shot about 500 phots this weekend, in very well lit outdoors and a porch under a shade.. All were shot using AF and 1 out of 3 images were OOF.. I usually shot pretty fast with nearly a second between two scenes..
..

Don't forget to let time to the focus system, wait for the bip and the sign on the viewfinder
10-03-2008, 11:52 AM   #7
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QuoteOriginally posted by throndor Quote
I have shot about 500 phots this weekend, in very well lit outdoors and a porch under a shade.. All were shot using AF and 1 out of 3 images were OOF
It would be interesting to see some of the problematic images. Since many were in focus, it seems unliekly to be a front-focus or back-focus problem in general. Much more likely a case of the camera choosing to focus on something other than the subject you intended, and you were shooting to fast to notice the little red dots lighting up to show you what it was in fact focusing on (or you didn't know what they meant...).

I don't trust a camera to ever guess exactly what I want to focus on. I have my camera set to always use the center point only. Others have it set where they select the focus point themselves. But letting the camera decide what to focus on is just asking for trouble - cameras cannot read minds.
10-03-2008, 06:15 PM   #8
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I guess your split screen cannot be aligned properly as it is not designed to use in your Pentax DSLR, they just cut those old focusing screen from an unknown body to the same size and fit it in - this would affect both metering accuracy and it would not focus correct.

I suggest you first see if your body's AF is correct or just in case the new cheaply built focusing screen is. For the later case, it is unfortunately, then you need to find Pentax to correct the body error. But I bet the most likely case if the former case: it is just that the cheaply cut focusing screen won't work, just trash it if so, I don't think they will give you a working replacement nor refund.

For more about the technical information for all those alignments and how those can be calibrated, you can read my old technical article written (lengthy):-

RiceHigh's Pentax Blog: Focus Calibrations for (Pentax) (D)SLR Bodies and Lenses

QuoteOriginally posted by BBear Quote
Using center spot for AF, sometimes I aim at something and I can tell thru my Chinese Split Screen that the AF has not fully closed on the subject.

Whenever this happens, I use the quickshift to manually adjust the focus.

I figure this was normal and thats what the quickshift is there for (ironically available for Pentax!)...


I havent actually tried to compare the AF the camera gives me versus my manual adjustments, to see who is actually correct.


Question is, if the splitscreen shows me the subject is not focused but the AF indicates it is, whats the deal?


Thanks!


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