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10-04-2012, 06:16 AM   #1
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Calibrating AF on an *ist DS

I need to fine tweak the AF calibration on my *ist DS. Rather than use the fastest AF lens I have for the job (a mediocre f3.5 zoom) I was thinking since the same circuit that tells the camera it's in focus when in AF also tells you you're in focus (focus confirmation) when using a MF lens, I could use a much faster (hence, tiny DOF) and much sharper 50mm f1.4 lens to do the job with more accuracy.

Whaddya think?

Bobbo :-)

10-04-2012, 07:07 AM   #2
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You can certainly use focusing confirmation for MF, but it's not all that accurate. Also, I don't think the DS has any user-accessible AF calibration settings, unfortunately- those weren't made available until later bodies.

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10-04-2012, 07:28 AM   #3
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QuoteOriginally posted by Adam Quote
I don't think the DS has any user-accessible AF calibration settings, unfortunately- those weren't made available until later bodies.
Yep. You can do it via the debug mode. Here's how. I've already calibrated it once with the f3.5 AF lens I have, but I think it could be a little better.

Doesn't the MF focus confirmation rely on the same mechanism that tells the camera when it's in focus in AF mode?
10-04-2012, 09:15 AM   #4
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I think it should work. I don't know exactly how this works so I feel compelled to mention that an MF lens means center-point only. It's probably dependent on your testing setup too. can you easily return to your starting point? Then give it a try and see if you can get better results.

10-04-2012, 09:35 AM   #5
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QuoteOriginally posted by Just1MoreDave Quote
I think it should work. I don't know exactly how this works so I feel compelled to mention that an MF lens means center-point only. It's probably dependent on your testing setup too. can you easily return to your starting point? Then give it a try and see if you can get better results.
I always use center point anyway. Yeah, you can easily return to the numeric value where you were before. I was planning on shooting something with a lot of detail (like magazine print) head-on at the widest (1.4) f-stop to calibrate.
10-04-2012, 10:17 AM   #6
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You will have to be sure of approaching the focus confirm point from the same direction every time. There is enough slop in the confirm light turn on point to be off focus when using f1.4.
I've played around with this on my DS and a macro lens to find the spot that hits focus every time wide open.
10-04-2012, 02:59 PM   #7
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QuoteOriginally posted by wildlifephotog Quote
You will have to be sure of approaching the focus confirm point from the same direction every time. There is enough slop in the confirm light turn on point to be off focus when using f1.4.
I've played around with this on my DS and a macro lens to find the spot that hits focus every time wide open.
Great tip! Thanks! I also have an f3.5 105mm macro lens. I wonder between that one and the 50/1.4 which would have the more narrow DOF wide open to test with?

I'm just really a stickler for sharp focus. ;-)
10-04-2012, 05:23 PM   #8
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F 1.4 is going to have the least error to play with, at a normal distance.
F3.5 at 1:2 or 1:1 mag will have less at very close distance. It could be as little as a few mm.

10-04-2012, 06:22 PM   #9
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QuoteOriginally posted by wildlifephotog Quote
F 1.4 is going to have the least error to play with, at a normal distance.
F3.5 at 1:2 or 1:1 mag will have less at very close distance. It could be as little as a few mm.
I'll just whittle it down with both. :-)
10-05-2012, 12:43 PM   #10
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I don't know that anyone has definitely proven that the AF adjustments works with respect to confirmation in MF mode. I can see at least two ways it could have been implemented, and it seems quite possible the implementation night be different on older cameras than newer. One possibility is that it shifts the notion of what it thinks is in focus, in which case it should work for confirmation. The other is it changes the algorithm for driving the focus motor, in which case it will have no effect on confirmation.
10-06-2012, 06:41 AM   #11
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Just for the record in case anyone wants to know, the AF adjustment DOES affect the focus confirmation and catch-in-focus, at least on the *ist DS. I imagine it does the same on other Pentax models, but I can't be 100% sure.

So, using a very fast manual lens to finely calibrate the AF works. It was very close before, (only 10um off) but now it's right on the button.
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