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01-09-2007, 11:48 PM   #1
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Question: backfocus on *istDS

today i was taking some shots of a political demonstration at noon (cloudy, some raindrops, light not really good) with my DS plus the smca DA 16-45 ED-AL (WB set on "cloudy"). i focused on a placard and after taking the shot i saw that the writing on the placard was slightly blurry although i focused on one of the letters on the placard. i refocused with AF and did a slight adjustment manually (important: i have the katzeye screen non-optibrite version with microprism-collar AND split screen; the centre AF sensor is slightly above the split so this shouldn't matter...) and the shot was totally in focus.

back at home i printed out the focus test chart and did some test shots with 3 different AF lenses (18-55 kit, 16-45 and the tamron 70-300 in macro mode) and focus was appr. between 6 and 10 mm to the back with all 3 lenses.

what should i do? is this really an issue in everyday shooting? might the katzeye play a role? when using a pentax lens, manual correction is easy, nevertheless a really accurate AF would be much better.
thx for any comments!

01-10-2007, 03:15 AM   #2
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Judging for all the complaints I've heard about AF inaccuracies with all brands of DSLRs, I think none of the AF systems are perfect. There is a range of acceptability built in to these systems. Whether it is acceptable to you is a personal decision.

A couple of points:
  1. First, I think this is noticable only in situations of very narrow DOF. Fast or long lenses.
  2. Second, when you shoot a focus chart, you are at very close range, and shooting under tungsten light. Tungsten apparently effects the AF sensors, and as DOF is a function of focus distance, DOF is even narrower.
  3. Third, if you haven't noticed it until now, in normal shooting, it probably is not a problem.
When all the stuff broke about th K10D front focusing, I test 5 of my lenses. 2 (35 f2.0, 50 f1.4) front focused on the test chart. I use both lenses on both the K10 and my DS routinely in everyday shooting, with no trace of the effect. When focus is critical, I manual focus. If you don't use spot metering, keep the Katz Eye, it is a huge help in critical focusing.

Hope this helps some
Cheers
-Alan
01-10-2007, 08:16 AM   #3
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It is difficult to draw any conclusion w/o testing with more cameras and lenses. My DA16-45/4 AF precisely at 45mm with the DS, but backfocus quite badly at the wide end. I sent both to Japan for repair and they had a hard time detecting the problem. They eventually did and serviced the lens. But it came back with pretty much the same problem. I have learnt to AF at 45mm then zoom out to whatever focal length to take the shot. This mean grab shot beyond 45mm is out of the question. Now, the twist is that this exact zoom lens AF precisely with my pair of Z-1p at all focal length. And my DS AF fine with all my other lenses too. Who what exactly happened? I don't have a clue.
01-10-2007, 08:44 AM   #4
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The main problem I see with your testing, stern, is that you didn't replicate the original situation in a controlled environment with your testing. Scientifically, you have not proven that you have focusing problems with the use of the Jackson testing. My guess, echoing Alan, if you haven't noticed a problem under the course of everyday shooting until now, you probably don't have a problem. Things do occasionally get out of alignment; you can look into Pentax service centers if you are that worried.

As for your trouble at the demonstration (and I'm speculating because you didn't say anything except the light was bad), I think you just found the limitation of the autofocus system. Your DS (assuming you have the original and not the DS2) manual states: "The autofocus system is highly precise, but not perfect." Page 101 lists half-a-dozen instances where it can be fooled, and there are several others as well. The takeaway? "In such a case, set the focus mode switch to MF and use the manual focus mode to focus the lens on the subject." (p. 101, *ist DS manual)

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