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04-18-2010, 08:19 PM   #1
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Timichango's Avatar

Join Date: Feb 2010
Location: Vancouver
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Posts: 62
Help with AF calibration for WR 18-55 kit lens

Hey all, few quick questions about AF calibration.

Before today, I'd never really considered messing with the AF calibration, as I've been doing a lot of MF shooting with some A-series lenses, and when I've used af with the WR, it's generally seemed fine. Today while hanging out on my patio, I tried taking some shots of the deck, focusing on some tibetan prayer flags about 10 feet away. I was shooting at the wide end of the lens.

I noticed when zooming into the resulting shot, that the flags were a bit on the blurry side. I decided to try focusing while zoomed in, and then zooming out and taking the shot. Bam - perfect focus. I subsequently spent some time farting around with the AF calibration, and ended up with a +4 calibration in order to nail AF of the flags at the wide end of the lens.

I subsequently came inside and did a bit of reading up on AF calibration, and ended up running a battery of AF tests using Yvon Bourque's AF test charts and instructions, and all of my results seem to indicate that the lens is accurate at it's widest focal length with 0 calibration, or possibly even a bit of negative calibration (-2), which runs contrary to what I was experiencing earlier. I performed the tests under fluorescent lighting in order to avoid any tungsten-induced inaccuracies, even though I'm shooting a K-7.

  • how can the lens be exhibiting perfect, or slightly front-focus at close distances, but back focus at larger distances?
  • is "zoom-focus-unzoom" a reasonable (if inconvenient) workaround?
  • how can I resolve this discrepancy in a way that yields the most accurate AF results at all focal lengths?
Thanks in advance... I'm pretty perplexed by this!

-Tim

04-18-2010, 09:49 PM   #2
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Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
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Posts: 10,686
When shooting wide angle, it becomes extremely likely that more than one object is in range of whatever focus sensor you have chosen. So what you are experiencing may not be FF at all - it's more likely just a case of the camera choosing to focus on something in front of whatever you intended. feel free to post samples if you don't believe this could be the case, but especially when shooting wide, that potential is almost always there.
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