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