A Question of Focus

The autofocus on my new 40d has never seemed quite right to me - on many occasions, after loading the card onto the computer and viewing the images full screen, I've been disappointed to learn that what I thought was my focal point was, in fact, just that little bit soft.

It was starting to really bug me, so I decided today to actually test it, to see if I was simply imagining the whole thing. I printed this test chart and shot a few frames with four different lenses.

Turns out I'm not going crazy. The shot above (taken with my 50mm@1.4) looks to me to be sharpest somewhere around 2-4mm, even though I was focussing on the black "Focus Here" bar on the right (this is actually a crop - the bar was dead center in the original, so I wasn't shooting at an odd angle).

Results were similar with almost every lens - a tendency to slightly forward-focus:

-- The Canon 28-135 that came with the camera was off about the same amount as the Canon 50mm/1.4 above.

-- Couldn't really tell with the 17-55, because pretty much the whole world's always in focus with that lens, anyway.

-- My Tamron 28-300, which I've used for years with my old 300d with nary a (noticed) problem, turned out to be way off at 28mm.

-- The only shot I got that really looked dead on was when I stopped the 50mm down to 3.5 - strangely, instead of the area of focus growing equally on both sides, it seemed to shift backward as I stopped down the lens.

Camera was, of course locked down on a tripod, so no movement there (and it shouldn't matter anyway, as the camera was confirming positive focus on the black bar in every instance).




Solution? Off to Canon, I guess, for calibration. I assume they'll fix something like this under warranty.


:-(




Sorry for the overly-technical blip, by the way. Just wanted to vent my frustration, and was curious if any other 40d users (there are at least 3 or 4 of us here, I think) had experienced the same thing....



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All that said, I did manage a nice shot of tonight's lunar eclipse with it.