After some additional testing of my 40D and 24-105L lens using the PDF file at
Home, I looks like I have been getting some front focus. My pictures look "nice" but something has always seemed "off" to me concerning its focusing performance.
I tested the 24-105 at each marked focal length (24, 35, 50, 70, and 105) at f4.0 with mirror lockup and 2s delay on a tripod. At each focal length, I made three exposures using auto focus, forcing the camera to auto focus again for each exposure. Then I did the same thing, but with manual focus and used the 10x display on the LCD. This provided 6 exposures for each focal length. All exposures were recorded RAW and viewed at 100%.
The results were very clear and consistent. The camera/lens combination produced front focus for each and every auto focus exposure at all focal lengths. The amount of front focus varies somewhat from about +8mm at 24mm FL gradually diminishing to about +4mm at 105mm FL. I also see some CA on vertical lines (but not on the horizonal lines) beginning at about 50mm FL (very minor), becoming more pronounced at 70mm and being very obvious at 105mm (still only on vertical lines, not on horizontal ones).
Wondering if it is the camera or the lens, I tried this again using my 50mm f1.8 II at f1.8 and again at f4.0. Same results, but no CA at all with the 50mm lens.
Two overall results:
1) On my camera and lenses, auto focus consistently produced front focused exposures by an average of about 6mm using this test chart, and
2) I was consistently able to produce a better manual focus than the camera could using auto focus. I guess this is a function of my first result.
These results aren't really surprising after looking at the many exposures I've made and doing lots of reading here and across the web. The front focus problem doesn't seem to be "way" off, but its off by enough for me to notice. Its too bad that I'm taking the camera on vacation with me in 2 weeks, or I'd send both the 24-105 lens and the body in for calibration right away. The 50mm is old enough and inexpensive enough that I'm not going to bother doing anything with it, and given the consistency of the results across lenses, it may very well be the body that needs a little tweaking.
Just thought I'd share my results.
Eric