Quote Originally Posted by DavidEccleston


I recently picked up a 24-105mm. I find it fairly soft at f/4, and sharp at f/5.6 (it may go sharp before f/5.6, I haven't had time to test everything yet). With that in mind, I'm thinking I would have been better off with the 15-85mm.



According to the reviews, the 24-105mm f/4L IS is actuallyquite sharpwide-open at f/4 (see [url="http://www.The-Digital-Picture.com/Reviews/Canon-EF-24-105mm-f-4-L-IS-USM-Lens-Review.aspx]Bryan's review[/url], or the [url="http://www.photozone.de/canon-eos/188-canon-ef-24-105mm-f4-usm-l-is-lab-test-report--review]photozone.de review of the lens tested on APS-C[/url].If I had to guess, I'd say you probably have an autofocus issue with that lens on your body. See the [url="http://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths]"This lens is soft" and other myths[/url] article for a more complete description of the issue. I suspect what is happening is that you're noticing the AF issue at f/4 because of the thinner DoF, and it 'gets sharp' at f/5.6 because the DoF is then deep enough to encompass the front- or back-focusing.


Unfortunately, reading your profile you have a T1i, so if the focus adjustment is the issue, all you can to correct it would be to send the lens and probably the body to Canon for calibration, or get a different copy of the lens and hope you get lucky. In fact, the lack of AF Microadjustment was one of the factors that prompted my very recent upgrade from the T1i to the 7D (myEF 100mm f/2.8<span style="color: red;"]LMacro IS USM was front-focusing on my T1i). When I checked my lenses on the 7D, I found most of them benefited from some degree of AF microadjustment - it was clearly lens-specific, since the adjustments applied ranged from +2 to -5, and some (such as the EF 85mm f/1.8) required no adjustment. Since each unit of AF adjustment is 1/8 of the DoF at the maximum aperture, the lens that needed the +5 adjustment would have been slightly out of focus wide open, but in focus stopped down by one stop, based on the math.


So, before giving up on the lens, you might consider contacting Canon Service (especially if you bought the lens new, in which the warranty would cover the calibration).


Just my 2&cent;.


--John