The Canon 24-105 was one of the first lenses I purchased when I upgraded to the Canon 7D last year. I’d heard mixed reviews about the auto-focus with that particular lens, but a friend of mine uses it all the time and would consistently get really sharp images. So when I noticed I wasn’t really getting sharp images (especially when compared to my crazy tack-sharp Canon 70-200 ƒ/4L IS lens) I was beginning to get disappointed.
At first I thought that maybe I was somehow introducing camera shake when I was hand-holding the photos, or I wasn’t nailing the eyes (or wherever I wanted the focus point to be) so most of the time I just thought it was me. But after I was continially getting soft images and also after doing a recent food shoot with my camera on a tripod with the IS turned off I noticed that the images were still not sharp so I could rule out human-error as a contributing factor. I know this lens has the potential to create sharp images at f/4 (which is where the aperture on my camera is usually set) so I thought it would be a good idea to test out the micro-adjustment feature on the 7D.
I enlisted my friend’s help to go through this process and he suggested we photograph our eyes to do the testing since it’s something we photograph regularly and are used to looking at on the computer. We positioned a mono-light to my left (those are my creepy-looking eyes in the photo above) and set the lights to 1/160 sec at f/4. Then we started by going into the camera’s “Custom Functions” tab and found the “AF Microadjustment” menu item. We moved the micro-adjustment incrementally until we found the most in-focus correction (it ended up being +9 on my camera). The image on the left is what my camera was defaulted to, and the image on the right is the corrected version and sharpest focus I can get with that lens.
I don’t really know the mechanics of what AF micro-adjusting on the camera does … I guess the best explanation is that it’s similar to the “exposure compensation” feature, but for focusing. All I know is that I now have much more in-focus shots because of those 20 minutes of testing.
Now, you might be looking at the above images and think that I’m crazy to even care about the sharpness of the image on the left. It’s really not too bad, but I do my best to get very clean images and work on my images at 100% most of the time and focus is one of the most important technical aspects of my images when I’m deciding which ones to “keep” and edit. Another thing to understand is that changing the micro-adjustment makes it so that what I’m focusing on is the part that will be in-focus. When the focus was off it wasn’t just soft all-over, the focus plane was in the wrong spot (like I would focus on the eye and the nose would be in-focus instead). With my line of work my images have to be as close to technically perfect as possible and having accurate focus is extremely important.