I suspect that many of you won’t be familiar with the term “AF micro-adjustment.” And there’s no reason you should be, unless you’ve used one of Canon’s top series cameras, like the EOS 5D. This is a feature that is just starting to trickle down into the mid-range EOS bodies – the 50D was the first in its series to include it, and now the 7D as well.
AF micro-adjustment allows you to “fine tune” your lenses individually for maximum performance. Virtually no lens out there is tuned perfectly; each one suffers from some level of front or back focus. usually it’s such a small amount that you’d hardly notice it (although sometimes you’d be surprised). The AP micro-adjustment feature lets you log each lens (up to a limit of, I think, 20). Then, through the use of a target image on an LCD screen, you perform a sequence of checks to determine the amount of micro-adjustment necessary for optimum performance.
The directions that I use are a modified version of the ones posted by on the Will Shoot Photography blog. The target images come from Open Photography Forums.
Click here for a PDF of directions.
Do not be discouraged! The directions look somewhat drawn out and complicated, but once you’ve done a lens or two, you start to figure out how it all works and then it goes quickly. I can do a lens in about 15 minutes, including setup and cleanup times.
The real trick is patience. Repeat the focus analysis steps as many times as you need to be sure that an adjustment is needed. There is no good scale for how much adjustment is needed, so go with the obvious – it there is only a little focus discrepancy, adjust by only a click or two; if there is a large discrepancy, adjust by more. Then re-test the lens with the micro-adjustment applied to see if you’ve eliminated the issue.
Back when we were all comparing the 40D and 50D, this was one of the few features that really set them apart (at least in my opinion). Frankly, there are a lot of features appearing on DSLRs these days that I couldn’t care less about (like, HD movie mode, for example), but this is one of the good ones. If you’ve got it, be sure to use it. It’d be foolish not to.
On my 50D, my best lens was my Canon 28-135mm IS, which didn’t require any adjustment – it was spot-on in focus. My Sigma 70-200 needed 2 points alone; with the dreaded teleconverter mounted, it needed about 5 points. The Tokina 12-24, Canon 17-40, and cheap Canon wide angle all needed between 1 and 3. (Looking at the figures other photogs have published online, I guess mine are really good – looks like 4-7 is a common range, even for good lenses!)
Granted, with a 1 point adjustment, I’m not sure I can really see a difference. But I’m willing to trust that there is one. And since it takes 15 minutes and doesn’t cost anything extra, I figure that any improvement is good.
