Tuesday, May 5th 2009
While reading about the Canon EF-S 17-55mm, which I’m considering getting second hand, I saw an autofocus test where it performed flawlessly. That is, finding perfect focus 20 out of 20 times. Not all other lenses tested did though. The Sigma 18-50mm f/2.8 and the Tamron 17-50mm f/2.8 missed a few. (See the test towards the end of this page.)
This got me curious about how well my own lens focuses. (Obviously, the camera plays a part here too, probably a fairly major part.) Anyway, I set up a similar test with my Sigma 17-70mm on my Canon 30D.
I put the camera on my tripod, about 1.5 meters from a frame with our wedding photos. I aimed the centre focus point at the dark edge of a photo, like this:
Then I manually set the focus at infinity (or beyond infinity actually), activated the autofocus again, and simply pressed the shutter-release (cable).
I repeated the process twelve times, and then did another twelve setting the focus at the closest focusing distance and another twelve setting it at about 0.7 meters.
In all three cases, the lens got 12/12 sharp, focused shots, like this 100% crop here:
These shots were shot at 48mm, 1/20th sec, f/4 and ISO 400, to give you a feeling of the amount of light. The room wasn’t dark, not very bright either.
Not a very thorough test, but at least it shows the autofocus is no where near lousy, as some people try to make out about this lens.