Looking Up Trees - Soft Corners at f/3.5

Looking Up Trees, originally uploaded by blakophoto.

17mm, f/3.5, 1/60sec, ISO100, -1/3eV

I took this while walking around a local park, testing out my new Canon 17-55mm f/2.8 IS lens with my Canon 40D body. I am completely in love with this lens. However, with this shot, I was very disappointed to see soft corners, when I knew from this DoF calculator that the whole frame should be in focus for f/3.5 and the point I focused on. The soft corners are visible at low res, obvious when zoomed in.

My first thoughts were that I had a bad copy of the lens. But, after reading, and evaluating the MTF charts for 18mm, I learned what I should have already known. Even good lenses might have a tough time with corners when wide open.

I found this great review on the lens, comparing it to several other in the focal range. Of note were the
18mm test corner shots at f/2.8 and f/5.6. At f/2.8, the corners are very soft, but at f/5.6, they're very sharp. It's interesting to then compare corner shots of this lens to the non-IS kit lens at 18mm, f/5.6 - what an improvement.

So, if I paid all this cash for the f/2.8 model, but have to use f/5.6 for shots like this, then what did I pay for? Well, it's still great for portraits and to stop motion in a pinch - it's one of the best walking around lenses you can buy for FOVCF bodies.

For portraits, f/2.8 blurs backgrounds well - but not as well as my f/1.4 50mm, and the MTF charts at 50mm heavily favor the f/1.4 50mm. So, if I'm specifically shooting portraits of one or two people, I'll probably reach in the bag for my fast 50. But, this lens will probably spend most of its time out of the bag.