I was discussing this lens with another photographer who was interested in buying one, but hadn’t had much luck with Sigma in the past. He looked at some of the shots I took with it and liked them, but thought that the edges looked soft, which got me wondering… Looking at the areas he thought were soft I couldn’t tell if the effect was just a result of wide angle distortion, or if there really was a possible focal difference between the center and the edges indicating there was something wrong with the lens. To find out, I put together this test.

This is a shot taken on my 1Ds at 15mm, aperture was f11 or f16. The exposure is on the hot side because this was actually used as part of a blended composite I made. It does illustrate how this lens performs pretty well though. All shots have default Lightroom sharpening applied.
This shot can be used as the “key” for the other shots:

Here is crop number 1:

You can clearly see the ballesters on the bridge, and make out enough detail in the foliage.

Here’s crop number 2:

Again, the tree detail is pretty good. I’m actually pleasantly surprised at how well this lens resolves detail at the edges even at 15mm.

Here’s crop number 3:

This is taken right from the center of the lens, where you would expect it to be the sharpest. Not surprisingly it does seem to resolve slightly more detail than the edges. The difference is surprisingly small for a lens this wide (and in this price range) though.

Here is crop number 4:

Again, it shows that there really doesn’t seem to be any focal difference between the center and edges.

If nothing else, this test confirmed that this lens has surprising performance (particularly when stopped down). Even at 15mm on a full frame camera, I’m very impressed with it’s performance.

*Additional Note* One other thing I should mention to make this a more complete review is the flare. This lens like any other will flare when the sun hits it, but in ways other lenses don’t… “Good flare” and “Bad flare” are subjective of course, but this lens has what is probably best described as “unique flare”. Instead of getting really washed out, or having multiple rings appear, this lens flares with a single big deep purple spot. I don’t have an example handy, but if you search you can probably find one. It’s a strange effect…I can’t think of another lens that flares quite like this one. Best way to avoid it–keep direct sun out of the frame . I’ve used my hand to shield it, and that can work too if the sun is at the edge of the frame.

One other thing–for some reason this lens will produce much more saturated skies when the sun is at a low angle in the sky…it almost looks like there’s a polarizer on it when the sun gets low. Is that good or bad? Depends I guess…personally I really like the look of it. In many ways, this lens is kind of a one of a kind…