This is the distortion test. It appears that there is a small amount of barrel distortion, much less than the 16-35 zoom or the 17-40 zoom (as expected from a prime). Lighting was poor in this image, so it should not be looked at as a test for vignetting. I was a bit hasty in setting up the test. I would say that the boundaries are straight enough for architectural work. Note that this image is at f/2.8. Sharpness is excellent in the center, and still good-to-very good in the corners.