Originally posted by rupi Did the "brick wall test" with the 18-55 ALII lens and I think it is OK.
I guess I was expecting to high from this lens. It is decent, but not great.
If I want some sharpness at 18mm I have to shoot at f8.
Part of that is the lens getting sharper as you stop down, but part of it is also that the focus "plane" with most lenses is not completely flat. If you focus on a given spot one wall right in front of you, a lens really wants everything *at the same distance* to be in focus. But the sides of the wall are not at the same distance - they are farther away from you. The wall would have to curve toward you on both sides in order for the sides to be the same distance from you as the center.
Now, most lenses attempt to correct for this in their designs to *some* extent, but aside from macro lenses, most still focus on a curve. So the soft corners you see at wide apertures are at least partially the result of them being further away from you, and hence outside the focus zone. As you stop down DOF gets big enough that the corners becomes in focus.
Which is to say, virtually *all* lenses will do this to some degree; it's not a sign of a bad lens, but rather, of a bad test. Shoot a walls that is curved in the way the lens expects and the corners will look better even wide open. But of course, not perfect - lenses *are* usually sharper in the center.