Originally posted by ChuckB28 
Ah... That might have been my problem. I was shooting at f3.5 for many of the pictures.
Lenses get better stopped down compared to wide open. Oversimplified, as the aperture closes, stray light rays are reduced and/or straightened out. You get sharper photos with better contrast at smaller apertures. Then lenses get worse as the aperture gets really small. Generally, wide open is the worst, f8 is pretty good, f22 is not good (and you never have that much light anyway).
Zoom lenses are compromise designs so they are not perfect everywhere. Usually at least one end of the zoom is not that great. You can get a slightly better photo with the kit lens at 21mm than 18mm, assuming everything fits into the frame.
Lenses are better in the center of the frame than at the corners. Often the corners are darker (vignetting), or just not as sharp. The better lenses and lenses designed for 35mm are better in the corners. Lenses can also focus in a curved or flat field. Macro lenses are designed to focus in a really flat field, and having very few other defects mentioned here. The Pentax 50mm 1.4 lenses have a more curved field of focus.
Here are some test images that'll show what I'm talking about. I tested my kit lens with four other lenses at 24mm, shooting a newspaper taped to a wall about 8 feet away. Then I crop just the center out of each image and show them side by side. The kit lens is wide open in the first series, and so is the DA 16-45mm f4 zoom. The lenses at either end are really good prime lenses, which have a big advantage here- they are stopped down one or two stops from wide open. (The one in the center is not that good.) The second series shows the difference at f8.
Similar Threads 
