Originally posted by kmhtax As far as the damaged HD coating, I don't think it is the reason because I later clicked some photos with another non HD lens and I got the same artifacts.
you can usually see "damaged hd coating" by shining a flashlight into the lens, at an angle to the direction that you are viewing it.
look at the photo comparison of different apertures here:
photo technique magazine
you can clearly see that f/16 has more blur than f/5.6... and that's on a camera with a full-frame sensor, your aps-c crop sensor camera will be worse than that... f/8 on crop is roughly equal to f/11 on ff.
1)don't close the aperture down more than f/8 unless you absolutely have to
2)turn the iso way down