Artifacts on the image at high f-stops usually means a dirty sensor. Most digital cameras now have an ultrasonic cleaning cycle that shakes the dirt off. I stopped using m Canon 10D years ago because id didn't have a cleaning cycle and the sensor had to be manually cleaned. Try shooting the same image at a high f-stop and a low f-stop and see if the artifacts disappear. A dirty lens usually only leads to blurry images because the lens surface isn't in focus.