I have a D70, and I've noticed that there are quite a few small black spots scattered throughout any shot I take. I looked at my lens the other day and noticed a tiny bit of dirt UNDER the first lens element. I clean my lens after every use (dust the glass off and wipe the body down). The only reason I could think of is the day that I spent at a rock crushing plant. I have a zoom lens so perhaps some of the dirt could've found it's way into the lens. But I still have NO idea how it couldn't gotten up to the second element. Is this an expensive thing to get taken care of?