I have been reading and reading to learn about lenses,(hence: mm,f stop,aperture,dof, and such). I want to know why a person wouldnt just buy a lens that covers a wider range is it price,function , size? you can buy an 18-200 2.8-4.5 or just a 70-200 f 2.8 why wouldnt you just buy the 1st. one? then there is a 70-300 wouldnt that be better yet? I am confused at all the different numbers . I do understand that a macro lens is a very small like 50mm and a low f1.8, but the other lenses got me confused.