Coins can be tough. It usually helps to have them on a non distracting backround, such as a sheet of black or white paper.
As others pointed out, you want to be in a macro mode ... And sometimes they do very nice with a flash if you have the right angle. Obviously, if the light bounces right back to the lens the pic will be useless.
Scanners are Ok too, no focusing or light considerations but from what I have seen, they do not capture detail or colors the way a camera can.