Aperture is a veryyy sensitive thing, and you have to be very careful, and you have to be sure of the right size in order too get the best picture quality. Shutter speed too. It depends on how fast your subject is moving, (action shot, portrait, etc.) and what kind of picture you want, with a bit of blur or clear as day. You have to be exact with the numbers, and if you are not, you could miss a great photo oppurtunity.