This chapter will teach you how to control and predict which details are lost. The Controls Modern 35mm cameras have an intimidating array of buttons. The two controls that affect exposure are aperture and shutter speed. If neither the subject nor the camera are moving, the shutter speed is not very important. Aperture, however, affects the depth of field and therefore which portions of the image will be in focus.
Aperture is the degree to which the iris or diaphragm inside the lens is opened.
Lenses are designed for maximum light-gathering capability. A lot of expense and weight went into making your lens fast or good at gathering light. The first reason to stop down a lens is that the world might simply be too bright. A more interesting reason is for aesthetic control of sharpness. The f-number is the lens length divided by the diameter of the aperture opening.