Reducing the aperture size has a very important effect on the image; apart from altering exposure it increases depth of field. The more the aperture is stopped down, the greater the depth of field, so the aperture becomes an essential tool for managing the amount of sharpness in an image. For a quick guide, lenses have a depth-of-field scale engraved on the barrel, showing the distances from the camera (usually in feet and meters) within which everything will appear sharp at any given aperture.
When front-to-back sharpness is important in a photograph, the usual technique is to stop down the aperture. This in turn cuts down the light reaching the film, so that the photographer then has to turn to the camera's other exposure control - the shutter.
To standardize the aperture measurements between lenses, a system of f- numbers is used. The sequence reads: f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, fl6, f22, f32, f45, f64, although any one lens covers only a part of this range. Each number in the sequence allows twice the exposure of the adjacent higher number, and half that of the adjacent lower number. Thus, f5.6 gives double the exposure of f8, and half that of f4.
