- Relative aperture of a lens
- The covering power
- Depth of field
The main function of the camera lens is to focus the image sharply on the film. But they do a lot more than that: the lens aperture allows us to control the amount of light reaching the film and the depth of the field. Lenses are termed as ?standard', ?wide-angle' or ?tele-photo' according to the focal length. But these are relative terms and differ from format to format.
[...] For example, a 135 mm lens of a 35 mm camera will cover negative size 1 X 1 inches whereas 90 mm lens of a 4 X 5 inch camera will cover a negative size 4 X 5 inch. Unless they are very expensive, lenses used in photography suffer from certain flaws which create distortions in the image. Distortion comes in two basic forms: pincushion and barrel. Focusing Less expensive are the point and shoot cameras with focus-free lenses in which the focus is permanently set. [...]