Focal Length & Aperture
The two fundamental parameters of a camera lens
With the lens focused at infinity, the Focal Length is the distance (specified in mm) measured from the centre of the front lens element to the focal point in the camera, indicated by the Focal Plane Mark on the camera body. The focal length determines the angle-of-view, short focal lengths giving a wider AOV than longer focal length lenses.
The aperture is the opening of the lens that determines the volume of light that is let through to expose the film. The aperture is variable and the size is indicated by a series of numbers known as f-stops or f-numbers, which are found on the aperture ring of the lens. The aperture is controlled by the diaphragm, and their relationship can be compared to that of the iris and the pupil of the eye. The iris (diaphragm) controls the size of the pupil (aperture). In a dimly-lit room, the pupil needs to be wide open to allow lots of light through to register an image; the opposite applies in bright light, when it has to shrink to minimise the intensity of light.