camera lens is marked 8cm.fl.6cm this means that aperture of lens is........???AND WHY????

The f-number (f/#) is often notated as N and is given by

where f is the focal length, and D is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16. The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.

The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

In other types of optical system, such as telescopes and binoculars, the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).