What is lens aperture in digital cameras?

There is no difference between the aperture in a film camera or in a digital camera.

The aperture is the diameter of the adjustable iris that controls the amount of light passing through the lens and onto the film plane or digital sensor. An aperture is expressed as an f-stop, which is equal to Focal_Length / Aperture_Diameter, and is written: f/number.

For example, if a lens has a focal length of 50mm - the 'normal' 35mm film camera lens - and the aperture diameter is 5mm, then the f-stop is 50/5 or f/10.

If you know what f-stop a lens is set to, you can determine the physical diameter of the aperture. For example, with an aperture of f/11:
- On a 50mm lens, the aperture is 4.55mm in diameter (50/11).
- On a 100mm lens, the aperture is 9.09mm in diameter (100/11).
- On a 20mm lens, the aperture is 1.82mm in diameter (20/11).

F-stops are used because they are a standard way to express the amount of light passing through a lens. The amount of lens passing through a 50mm lens set to an aperture of f/11 is the same as the amount of light passing through a 300mm lens set to f/11. This is far simpler than measuring the diameter of the aperture to calculate relative levels of illumination..