Fnummers

Fnummers, also written as f-nummers or f-stops, are a standard way to describe the aperture of a photographic lens. They are dimensionless numbers defined as the ratio of the lens’s focal length f to the diameter D of the entrance pupil: N = f / D. The higher the f-number, the smaller the opening, and the less light reaches the sensor; conversely, a lower f-number means a larger aperture and more light.

Fnummers influence two main image properties: exposure and depth of field. Because the aperture area scales with the square of the diameter, exposure is roughly proportional to 1/N^2 for a given shutter speed and ISO. In practice, standard stops change the light by about a factor of two: moving from f/2 to f/2.8 or from f/4 to f/5.6 halves or doubles the amount of light reaching the sensor.

Typical lenses offer a practical range from around f/1.4 to f/22, though exact ranges vary by lens design. The aperture also interacts with sharpness: very large openings yield shallow depth of field, while very small openings increase depth of field but can introduce diffraction, reducing overall sharpness.

In Dutch usage, fnummers are discussed primarily in photography and optics contexts to describe lens aperture. In English and many other languages, the term f-number or f-stop is common. The underlying concept remains the same: the f-number is determined by the lens’s focal length and entrance-pupil diameter and is used to estimate exposure and focus characteristics.