F NumberEdit
The F Number is a fundamental concept in optics and photography that describes the size of a lens’s aperture relative to its focal length. In practical terms, it tells you how much light can pass through the lens material to form an image, and it also governs the depth of field and the overall look of a photograph. The F Number is commonly written as N and is defined by the ratio N = f / D, where f is the focal length of the lens and D is the diameter of the entrance pupil—the effective aperture seen from the subject. This definition applies across formats and camera systems, making the F Number a universal tool for predicting exposure and image quality. For more technical background, see f-number and aperture (optics).
In many cameras, the F Number is part of what is known as the exposure triangle, along with shutter speed and ISO. A smaller F Number (a larger aperture) lets in more light, enabling faster shutter speeds or lower ISO values, while a larger F Number (a smaller aperture) reduces light and increases sharpness from edge to edge in many lenses. The relationship is not just about brightness; it also shapes the scene by affecting depth of field, which is the range of distances within a photo that appear acceptably sharp. See depth of field and exposure (photography) for related concepts.
Definition and mathematical form
The F Number is a dimensionless quantity that encapsulates two key optical properties: focal length and entrance pupil diameter. The entrance pupil is the image of the aperture stop as seen from the front of the lens, which means the actual physical aperture in the lens may be magnified or reduced by the optical design. In formula terms, N = f / D, linking how the same focal length behaves with different aperture diameters. When the focal length f stays constant, increasing D lowers N and brightens the image; conversely, decreasing D raises N and darkens the image. See focal length and entrance pupil for more.
In practice, photographers speak of stops, with each stop change corresponding to a doubling or halving of light reaching the sensor or film. Common stops include f/2, f/2.8, f/4, f/5.6, f/8, and so on, often modernized into digital and mirrorless terminology as users adjust the F Number to balance exposure, motion blur, and depth of field. For a deeper look at how stops relate to exposure, see EV (exposure value).
Practical implications for image making
Brightness and exposure: Because light reaching the image plane is proportional to the area of the aperture (which scales with D^2), changes in the F Number translate into predictable exposure changes. Halving the light typically corresponds to increasing the F Number by about one stop (for example, from f/4 to f/5.6). See exposure.
Depth of field: Lenses set to smaller F Numbers (larger apertures) produce a shallower depth of field, isolating subjects from backgrounds. This effect is widely used in portraiture and scene separation. Stopping down to larger F Numbers (smaller apertures) increases depth of field, beneficial for landscape and architectural photography. See depth of field and aperture.
Image quality and aberrations: Aperture size interacts with lens design to influence sharpness, vignetting, diffraction, and chromatic aberration. At very small apertures, diffraction can soften image detail, while very large apertures may introduce aberrations and falloff toward the edges. See diffraction, lens, and aperture (optics).
Formats and equivalence: The same F Number can produce different depth of field and field of view on different sensor sizes or format types, due to geometric factors. This is a common topic in discussions of full-frame sensor and equivalence arguments. See sensor and format (optics).
Variations and related topics
F-number vs f-stop: The terms are used interchangeably in many contexts, with some prefer a strict distinction between the symbolic N and the concept of stops used in exposure planning. See f-stop.
Circle of confusion: The practical measure of acceptable sharpness for a given print size and viewing distance is captured by the circle of confusion, which connects aperture, focal length, and subject distance to perceived sharpness. See circle of confusion.
Hyperfocal distance: A useful calculation that ties together focal length, aperture, and acceptable depth of field for a given scene. See hyperfocal distance.
Historical and technical notes
The F Number emerged as a standardized way to express aperture across diverse optical systems, enabling photographers and engineers to compare lenses and optimize exposure without relying on absolute measurements of physical aperture diameter alone. The system has endured through film-to-digital transitions, with manufacturers and imaging professionals continuing to rely on the same core relationships to guide lens design, product marketing, and practical shooting decisions. See lens design and aperture.
In the market for photographic gear, competition among manufacturers has driven refinements in coating, precision manufacturing, and quality control, contributing to more consistent F Numbers across a range of lenses and price points. Consumers benefit from clearer performance expectations and interchangeable systems that preserve the integrity of exposure calculations across brands. See consumer electronics and industrial design.