Aperture is the adjustable opening inside a camera lens through which light passes before reaching the camera sensor or, in film photography, the photographic film. It is one of the three principal exposure controls alongside shutter speed and ISO, and it also has major effects on depth of field, background rendering, diffraction and the overall visual character of a photograph.
Aperture is normally described by an f number, written in forms such as f 1.4, f 2.8, f 4, f 5.6, f 8, f 11 and f 16. The apparently counterintuitive feature of this notation is that a smaller f number represents a larger physical opening, while a larger f number represents a smaller opening.
The f number is derived from the ratio between the lens focal length and the effective diameter of the entrance pupil, so an aperture of f 2 on a 50 millimetre lens corresponds to a substantially different physical opening from f 2 on a 200 millimetre lens, even though both have the same relative aperture. Aperture therefore describes a relative optical relationship rather than simply the diameter of a hole in the lens.
The mechanical aperture is commonly formed by an iris diaphragm consisting of overlapping blades that move to enlarge or reduce the opening. Modern interchangeable lenses may have several curved blades designed to create a particular opening shape, and the number and geometry of those blades can influence the appearance of out of focus highlights and the smoothness of defocused areas. The opening is usually controlled electronically by the camera, although some lenses permit manual aperture adjustment through a physical ring.
In a fully automatic exposure mode, the camera can select an aperture according to its metering calculations and the photographer's chosen parameters. In aperture priority mode, the photographer selects the aperture while the camera determines a corresponding shutter speed, subject to the camera's metering system and ISO settings. Manual exposure allows aperture, shutter speed and ISO to be controlled independently, although their photographic consequences remain interconnected.
Aperture has a direct relationship with exposure because a larger opening allows more light to reach the sensor during a given shutter interval, while a smaller opening allows less light. The conventional full stop sequence progresses approximately through f 1, f 1.4, f 2, f 2.8, f 4, f 5.6, f 8, f 11, f 16 and f 22, with each full stop representing approximately a doubling or halving of the amount of light admitted, depending on the direction of the change.
Moving from f 4 to f 2.8 therefore increases light by approximately one stop, while moving from f 4 to f 5.6 reduces light by approximately one stop. Modern cameras frequently permit third stop or half stop adjustments, allowing much finer exposure control. Aperture interacts with shutter speed and ISO through the exposure triangle, meaning that changing aperture normally requires another exposure parameter to change if the photographer wants to maintain approximately the same overall exposure.
One of the most important consequences of aperture is its effect on depth of field, meaning the range of distances around the focused subject that appears acceptably sharp. A wider aperture such as f 1.4 or f 2 generally produces a shallower depth of field, while a narrower aperture such as f 8 or f 16 generally produces greater depth of field under comparable framing and focusing conditions.
Depth of field is also strongly affected by focal length, subject distance, sensor format and the final viewing conditions, so aperture alone does not determine exactly how much of a scene will appear sharp. Portrait photographers may use a wide aperture to isolate a subject from its background, while landscape photographers may select a narrower aperture to maintain useful sharpness from foreground to distant scenery. Macro photography presents especially shallow depth of field because focusing distances are very short, making aperture selection particularly consequential.
Aperture also affects optical image quality. At very wide apertures, many lenses exhibit characteristics such as reduced edge sharpness, vignetting, spherical aberration, chromatic aberration or reduced contrast, although modern optical designs and computational corrections can reduce some of these effects. Stopping a lens down can improve sharpness across portions of the frame because optical aberrations may be reduced, but closing the aperture too far introduces diffraction.
Diffraction occurs when light passing through a small opening spreads according to wave behaviour, reducing fine detail and making the image appear softer. The aperture at which diffraction becomes visually significant depends on sensor resolution, viewing conditions, lens design and the intended output size. Consequently, there is often a practical range in which a lens provides a useful balance between depth of field and optical sharpness rather than one universally ideal aperture.
Aperture strongly influences background appearance, but aperture and bokeh are not identical concepts. A wide aperture can reduce depth of field and increase the amount of background blur, but the character of that blur depends on lens design, focal length, focusing distance, background distance, aperture shape and optical aberrations. Specular highlights in the out of focus region may appear as discs or other shapes corresponding approximately to the effective aperture opening.
Some lenses use rounded diaphragm blades to maintain more circular openings, while other designs produce polygonal highlights when stopped down. Catadioptric lenses can produce distinctive ring shaped highlights because of their optical construction. The visual quality of blur therefore depends on the entire optical system rather than simply on the numerical f number.
Aperture choice is consequently a compositional and technical decision rather than merely an exposure adjustment. A photographer photographing a moving subject in low light may select a wide aperture to admit more light and permit a faster shutter speed, while accepting reduced depth of field. A landscape photographer may stop down to increase depth of field but may need a slower shutter speed or higher ISO.
A product photographer may use a relatively narrow aperture to maintain sufficient object sharpness, potentially combining several focused images through focus stacking when ordinary depth of field is inadequate. Street, documentary and event photographers may choose intermediate apertures to provide a useful margin of focusing tolerance. The correct aperture therefore depends on the subject, light, desired depth of field, lens characteristics, motion and aesthetic intention rather than on a universal rule.
Common misconceptions about aperture include the belief that a larger f number always means more light or that a wide aperture automatically produces better photographs. In reality, aperture is a tradeoff involving exposure, depth of field, optical behaviour and artistic intent. Another misconception is that f numbers directly represent physical aperture diameter without reference to focal length.
The physical entrance pupil diameter can be estimated by dividing focal length by f number, so a 100 millimetre lens at f 2 has an entrance pupil of approximately 50 millimetres, whereas a 50 millimetre lens at f 2 has an entrance pupil of approximately 25 millimetres. Aperture remains important in film cameras, digital cameras, interchangeable lens systems and many computational cameras, although fixed aperture systems and smartphones may simulate some depth of field effects computationally. Understanding aperture therefore requires understanding both its optical definition and its practical relationship with exposure, focus and image rendering.
