Correct exposures rely on three inter related parameters: aperture, shutter speed and ISO sensitivity. Change one parameter and its influence will affect at least one of the others. This relationship is often depicted in an ‘Exposure Triangle’ graphic, which is shown on this page.
Lens aperture
The aperture of a lens controls how light passes through to the sensor that collects the signal. Unlike your eyes, where the iris diameter adjusts automatically in response to changes in ambient lighting, lens apertures must be adjusted, either by the camera when one of the auto-exposure modes are selected or by the photographer in manual exposure mode.
Aperture settings are specified in f-stops, which are expressed as fractions that indicate the ratio of the focal length setting to the effective diameter of the opening in the iris diaphragm. Wide aperture settings (which are somewhat counter-intuitively denoted by small numbers) let in a lot of light,
