Aperture is a term given to a hole or opening. In optics it is the name given to the hole or opening which light is admitted, it determines the angle of a bundle of rays that come to focus on the image. While in photography it refers to the size of the opening in the lens that determines the amount of light falling onto the sensor or film.
The aperture stop of a lens can be adjusted to control the amount of light that reaches the image sensor, and in combination with the shutter speed the aperture size will regulate the degree of exposure to light. To help control the amount of light coming through the lens, lenses also have a device called a diaphragm and this functions much like the iris of your eye, it controls the diameter of the lens opening. Reducing the aperture size increases the depth of field which determines how much of the image is in focus. The amount of light captured by the lens is proportional to the area of the aperture. Typically fast shutter speeds require larger apertures and slower speeds require smaller apertures to avoid excessive exposure.
Generally apertures are specified as the f-number, or f-stop, such as f2.8 or f32 at the other extreme, the ratio of focal length to effective aperture diameter. Because f-numbers are fractions of the focal length, higher f-numbers represent smaller apertures. The smaller the f-number the larger the hole and therefore the more light that gets through to the sensor.
Lenses are typically quoted with maximum apertures (ie. f2.8) , with fixed focal length lenses with large apertures, favoured by photojournalists who often work in dim light and have little time or no ability to introduce extra light via flash or other means to capture the image. A lens with a large maximum aperture is called a ‘fast’ lens because it allows faster shutter speeds, however they do tend to be larger and heavier. Zoom lenses tend to have a maximum aperture that varies through their range, which means that the diaphragm remains the same size while the focal length increases giving a higher f-number. Any with an aperture of f2.8 tend to be large and expensive. A more typical consumer zoom will have variable relative aperture, typically f3.5 to f6.3 allowing the lens to be much smaller in size.
Aperture Priority refers to a shooting mode used in modern cameras. It allows photographers to choose an aperture setting and the camera decides the shutter speed based on what ISO is set to give the correct exposure. It can also be referred to as Auto Exposure, A Mode, or Av Mode.
Diagram of Aperture holes showing the f-number in relation to size of hole
The F-numbers that we refer to as stops each reduce the light by 50%. As light spreads in both directions it decreases in power by a quarter for each relative step in distance. For this reason the f-numbers halve numerically every alternate stop. This means that the area of the opening each allow half the amount of light of the larger one above it. The diagram above illustrates this.
In summary, the aperture affects exposure and depth of field, a higher maximum aperture allows more light to hit the sensor of your camera, but a smaller minimum aperture is good when there is a lot of light or you want less depth of field.