The aperture of an optical system is a measurement of the size of the iris or light opening and therefore of how much light is allowed to the reach the point of focus. It does not refer to the mechanism itself that controls or determines that same size.
The aperture is measured in f-stops with small f-stop numbers indicating when the aperture is set at its’ widest opening, and small f-stops when set at it’s smallest. A wide aperture allows the maximum transmission of light through the optical system to the focus point, where as a narrow aperture allows for less light to reach the focus point. The effect of these aperture changes, wide to narrow and vice-versa on an image are two-fold.
- One is the effect varying aperture sizes have on light gathering power. Wide apertures allow for greater light-gathering power in low-light situations, as would be useful in dimly lit or night-time environments, whilst narrow apertures limit the light gathered, as would be useful in artificially brightly lit or daytime environments.
- The other is that the field of depth, or apparent depth of focus in an image is either shallow deep, depending on the aperture size. Wide apertures give rise to shallow fields of depth, where only a small proportion of the image is in clear focus, whereas narrow apertures give rise to deep fields of depth, where most of an image, front to back is in clear focus.
The aperture in a camera system is controlled by an aperture mechanism. This is a device, usually made up of multiple interleaving blades, whose position is adjusted to determine a given f-stop number. The typical range of f-stops for a lens can be between f/1.4 and f/16.