An aperture, in general, is any opening through which light, other forms of electromagnetic radiation, or streams of particles can pass. In photography, the aperture is a hole that allows light to pass through the lens onto the recording surface. A diaphragm aperture works like the iris in the human eye. It can be widened or narrowed automatically or according to a series of manually adjustable values called f-numbers or f-stops. The individual f-numbers represent the focal length of the lens divided by the diameter of the aperture. As the aperture narrows (larger f-number), the depth of field increases.
The term aperture is also applied to the diameter of the main mirror in a reflecting telescope, the objective lens in a refracting telescope, or the dish of a radio telescope. The larger the aperture, the greater the ability to collect light and detect faint objects, and the greater the resolving power. In the case of amateur telescopes, a rough guide for estimating the magnification that an aperture can handle well is 2× per mm, or 50× per inch.