Aperture

Chapter Nine

An aperture is a hole or an opening through which light travels. It is also called as adjustment of the lens opening measured as f-number,  which controls the amount of the light passing through the lens. The aperture f-numbers are obtained by the manufacturer at the time of manufacturing the lens as per following Formula :

F-number      =                            Focal Length

                                         -------------------------------------

                                               Diameter of aperture

Diagram of decreasing aperture sizes increasing f-number for "full stop" increments factor of two aperture area per stop

Most specifically the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plan. The aperture determines how collimated the admitted rays are which is of great important for the appearance at the image plan. If the admitted rays also pass through  lens highly collimated rays narrow aperture will result in sharpness at the image plan, while un collimated rays wide aperture will result in sharpness for rays with the right focal length only. This means that a wide aperture results in an image that is sharp around what the lens is focusing  on the  blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how many light reaches the image plan the narrower the aperture, the darker the image.

An optical system typically has many openings, or structures that limit the ray bundles ( ray bundles are also known as pencils of light ). These structures may be the edge of a lens or mirror , or a ring or other fixture that hold an optical element in place, or may be a special element such as diaphragm placed in the optical path to limit the light admitted by the system . In general, these structures are called stops, and aperture stop is the stop that determines the rays cone angle, or equivalently the brightness, at an image point

In some contexts, especially in photography and astronomy , aperture refers to the diameter of the aperture stop rather then the physical stop or the opening  itself. For example , in a telescope the aperture stop is typically the edges of the objective lens or mirror (Or of the amount that holds it). One then speaks of a telescope as having for example  a  100 centimeter aperture. Note that the aperture  stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system

Sometimes stops and diaphragms are called aperture, even when they are not the aperture stop of the system. The word aperture is also used in other contexts to indicate a system, which blocks off light outside a certain region. In astronomy for example, A photometric  aperture around a star usually corresponds to a circular around the image of a star within which the light intensity is summed.

Aperture also has an effect on Depth of Field   and  diffraction, the depth of field is the zone of acceptable sharpness, in front and behind of the subject on which the lens is focused. Depth of Field is dependent on the aperture, set the focal length of the lens and focusing distance. The smaller the aperture the shorter the lens and the farther the focusing  distance the greater the depth of field. Likewise, the higher the f-number, the smaller the opening , the less light the greater the depth of field, and the ore the diffraction blurs.

In next chapter we will discuss on Shutter Speed. If you have any question regarding Photography just feel free to ask. So stay with us and like us on facebook