Aperture
As a result, it also determines the ray cone angle and brightness at the image point (see exit pupil).
For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it).
This can be either unavoidable due to the practical limit of the aperture stop size, or deliberate to prevent saturation of a detector or overexposure of film.
In both cases, the size of the aperture stop determines the amount of light admitted by an optical system.
Generally, one would want the aperture to be as large as possible, to collect the maximum amount of light from the distant objects being imaged.
The size of the aperture is limited, however, in practice by considerations of its manufacturing cost and time and its weight, as well as prevention of aberrations (as mentioned above).
[5] Laser applications include spatial filters, Q-switching, high intensity x-ray control.
The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor.
In combination with variation of shutter speed, the aperture size will regulate the film's or image sensor's degree of exposure to light.
Reducing the aperture size (increasing the f-number) provides less light to sensor and also increases the depth of field (by limiting the angle of cone of image light reaching the sensor), which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus.
A lower f-number denotes a greater aperture which allows more light to reach the film or image sensor.
It permits the photographer to select an aperture setting and let the camera decide the shutter speed and sometimes also ISO sensitivity for the correct exposure.
The specifications for a given lens typically include the maximum and minimum aperture (opening) sizes, for example, f/0.95 – f/22.
[9] The fastest lenses for the common 35 mm film format in general production have apertures of f/1.2 or f/1.4, with more at f/1.8 and f/2.0, and many at f/2.8 or slower; f/1.0 is unusual, though sees some use.
For instance, both the current Leica Noctilux-M 50mm ASPH and a 1960s-era Canon 50mm rangefinder lens have a maximum aperture of f/0.95.
Accordingly, DSLR lens typically have minimum aperture of f/16, f/22, or f/32, while large format may go down to f/64, as reflected in the name of Group f/64.
[13] The amount of light captured by an optical system is proportional to the area of the entrance pupil that is the object space-side image of the aperture of the system, equal to: Where the two equivalent forms are related via the f-number N = f / D, with focal length f and entrance pupil diameter D. The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N. If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.
Nikon PC-E perspective-control lenses,[20] introduced in 2008, also have electromagnetic diaphragms,[21] a feature extended to their E-type range in 2013.
Optimal aperture depends both on optics (the depth of the scene versus diffraction), and on the performance of the lens.
Beyond a certain point, there is no further sharpness benefit to stopping down, and the diffraction occurred at the edges of the aperture begins to become significant for imaging quality.
In many living optical systems, the eye consists of an iris which adjusts the size of the pupil, through which light enters.
In rare cases in some individuals are able to dilate their pupils even beyond 8 mm (in scotopic lighting, close to the physical limit of the iris.
Some individuals are also able to directly exert manual and conscious control over their iris muscles and hence are able to voluntarily constrict and dilate their pupils on command.
[29] However, modern optical research concludes that sensor size does not actually play a part in the depth of field in an image.
[30] An aperture's f-number is not modified by the camera's sensor size because it is a ratio that only pertains to the attributes of the lens.
Every photosite on a camera's sensor requires a certain amount of surface area that is not sensitive to light, a factor that results in differences in pixel pitch and changes in the signal-noise ratio.
