Stopping down
Encyclopedia
In photography
, stopping down is to the act of increasing the f-stop number, thus decreasing the size (aperture) of the iris
of a lens
, for example, stopping down from f2 to f4. This increases the depth of field
of the image and allows less light to reach the film plane
. To compensate for the latter, it is necessary to increase the exposure time
, use a photographic film
with a higher ISO rating
, using or increasing the strength of artificial lighting, such as electronic flash
or, in digital camera
s, increasing the light sensitivity of the sensor.
In general, stopping down increases image sharpness. This is true for out-of-focus objects, and for cameras with inexpensive or simple lenses, because spherical aberration
, coma and astigmatism
) are less apparent at smaller apertures. High quality lenses are corrected for these aberrations, and usually give the sharpest images for in-focus objects at about one or two f-stops below maximum aperture. When using very small apertures, diffraction
reduces sharpness. One generally obtains a sharper image by stopping down one or two f-stops because lens designers optimise their designs for a compromise between admitting the most light to shorten the exposure time and producing a sharply focussed image. If the lens designer optimised for sharpness only, the lens would have a small minimum aperture and would be sharpest at full aperture, but would produce too dim an image for some poorly lit scenes.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, stopping down is to the act of increasing the f-stop number, thus decreasing the size (aperture) of the iris
Diaphragm (optics)
In optics, a diaphragm is a thin opaque structure with an opening at its center. The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture...
of a lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
, for example, stopping down from f2 to f4. This increases the depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
of the image and allows less light to reach the film plane
Film plane
A film plane is the area inside any image taking device with a lens and a digital sensor or film; such as a camera. The film plane varies in distance from the lens focal point in each manufacturer...
. To compensate for the latter, it is necessary to increase the exposure time
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
, use a photographic film
Photographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
with a higher ISO rating
Film speed
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system....
, using or increasing the strength of artificial lighting, such as electronic flash
Flash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...
or, in digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
s, increasing the light sensitivity of the sensor.
In general, stopping down increases image sharpness. This is true for out-of-focus objects, and for cameras with inexpensive or simple lenses, because spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
, coma and astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
) are less apparent at smaller apertures. High quality lenses are corrected for these aberrations, and usually give the sharpest images for in-focus objects at about one or two f-stops below maximum aperture. When using very small apertures, diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
reduces sharpness. One generally obtains a sharper image by stopping down one or two f-stops because lens designers optimise their designs for a compromise between admitting the most light to shorten the exposure time and producing a sharply focussed image. If the lens designer optimised for sharpness only, the lens would have a small minimum aperture and would be sharpest at full aperture, but would produce too dim an image for some poorly lit scenes.