Distortion (optics) in the context of "Astronomical seeing"

The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken note of or to be taken note of by the observer, at any distance more than least distance of distinct vision. In optics, refraction of a ray due to use of lenses can cause distortion. Shadows, patterns and movement can also influence line of sight interpretation (as in optical illusions).

The term "line" typically presumes that the light by which the observed object is seen travels as a straight ray, which is sometimes not the case as light can take a curved/angulated path when reflected from a mirror, refracted by a lens or density changes in the traversed media, or deflected by a gravitational field. Fields of study feature specific targets, such as vessels in navigation, marker flags or natural features in surveying, celestial objects in astronomy, and so on. To have optimal observational outcome, it is preferable to have a completely unobstructed sightline.

A fisheye lens is an ultra wide-angle lens that produces strong visual distortion intended to create a wide panoramic or hemispherical image. Fisheye lenses achieve extremely wide angles of view, well beyond any rectilinear lens. Instead of producing images with straight lines of perspective (rectilinear images), fisheye lenses use a special mapping ("distortion"; for example: equisolid angle, see below), which gives images a characteristic convex non-rectilinear appearance.

The term fisheye was coined in 1906 by American physicist and inventor Robert W. Wood based on how a fish would see an ultrawide hemispherical view from beneath the water (a phenomenon known as Snell's window). Their first practical use was in the 1920s for use in meteorology to study cloud formation giving them the name whole-sky lenses. The angle of view of a fisheye lens is usually between 100 and 180 degrees, although lenses covering up to 280 degrees exist (see below). Their focal lengths depend on the film format they are designed for.

A distorting mirror, funhouse mirror or carnival mirror is a popular attraction at carnivals and fairs. Instead of a normal plane mirror that reflects a perfect mirror image, distorting mirrors are curved mirrors, often using convex and concave sections to achieve the distorted effect. Because of their distorting properties, they are sometimes featured in fiction as a literary device, such as in Hans Christian Andersen's 1844 fairy tale The Snow Queen.

Distorted mirrors are used as a metaphor for describing Wikipedia, personalized medicine and social media which rather than reflecting society, tend to distort our perceptions of reality and social norms.

A toy camera is a simple, inexpensive film camera. Despite the name, toy cameras are fully functional and capable of taking photographs, though with optical aberrations due to the limitations of their simple lenses. From the 1960s onward, there has been interest in the artistic use of such cameras or recreation of this style, both with cameras originally designed for children, and others originally intended as mass-market consumer cameras.

Many professional photographers have used toy cameras and exploited the vignetting, blur, light leaks, and other distortionsof their inexpensive lenses for artistic effect to take award-winning pictures.Toy camera photography has been widely exhibited at many popular art shows, such as the annual "Krappy Kamera" show at the Soho Photo Gallery in the Tribeca neighborhood of New York City. Various publications such as Popular Photography magazine have extolled the virtues of the Diana camera in its own right as an "art" producing image maker. Several books have also featured the work of toy cameras such as The Friends of Photography's The Diana Show, Iowa by Nancy Rexroth, and Angels at the Arno by Eric Lindbloom.

The Holga is a medium format 120 film camera, made in Hong Kong, known for its low-fidelity aesthetic.

The Holga's low-cost construction and simple meniscus lens often yields pictures that display vignetting, blur, light leaks and other distortions. The camera's limitations have brought it a cult following among some photographers, and Holga photos have won awards and competitions in art and news photography. As of July 2017, the camera was in production after being unavailable for two years.