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The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.. The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power.
Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the image diagonal. Wide-angle lens: angle of view wider than 60° and focal length shorter than normal. Long-focus lens: any lens with a focal length longer than the diagonal measure of the film or sensor. [10] Angle of view is narrower.
Optically designed as a reverse telephoto to enable the lens to fit into a standard mount as the focal length can be less than the distance from lens mount to focal plane. Long-focus lens - a lens with a focal length greater than the diagonal of the film frame or sensor. Long focus lenses are relatively simple to design, the challenges being ...
Using a positive lens of focal length f, a virtual image results when S 1 < f, the lens thus being used as a magnifying glass (rather than if S 1 ≫ f as for a camera). Using a negative lens ( f < 0 ) with a real object ( S 1 > 0 ) can only produce a virtual image ( S 2 < 0 ), according to the above formula.
In modern usage the term more commonly indicates simply that a film is of a substantial length or running time, as distinguished from short films, though what is considered "feature length" can vary and has changed over time. feature length The minimum running time necessary to be considered a full-length or feature film, as opposed to a short ...
The focal length of the lens determines the angle of view and, therefore, the field of view. Cinematographers can choose from a range of wide-angle lenses , "normal" lenses and long focus lenses , as well as macro lenses and other special effect lens systems such as borescope lenses.
Log-log graphs of lens focal length vs crop factor vs equivalent focal length for 35 mm film or image sensor size (= the focal length multiplied by a crop factor) vs diagonal, horizontal and vertical angles of view for film or sensors of 3:2 and 4:3 aspect ratios.
If the medium surrounding an optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from each principal plane to the corresponding focal point is just the focal length of the system. In the more general case, the distance to the foci is the focal length multiplied by the index of refraction of the medium.