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For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens.For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot.
For a thin lens in air, the distance from the lens to the spot is the focal length of the lens, which is commonly represented by f in diagrams and equations. An extended hemispherical lens is a special type of plano-convex lens, in which the lens's curved surface is a full hemisphere and the lens is much thicker than the radius of curvature.
Diagram illustrating the flange focal length of an SLR–type and a mirrorless–type camera. For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), back focus [1] or register, depending on the usage and source) of a lens mount system is the distance from the ...
A 100 mm focal length f / 4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f / 2 lens has an entrance pupil diameter of 50 mm. Since the area is proportional to the square of the pupil diameter, [6] the amount of light admitted by the f / 2 lens is four times that of the f / 4 lens.
The effective focal length is nearly equal to the stated focal length of the lens (F), except in macro photography where the lens-to-object distance is comparable to the focal length. In this case, the magnification factor ( m ) must be taken into account: f = F ⋅ ( 1 + m ) {\displaystyle f=F\cdot (1+m)}
The depth of field, and thus hyperfocal distance, changes with the focal length as well as the f-stop. This lens is set to the hyperfocal distance for f /32 at a focal length of 100 mm. In optics and photography, hyperfocal distance is a distance from a lens beyond which all objects can be brought into an "acceptable" focus.
In order to reach a minimal focal distance, the aperture and the focal length of the lens are reduced (a slow wide-angle lens), to make the hyperfocal distance small. This allows the depth of field to extend from a short distance to infinity.
35 mm equivalent focal length. The resulting images from 50 mm and 70 mm lenses for different sensor sizes; 36x24 mm (red) and 24x18 mm (blue) In photography, the 35 mm equivalent focal length is a measure of the angle of view for a particular combination of a camera lens and film or image sensor size. The term is popular because in the early ...