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An internal focusing lens (abbreviated as IF) is a
Starting in the 1960s, optical designers began developing ''floating element'' lenses, which incorporate elements from zoom lens design and automatically adjust the internal distance between element(s) and group(s) as the lens is focused to reduce
photographic lens
A camera lens, photographic lens or photographic objective is an optical lens (optics), lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to Imaging, make images of objects either on photographic film ...
design in which focus
Focus (: foci or focuses) may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film
*Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel
*Focus (2015 ...
is adjusted by moving internal lens element(s). In contrast, traditional ''unit focusing'' lenses are focused by adjusting the distance between the entire set of lens elements and the imaging plane, usually through a helicoid or rack and pinion
rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert between rotational motion and linear motion: rotating the pinion causes the rack to be d ...
mechanism.
Characteristics
Unit focusingprime lens
In film and photography, a prime lens is a fixed focal length photographic lens (as opposed to a zoom lens), typically with a maximum aperture from f2.8 to f1.2. The term can also mean the primary lens in a combination lens system.
Confusion ...
es do not rotate or shift the front lens element; the entire lens moves in a linear direction perpendicular to the imaging plane. As an alternative, some prime and zoom lens
A zoom lens is a system of camera lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed-focal-length (FFL) lens (''prime lens'').
A true zoom lens or optical zoom lens is a type of '' parfocal ...
es use ''front cell focusing'', in which only the front element(s) of the lens are moved to adjust focus; however, with front cell focusing, typically the front element(s) rotate as focus is adjusted.
Starting in the 1960s, optical designers began developing ''floating element'' lenses, which incorporate elements from zoom lens design and automatically adjust the internal distance between element(s) and group(s) as the lens is focused to reduce astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...
, improving lens performance at close ranges. As a parallel evolution, lenses were developed in which internal element(s) could be adjusted to vary focus distance, which has significant advantages in lens size, handling, and sealing. This also means internal focusing lenses do not rotate or shift the front lens element. This makes it easy to use, for example, a screwed-in polarizing filter or a petal shaped lens hood
In photography, a lens hood or lens shade is a device used on the front end of a photographic lens, lens to block the Sun or other light source(s) to prevent glare (vision), glare and lens flare. Lens hoods may also be used to protect the lens fr ...
without having to readjust those items when the lens is focused. For macro photography
Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is g ...
, using an internal focus lens also reduces the risk of the front of the lens accidentally hitting the subject during focusing as the front element does not move. The same advantages apply to ''rear group internal focusing'', in which the rear elements are moved to vary focus.
The physical size of an internal focusing lens does not change during focus, nor does the front of the lens rotate. This is particularly useful for large lenses, keeping the size more compact, or when using filters or accessories mounted on the front of the lens that may require careful alignment.
One issue internal focusing lens can have is that the true focal length of the lens is reduced when not focused at infinity.
See also
* Breathing (lens)References
Photographic lenses {{Photo-stub