A parfocal lens is a
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
that stays in
focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
when
magnification/
focal length is changed. There is inevitably some amount of focus error, but too small to be considered significant.
Microscopy
Parfocal
microscope objectives stay in focus when magnification is changed; i.e., if the
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
is switched from a lower power objective (e.g., 10×) to a higher power objective (e.g., 40×), the object stays in focus. Most modern bright-field microscopes are parfocal.
Photography
''
Zoom lens
A zoom lens is a mechanical assembly of lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed-focal-length (FFL) lens (see prime lens).
A true zoom lens, also called a parfocal lens, is one ...
es'' (sometimes referred to as "true" zoom) are ideally ''parfocal'', in that focus is maintained as the lens is zoomed (i.e., focal length and magnification changed), which is convenient and has the advantage of allowing more accurate focusing at maximal focal length then zooming back to a shorter focal length to compose the image.
[ ]
Many zoom lenses, particularly in the case of fixed-lens cameras, are actually ''
varifocal lenses'', which gives lens designers more flexibility in optical design trade-offs (e.g., focal length range, maximal aperture, size, weight, cost) than parfocal zoom, which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming"), making operation practically indistinguishable from a parfocal zoom.
Cinematography
Zoom lenses used for
moviemaking applications must have the parfocal ability in order to be of practical use. It is almost impossible to stay in correct focus (as done manually by the
focus puller
A focus puller or first assistant camera (1st AC) is a member of a film crew's camera department whose primary responsibility is to maintain the camera lens's optical focus on whatever subject or action is being filmed.
"Pulling focus" refers to ...
) while zooming. This could be possible theoretically, but if the subject and/or camera also move, there are too many variables to correct consistently.
Telescopy
Parfocal telescope
eyepiece
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
s stay in focus when magnification is changed; i.e., if the
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
is switched from a lower-power eyepiece (e.g., 10×) to a higher-power eyepiece (e.g., 20×), or vice versa, the object stays in focus.
References
* {{cite web, url=http://www.tokina-usa.com/glossary.html , title=Lens Glossary , accessdate=2007-11-18 , work=Tokina Industrial, Inc.
Lenses