Parfocal
Encyclopedia
A parfocal lens is a lens
that stays in focus
when magnification
/focal length
is changed. There is inevitably some amount of focus error, but small enough to be considered insignificant.
stay in focus when magnification is changed; i.e., if the microscope
is switched from a higher power objective (e.g., 40×) to a lower power objective (e.g., 10×), the object stays in focus.
es (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and 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 lens
es, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and 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.
stay in focus when magnification is changed; i.e., if the telescope
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.
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
that stays in focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
when magnification
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
/focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
is changed. There is inevitably some amount of focus error, but small enough to be considered insignificant.
Microscopy
Parfocal microscope objectivesObjective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...
stay in focus when magnification is changed; i.e., if the microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
is switched from a higher power objective (e.g., 40×) to a lower power objective (e.g., 10×), the object stays in focus.
Photography
Zoom lensZoom lens
A zoom lens is a mechanical assembly of lens elements for which the focal length can be varied, as opposed to a fixed focal length lens...
es (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and 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 lens
Varifocal lens
A varifocal lens is a camera lens with variable focal length in which focus changes as focal length changes, as compared to parfocal zoom lens, which remains in focus as the lens zooms . Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses....
es, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and 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.
Telescopes
Parfocal telescope eyepiecesEyepiece
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...
stay in focus when magnification is changed; i.e., if the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
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.