Physics:Parfocal lens
A parfocal lens is a lens or optical system that remains approximately in focus when its magnification or focal length is changed. In practice, some focus shift is usually present, but it is small enough that little or no refocusing is needed.[1]
The term is used in microscopy, photography, cinematography, astronomy, and other optical applications in which magnification may be changed during observation or image capture. [2]
Microscopy
In microscopy, parfocality refers to the property of a set of objective lenses or eyepieces that allows a specimen to remain nearly in focus when magnification is changed. For example, when a microscope is switched from a lower-power objective (such as 10×) to a higher-power objective (such as 40×), only slight focus adjustment is typically needed.[3]
Most modern laboratory bright-field microscopes are designed to be approximately parfocal. In microscope design, parfocality is supported by the standardization of optical and mechanical dimensions, so that objectives of different magnifications can be interchanged without major refocusing. [4]
Photography
In photography, a zoom lens is described as parfocal if it maintains focus while its focal length is changed. This allows the photographer or camera operator to focus at one focal length and then reframe by zooming without substantial loss of focus.[5]
Many consumer zoom lenses are instead varifocal, meaning that focus changes as focal length changes. In still photography this is often compensated for by autofocus systems or by electronic correction within the camera, so the distinction may not be obvious in normal use. [6]
Cinematography
In cinematography, parfocal behavior is especially important in zoom lenses used during a shot, because visible focus shift while zooming is usually undesirable. Professional cinema and broadcast zoom lenses are therefore generally designed to maintain focus throughout the zoom range. [7]
Whether a lens remains effectively parfocal in use may also depend on correct setup. A lens designed to be parfocal can appear to lose focus during zooming if its back focus, or flange focal distance adjustment, is not properly set for the camera body on which it is used.[8]
Astronomy
The term is also used informally in amateur astronomy to describe sets of telescope eyepieces that require little or no refocusing when exchanged. In practice, eyepieces marketed as parfocal may still require small focus adjustments depending on the telescope and optical accessories used.
Other applications
Parfocal optical systems are also used in a range of technical and observational instruments where it is useful to maintain focus while changing magnification or optical configuration. Examples include some binoculars, industrial imaging systems, machine vision equipment, and optical inspection instruments.
In such applications, parfocality can reduce the need for repeated manual refocusing and help maintain more consistent viewing or image capture conditions.[9]
Distinction from varifocal lenses
A parfocal lens should be distinguished from a varifocal lens, in which focus changes as focal length changes. Although the terms are sometimes confused in consumer marketing, the distinction is operationally important in applications such as microscopy, video production, and observation optics. [10]
See also
References
- ↑ "parfocality". https://www.microscope.healthcare.nikon.com/resources/glossary/parfocality.
- ↑ "What Is a Parfocal Objective" (in en). https://microscopeinternational.com/what-is-a-parfocal-objective/.
- ↑ "parfocality". https://www.microscope.healthcare.nikon.com/resources/glossary/parfocality.
- ↑ "How to Parfocal Microscope Objectives | Microscope World Resources". https://www.microscopeworld.com/how-to-parfocal-microscope-objectives/.
- ↑ "Parfocal Lenses". 2003-05-29. https://web.archive.org/web/20071007070643/http://www.rogercavanagh.com/helpinfo/30_parfocal.stm.
- ↑ "Best Lenses - All Categories" (in en). https://toptechchoices.com/us/en/best/lenses/.
- ↑ on, Kyle DeGuzman (2021-05-09). "What is a Parfocal Lens — The Advantage of Parfocal Lenses" (in en-US). https://www.studiobinder.com/blog/what-is-a-parfocal-lens-definition/.
- ↑ "How to Back Focus a Lens". 2024-08-28. https://www.fujinon.com/how-to-back-focus-a-lens/.
- ↑ "Basic Microscopy Concepts - Parfocality Adjustment for Upright Microscopes" (in en). https://moticmicroscopes.com/blogs/articles/basic-microscopy-concepts-parfocality-adjustment-for-upright-microscopes.
- ↑ Vistek (2017-04-03). Parfocal vs Varifocal lenses | what's the difference and why does it matter?. Retrieved 2026-03-30 – via YouTube.
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