# Internal focusing

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An **internal focusing** lens (abbreviated as **IF**) is a [photographic lens](/source/Photographic_lens) design in which [focus](/source/Focus_(optics)) 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](/source/Rack_and_pinion) mechanism.

## Characteristics

Unit focusing [prime lenses](/source/Prime_lens) do not rotate or shift the front lens element; the entire lens moves in a linear direction perpendicular to the imaging plane.[1]: 86–87[2]: 194 As an alternative, some prime and [zoom lenses](/source/Zoom_lens) use *front cell focusing*, in which only the front element(s) of the lens are moved to adjust focus;[1]: 88[2]: 195 however, with front cell focusing, typically the front element(s) rotate as focus is adjusted.[3]

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](/source/Astigmatism_(optical_systems)), improving lens performance at close ranges.[4] 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.[2]: 195 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](/source/Polarizing_filter_(Photography)) or a petal shaped [lens hood](/source/Lens_hood) without having to readjust those items when the lens is focused. For [macro photography](/source/Macro_lens), 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.[5] The same advantages apply to *rear group internal focusing*, in which the rear elements are moved to vary focus.[6][7]: 50–53

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)](/source/Breathing_(lens))

## References

1. Ray, Sidney F. (1992). [*The Photographic Lens*](https://archive.org/details/photographiclens0000rays/). Second ed. Oxford: Focal Press. ISBN 0-2405-1329-0. LCCN 91-38092.

1. Ray, Sidney F. (2002). ["20: Focusing an optical system"](https://archive.org/details/appliedphotograp0000rays/). *Applied Photographic Optics*. Third ed. Oxford: Focal Press. pp. 192–200. ISBN 0-240-51540-4.

1. Kilpatrick, David (1990). [*Using Zoom Lenses with Your 35mm SLR*](https://archive.org/details/usingzoomlensesw0000kilp/page/86/mode/2up). First ed. Surrey: Fountain Books. p. 86. ISBN 0-86343-195-X.

1. Ohshita, Kouichi. ["NIKKOR - The Thousand and One Nights No.14: NIKKOR-N Auto 24mm F2.8"](https://imaging.nikon.com/imaging/information/story/0014/). *Nikon Imaging*. Retrieved 2 April 2025.

1. Atkins, Bob. ["Measuring focal length"](http://www.bobatkins.com/photography/technical/measuring_focal_length.html). Retrieved 2 April 2025.

1. ["FD400mm f/4.5 S.S.C."](https://global.canon/en/c-museum/product/fd181.html). *Canon Camera Museum*. Retrieved 2 April 2025. Lenses which carry out focusing by moving one or more lens groups behind the diaphragm position are called rear focusing lenses. As the lens length does not change during focusing and a compact, lightweight design is possible with rear focusing system, hand-held shooting is possible even with such a super telephoto lens of 400mm.

1. ["Canon Interchangeable Lenses"](https://archive.org/details/canon-fd-lenses-1981-12/page/n25/mode/2up). Canon Inc. December 1981. Retrieved 2 April 2025.

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Adapted from the Wikipedia article [Internal focusing](https://en.wikipedia.org/wiki/Internal_focusing) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Internal_focusing?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.