Editing Binoculars (section)

=== Focus and adjustment ===
[[File:The new Steiner Military 8x30 R binoculars MOD 45158985.jpg|thumb|Independent focusing binoculars as used by the British military]]
[[File:Binocular Eschenbach Derby Compact 8x40 Ww.jpg|thumb|Porro type, external eyepiece bridge central-focusing binoculars with a rotating diopter on the right eyepiece allowing to adjust refractive differences between the viewer's left and right eyes]]

Binoculars have a [[focus (optics)|focusing]] arrangement which changes the distance between eyepiece and objective lenses or internally mounted lens elements. Normally there are two different arrangements used to provide focus, "independent focus" and "central focusing":
* ''Independent focusing'' is an arrangement where the two telescope tubes are focused independently by adjusting each eyepiece. Binoculars designed for harsh environmental conditions and heavy field use, such as military or marine applications, traditionally have used independent focusing.
* ''Central focusing'' is an arrangement which involves rotation of a central focusing wheel to adjust both telescope tubes together. In addition, one of the two eyepieces can be further adjusted to compensate for differences between the viewer's eyes (usually by rotating the eyepiece in its mount). Because the focal change effected by the adjustable eyepiece can be measured in the customary unit of refractive power, the [[dioptre]], the adjustable eyepiece itself is often called a ''dioptre''. Once this adjustment has been made for a given viewer, the binoculars can be refocused on an object at a different distance by using the focusing wheel to adjust both tubes together without eyepiece readjustment.<br />Central focusing binoculars can be further subdivided into:
** ''External focusing'', which focuses binoculars by moving the eyepieces, where the volume of the binoculars always changes. During this process, external air and also small dust particles and moisture can be drawn into or pressed out of the binoculars. It is hard to seal or waterproof such systems and in case the eyepieces are moved by a central focuser shaft and external eyepiece arms bridge construction, this construction can (accidentally) get bent/deformed that can result in disabling misalignment.
** ''Internal focusing'', which focuses binoculars by moving internal mounted optical lenses located between the objective lens group and the prism assembly – or rarely located between the prism assembly and eyepiece lens assembly<ref name="auto"/><ref>{{Cite web |url=https://patents.google.com/patent/US3484149A/en |title=US Patent US3484149A Center focusing prism binocular and reticle |access-date=2022-09-17 |archive-date=2022-09-20 |archive-url=https://web.archive.org/web/20220920170348/https://patents.google.com/patent/US3484149A/en |url-status=live }}</ref> – within the housing without changing the volume of the binoculars. The addition of a focusing lens reduces the light transmission of the optical system contained in the telescope tube somewhat. Internal focusing is generally considered the mechanically more robust central focusing solution and with the help of an appropriate seal like O-rings air and moisture ingress can be prevented, to make binoculars fully waterproof.<ref>{{Cite web |url=https://binocularsky.com/binoc_basics.php |title=Binocular Basics |access-date=2022-07-31 |archive-date=2022-02-28 |archive-url=https://web.archive.org/web/20220228172235/https://binocularsky.com/binoc_basics.php |url-status=live }}</ref>

With increasing magnification, the [[depth of field]] – the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image – decreases. The depth of field reduces quadratic with the magnification, so compared to 7× binoculars, 10× binoculars offer about half (7² ÷ 10² = 0.49) the depth of field. However, not related to the binoculars optical system, the user perceived practical depth of field or depth of acceptable view performance is also dependent on the [[Accommodation (eye)|accommodation ability]] (accommodation ability varies from person to person and decreases significantly with age) and light conditions dependent effective pupil size or diameter of the user's eyes.
There are "focus-free" or "fixed-focus" binoculars that have no focusing mechanism other than the eyepiece adjustments that are meant to be set for the user's eyes and left fixed. These are considered to be compromise designs, suited for convenience, but not well suited for work that falls outside their designed [[hyperfocal distance]] range (for hand held binoculars generally from about {{convert|35|m|yd|0|abbr=on}} to infinity without performing eyepiece adjustments for a given viewer).<ref>{{Cite web |url=https://www.bestbinocularsreviews.com/self_focusing_binoculars.php |title=Self Focusing Binoculars, Fixed Focus & Individual Focus Binoculars |access-date=2022-05-13 |archive-date=2022-05-31 |archive-url=https://web.archive.org/web/20220531201745/https://www.bestbinocularsreviews.com/self_focusing_binoculars.php |url-status=live }}</ref>

Binoculars can be generally used without eyeglasses by [[myopic]] (near-sighted) or [[hyperopic]] (far-sighted) users simply by adjusting the focus a little farther. Most manufacturers leave a little extra available focal-range beyond the infinity-stop/setting to account for this when focusing for infinity.<ref>{{Cite web |date=2022-12-29 |title=How To Use Binoculars With Glasses: Easy Guide with 6 steps |url=https://birdsatfirstsight.com/how-to-use-binoculars-with-glasses/ |access-date=2023-07-24 |website=Birds at First Sight |language=en-US}}</ref> People with severe astigmatism, however, will still need to use their glasses while using binoculars.

Some binoculars have adjustable magnification, ''zoom binoculars'', such as 7-21×50 intended to give the user the flexibility of having a single pair of binoculars with a wide range of magnifications, usually by moving a "zoom" lever. This is accomplished by a complex series of adjusting lenses similar to a [[Zoom lens|zoom camera lens]]. These designs are noted to be a compromise and even a [[gimmick]]<ref>{{cite book |first=Pete |last=Dunne |url=https://books.google.com/books?id=WfxnqueHQmEC&pg=PA54 |title=Pete Dunne on Bird Watching: the how-to, where-to, and when-to of birding |isbn=9780395906866 |publisher=Houghton Mifflin Harcourt |date=2003 |page=54 |access-date=2016-10-10 |archive-date=2016-12-27 |archive-url=https://web.archive.org/web/20161227130917/https://books.google.com/books?id=WfxnqueHQmEC&pg=PA54 |url-status=live }}</ref> since they add bulk, complexity and fragility to the binocular. The complex optical path also leads to a narrow field of view and a large drop in brightness at high zoom.<ref>{{cite book |first=Philip S. |last=Harrington |url=https://books.google.com/books?id=2lIwU313wgkC&pg=PT65 |title=Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using |isbn=9781118046333 |publisher=John Wiley & Sons |date=2011 |page=54 |access-date=2016-10-10 |archive-date=2016-12-27 |archive-url=https://web.archive.org/web/20161227225719/https://books.google.com/books?id=2lIwU313wgkC&pg=PT65 |url-status=live }}</ref> Models also have to match the magnification for both eyes throughout the zoom range and hold collimation to avoid eye strain and fatigue.<ref>{{cite book |first=Stephen |last=Tonkin |url=https://books.google.com/books?id=ac6wseOonlcC&pg=PT9 |title=Binocular Astronomy: The Patrick Moore Practical Astronomy Series |isbn=9781846287886 |publisher=Springer Science & Business Media |date=2007 |page=46 |access-date=2016-10-10 |archive-date=2016-12-28 |archive-url=https://web.archive.org/web/20161228002152/https://books.google.com/books?id=ac6wseOonlcC&pg=PT9 |url-status=live }}</ref>  These almost always perform much better at the low power setting than they do at the higher settings. This is natural, since the front objective cannot enlarge to let in more light as the power is increased, so the view gets dimmer. At 7×, the 50mm front objective provides a 7.14&nbsp;mm exit pupil, but at 21×, the same front objective provides only a 2.38&nbsp;mm exit pupil. Also, the optical quality of a zoom binocular at any given power is inferior to that of a fixed power binocular of that power.