Editing Binoculars (section)

=== Eyepieces ===
{{Main|Eyepiece}}
Binocular eyepieces usually consist of three or more lens elements in two or more groups.  The lens furthest from the viewer's eye is called the ''field lens'' or ''objective lens'' and that closest to the eye the ''eye lens'' or ''ocular lens''.  The most common [[Eyepiece#Kellner_or_"Achromat"|Kellner configuration]] is that invented in 1849 by [[Carl Kellner (optician)|Carl Kellner]].  In this arrangement, the eye lens is a plano-concave/ double convex achromatic doublet (the flat part of the former facing the eye) and the field lens is a double-convex singlet.  A [[Eyepiece#RKE|reversed Kellner eyepiec]]e was developed in 1975 and in it the field lens is a double concave/ double convex achromatic doublet and the eye lens is a double convex singlet.  The reverse Kellner provides 50% more eye relief and works better with small focal ratios as well as having a slightly wider field.<ref name="Tonkin2013">{{cite book|author=Stephen Tonkin|title=Binocular Astronomy|url=https://books.google.com/books?id=HSy8BAAAQBAJ&pg=PA11|date=15 August 2013|publisher=Springer Science & Business Media|isbn=978-1-4614-7467-8|pages=11–12|access-date=8 July 2017|archive-date=8 March 2020|archive-url=https://web.archive.org/web/20200308200835/https://books.google.com/books?id=HSy8BAAAQBAJ&pg=PA11|url-status=live}}</ref>

Wide field binoculars typically utilize some kind of [[Eyepiece#Erfle|Erfle configuration]], patented in 1921.  These have five or six elements in three groups.  The groups may be two achromatic doublets with a double convex singlet between them or may all be achromatic doublets.  These eyepieces tend not to perform as well as Kellner eyepieces at high power because they suffer from astigmatism and ghost images.  However they have large eye lenses, excellent eye relief, and are comfortable to use at lower powers.<ref name="Tonkin2013"/>

==== Field flattener lens ====
High-end binoculars often incorporate a [[field flattener lens]] in the eyepiece behind their prism configuration, designed to improve image sharpness and reduce image distortion at the outer regions of the field of view.<ref>{{Cite web |url=https://www.exploringoverland.com/overland-tech-travel/2021/3/21/be-your-own-optics-expert |title=Be your own optics expert |access-date=2022-04-14 |archive-date=2022-05-31 |archive-url=https://web.archive.org/web/20220531180129/https://www.exploringoverland.com/overland-tech-travel/2021/3/21/be-your-own-optics-expert |url-status=live }}</ref>