Editing Objective (optics) (section)

===Mechanical tube length===
Historically, microscopes were designed such that the objective lens would form an image in a specific plane near the eyepiece, which the eyepiece would re-image. Such microscopes were characterized by the ''mechanical tube length''; the distance between the mounting locations for the objective and the eyepiece.<ref>{{cite web |url=https://www.quekett.org/resources/understanding/objectives-ii#:~:text=The%20optical%20tube%20length%2C%20OTL,power%20have%20different%20focal%20lengths. |title=Understanding the microscope: 7. Objectives II |first=Jeremy |last=Sanderson |year=2010 |access-date=2025-05-18}}</ref> Early English microscopes used a mechanical tube length of {{convert|10|in|mm}}. In the 20th century most microscopes used the [[Royal Microscopical Society]] standard of 160 millimeters, while many [[Ernst Leitz GmbH|Leitz]] microscopes used 170 millimeters. Objectives had to be chosen to match the mechanical tube length of the microscope. 

Modern microscopes are often designed to use ''infinity correction'', in which the light coming out of the objective lens is [[Focus (optics)|focus]]ed at [[infinity]].<ref>{{Cite book|title=Photography with a Microscope|last1=Rost|first1=Fred|last2=Oldfield|first2=Ron|publisher=Cambridge University Press|year=2000|isbn=9780521770965|pages=83}}</ref><ref name=microscopeUInfinity>{{cite web | title=Infinity Optical Systems | url=https://www.microscopyu.com/microscopy-basics/infinity-optical-systems | website=MicroscopyU | publisher=Nikon Instruments Inc. | access-date=2025-05-17}}</ref> This is denoted on the objective with the [[infinity symbol]] (∞).