Editing Optical microscope (section)

===Objective lens===
{{Main|Objective (optics)}}

At the lower end of a typical compound optical microscope, there are one or more [[objective lens]]es that collect light from the sample. The objective is usually in a cylinder housing containing a glass single or multi-element compound lens. Typically there will be around three objective lenses screwed into a circular nose piece which may be rotated to select the required objective lens. These arrangements are designed to be [[Parfocal lens|parfocal]], which means that when one changes from one lens to another on a microscope, the sample stays in [[focus (optics)|focus]]. Microscope objectives are characterized by two parameters, namely, [[magnification]] and [[numerical aperture]]. The former typically ranges from 5× to 100× while the latter ranges from 0.14 to 0.7, corresponding to [[focal length]]s of about 40 to 2&nbsp;mm, respectively. Objective lenses with higher magnifications normally have a higher numerical aperture and a shorter [[depth of field]] in the resulting image. Some high performance objective lenses may require matched eyepieces to deliver the best optical performance.{{cn|date=December 2024}}

====Oil immersion objective====
[[File:Leica EpifluorescenceMicroscope ObjectiveLens.jpg|thumb|right|300px|Two Leica [[oil immersion]] microscope objective lenses: 100× (left) and 40× (right)]]
{{Main|Oil immersion}}

Some microscopes make use of [[oil-immersion objective]]s or water-immersion objectives for greater resolution at high magnification. These are used with [[index-matching material]] such as [[immersion oil]] or water and a matched cover slip between the objective lens and the sample. The refractive index of the index-matching material is higher than air allowing the objective lens to have a larger numerical aperture (greater than 1) so that the light is transmitted from the specimen to the outer face of the objective lens with minimal refraction. Numerical apertures as high as 1.6 can be achieved.<ref>{{cite web |url=http://www.olympusmicro.com/primer/anatomy/objectives.html |title=Microscope objectives |work=Olympus Microscopy Resource Center |first=Spring |last=Kenneth |author2=Keller, H. Ernst |author3=Davidson, Michael W. |access-date=29 October 2008 |url-status=live |archive-url=https://web.archive.org/web/20081101133539/http://www.olympusmicro.com/primer/anatomy/objectives.html |archive-date=1 November 2008  }}</ref> The larger numerical aperture allows collection of more light making detailed observation of smaller details possible. An oil immersion lens usually has a magnification of 40 to 100×.{{cn|date=December 2024}}