Editing Objective (optics) (section)

==Microscope objectives==
[[File:Leica EpifluorescenceMicroscope ObjectiveLens.jpg|thumb|Two Leica [[oil immersion]] microscope objective lenses; left 100×, right 40×.]]
The objective lens of a [[microscope]] is the one at the bottom near the sample. At its simplest, it is a very high-powered [[magnifying glass]], with very short [[focal length]]. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample.

===Magnification===
One of the most important properties of microscope objectives is their [[magnification]]. The magnification typically ranges from 4× to 100×. It is combined with the magnification of the [[eyepiece]] to determine the overall magnification of the microscope; a 4× objective with a 10× eyepiece produces an image that is 40 times the size of the object.

A typical microscope has three or four objective lenses with different magnifications, screwed into a circular "nosepiece" which may be rotated to select the required lens. These lenses are often color coded for easier use. The least powerful lens is called the ''scanning objective lens'', and is typically a 4× objective. The second lens is referred to as the ''small objective lens'' and is typically a 10× lens. The most powerful lens out of the three is referred to as the ''large objective lens'' and is typically 40–100×.

===Numerical aperture===
[[Numerical aperture]] for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40&nbsp;mm to 2&nbsp;mm, respectively.

===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]] (∞).

===Objective pupil diameter===
The objective pupil diameter<ref name=microscopeUInfinity />, also known as entrance pupil diameter<ref>{{cite web | title=Imaging Microscope Objectives, Dry | url=https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1044 | website=Thorlabs | publisher=Thorlabs, Inc. | access-date=2025-05-17}}</ref> or back aperture diameter, refers to the diameter of the rear opening of an objective lens. In dry infinity corrected objectives, this diameter <math>D</math> is

<math> D = 2 \times NA \times f_{obj}</math>

where <math>NA</math> is the numerical aperture, and <math>f_{obj}</math> is the effective focal length. Magnification <math>M</math> and effective focal length are related by<ref>{{cite web | title=Infinity Microscope Basics | url=https://asiimaging.com/docs/infinity_microscope_basics | website=Applied Scientific Instrumentation | publisher=ASI | access-date=2025-05-17}}</ref>

<math> f_{tube} = M f_{obj}</math>

where <math>f_{tube}</math> is the tube lens focal length. Tube lens focal lengths vary by manufacturer: Leica and Nikon typically use 200 mm, Olympus uses 180 mm, and Zeiss uses 165 mm<ref name=microscopeUInfinity />.

===Cover thickness===
Particularly in biological applications, samples are usually observed under a glass [[cover slip]], which introduces distortions to the image. Objectives which are designed to be used with such cover slips will correct for these distortions, and typically have the thickness of the cover slip they are designed to work with written on the side of the objective (typically 0.17&nbsp;mm).

In contrast, so called "metallurgical" objectives are designed for reflected light and do not use glass cover slips.

The distinction between objectives designed for use with or without cover slides is important for high numerical aperture (high magnification) lenses, but makes little difference for low magnification objectives.

===Lens design===
Basic glass lenses will typically result in significant and unacceptable [[chromatic aberration]]. Therefore, most objectives have some kind of correction to allow multiple colors to focus at the same point. The easiest correction is an [[achromatic lens]], which uses a combination of [[Crown glass (optics)|crown glass]] and [[flint glass]] to bring two colors into focus. Achromatic objectives are a typical standard design.

In addition to oxide glasses, [[fluorite lens]]es are often used in specialty applications. These fluorite or semi-apochromat objectives deal with color better than achromatic objectives. To reduce aberration even further, more complex designs such as [[apochromat]] and [[superachromat]] objectives are also used.

All these types of objectives will exhibit some [[spherical aberration]]. While the center of the image will be in focus, the edges will be slightly blurry. When this aberration is corrected, the objective is called a  "plan" objective, and has a flat image across the field of view.

===Working distance===
The ''working distance'' (sometimes abbreviated WD) is the distance between the sample and the objective. As magnification increases, working distances generally shrinks. When space is needed, special long working distance objectives can be used.

===Immersion lenses===
Some microscopes use an [[oil-immersion objective|oil-immersion]] or [[Water immersion objective|water-immersion]] lens, which can have magnification greater than 100, and numerical aperture greater than 1. These objectives are specially designed for use with [[Index-matching material|refractive index matching oil]] or water, which must fill the gap between the front element and the object. These lenses give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.<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 Oct 2008}}</ref>

===Mounting threads===
The traditional [[screw thread]] used to attach the objective to the microscope was standardized by the [[Royal Microscopical Society]] in 1858.<ref>{{cite journal|url=https://www.biodiversitylibrary.org/item/18835#page/276/mode/1up|journal=Journal of the Royal Microscopy Society|page=230|year=1915|title=Objective Screw thread|access-date=2021-12-01}}</ref> It was based on the [[British Standard Whitworth]], with a 0.8 inch diameter and 36 threads per inch. This "RMS thread" or "society thread" is still in common use today. Alternatively, some objective manufacturers use designs based on [[ISO metric screw thread]] such as {{nowrap|M26 × 0.75}} and {{nowrap|M25 × 0.75}}.