![Binocular 20101111 b](https://upload.wikimedia.org/wikipedia/commons/6/6e/Binocular_20101111_b.jpg)
In
optical engineering Optical engineering is the field of science and engineering encompassing the physical phenomena and technologies associated with the generation, transmission, manipulation, detection, and utilization of light. Optical engineers use optics to solve ...
, the objective is the optical element that gathers light from the object being observed and
focuses the
light rays to produce a
real image
{{citations needed, date=June 2019
In optics, an ''image'' is defined as the collection of focus points of light rays coming from an object. A real image is the collection of focus points actually made by converging/diverging rays, while a v ...
. Objectives can be a single
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
or
mirror
A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
, or combinations of several optical elements. They are used in
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
s,
binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
,
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
s,
camera
A camera is an Optics, optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), ...
s,
slide projector
A slide projector is an opto-mechanical device for showing photographic slides.
35 mm slide projectors, direct descendants of the larger-format magic lantern, first came into widespread use during the 1950s as a form of occasional home ...
s,
CD player
A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or aud ...
s and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.
Microscope objectives
![Leica EpifluorescenceMicroscope ObjectiveLens](https://upload.wikimedia.org/wikipedia/commons/1/1f/Leica_EpifluorescenceMicroscope_ObjectiveLens.jpg)
The objective lens of a
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
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
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
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 mm to 2 mm, respectively.
Mechanical tube length
Historically, microscopes were nearly universally designed with a finite mechanical tube length, which is the distance the light traveled in the microscope from the objective to the eyepiece. The
Royal Microscopical Society
The Royal Microscopical Society (RMS) is a learned society for the promotion of microscopy. It was founded in 1839 as the Microscopical Society of London making it the oldest organisation of its kind in the world. In 1866, the society gained it ...
standard is 160 millimeters, whereas
Leitz Leitz may refer to several German companies:
*Esselte Leitz GmbH & Co KG, founded by Louis Leitz in 1896, a German manufacturer of office products
**Louis Leitz (1846–1918), German inventor and founder of Esselte Leitz GmbH & Co KG
* Leitz GmbH & ...
often used 170 milliliters. 180 millimeter tube length objectives are also fairly common. Using an objective and microscope that were designed for different tube lengths will result in
spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
.
Instead of finite tube lengths, modern microscopes are often designed to use infinity correction instead, a technique in
microscopy
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
whereby the light coming out of the objective lens is
focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
ed at
infinity
Infinity is that which is boundless, endless, or larger than any natural number. It is often denoted by the infinity symbol .
Since the time of the ancient Greeks, the philosophical nature of infinity was the subject of many discussions amo ...
. This is denoted on the objective with the
infinity symbol
The infinity symbol (\infty) is a mathematical symbol representing the concept of infinity. This symbol is also called a lemniscate, after the lemniscate curves of a similar shape studied in algebraic geometry, or "lazy eight", in the terminol ...
(∞).
Cover thickness
Particularly in biological applications, samples are usually observed under a glass
cover slip
A microscope slide is a thin flat piece of glass, typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted (secured) on the slide, and then b ...
, 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 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
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
. 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
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane.
The most comm ...
, which uses a combination of
crown glass and
flint glass
Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractiv ...
to bring two colors into focus. Achromatic objectives are a typical standard design.
In addition to oxide glasses,
fluorite lens
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in cubic crystal system, isometric crystal habit, cubic habit, although octahedral and more complex isometric for ...
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
An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.
Explanation
Chromatic aberration is the phenomenon of differen ...
and
superachromat
The superachromat or superachromatic lens was first conceived and developed by Maximilian Herzberger as the ultimate well-corrected lens. The color shift curve of a superachromat is a quartic, meaning that in theory four separate colors can be ...
objectives are also used.
All these types of objectives will exhibit some
spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
. 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 or
water-immersion lens, which can have magnification greater than 100, and numerical aperture greater than 1. These objectives are specially designed for use with
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.
Mounting threads
The traditional
screw thread
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a ...
used to attach the objective to the microscope was standardized by the
Royal Microscopical Society
The Royal Microscopical Society (RMS) is a learned society for the promotion of microscopy. It was founded in 1839 as the Microscopical Society of London making it the oldest organisation of its kind in the world. In 1866, the society gained it ...
in 1858. 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
The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.
Th ...
such as and .
Photography and imaging
Camera lens
A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capab ...
es (usually referred to as "photographic objectives" instead of simply "objectives") need to cover a large focal plane so are made up of a number of optical lens elements to correct
optical aberration
In optics, aberration is a property of optical systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with th ...
s.
Image projector
A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen. Most projectors create an image by shining a light through a small transparent lens, but some newer typ ...
s (such as video, movie, and slide projectors) use objective lenses that simply reverse the function of a camera lens, with lenses designed to cover a large image plane and project it at a distance onto another surface.
Telescopes
![KeckObservatory20071013](https://upload.wikimedia.org/wikipedia/commons/7/78/KeckObservatory20071013.jpg)
In a telescope the objective is the lens at the front end of a
refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
(such as
binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
or
telescopic sight
A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a ''reticle'' – mounted in a focally appropriate po ...
s) or the image-forming
primary mirror
A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope.
Description
The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...
of a
reflecting or
catadioptric telescope
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses ( dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlights ...
. A telescope's light-gathering power and
angular resolution
Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolution. ...
are both directly related to the diameter (or "aperture") of its objective lens or mirror. The larger the objective, the brighter the objects will appear and the more detail it can resolve.
See also
*
List of telescope parts and construction Hardware Accessories
*Finderscope
* Iron sight
* Reflector (reflex) sight
* Cheshire collimator: A simple tool to collimate a telescope
Control
*Clock drive
* GoTo
Mechanical construction
*Mirror support cell
*Serrurier truss
*Silvering
Mounts
* ...
*
Etendue
Etendue or étendue (; ) is a property of light in an optical system, which characterizes how "spread out" the light is in area and angle. It corresponds to the beam parameter product (BPP) in Gaussian beam optics. Other names for etendue include a ...
References
{{Authority control
Lenses
Microscope components
Microscopy