Objective (optics)

Objective (optics)

Overview
In an optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...

 to produce a real image
Real image
In optics, a real image is a representation of an object in which the perceived location is actually a point of convergence of the rays of light that make up the image. If a screen is placed in the plane of a real image the image will generally become visible on the screen...

. Objectives can be single lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 or mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s, or combinations of several optical elements. They are used in microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

s, telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s, camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s, slide projector
Slide projector
A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows...

s, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

The objective lens of a microscope is the one at the bottom near the sample.
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Encyclopedia
In an optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...

 to produce a real image
Real image
In optics, a real image is a representation of an object in which the perceived location is actually a point of convergence of the rays of light that make up the image. If a screen is placed in the plane of a real image the image will generally become visible on the screen...

. Objectives can be single lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 or mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s, or combinations of several optical elements. They are used in microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

s, telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s, camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s, slide projector
Slide projector
A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows...

s, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

Microscope


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
Magnifying glass
A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle ....

, with very short focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

. 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.

A typical microscope has several objective lenses with different focal lengths screwed into a circular nose piece which may be rotated to select the required lens. The lenses are typically designed to be parfocal, which means that when one changes from one to another the sample stays in focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

.

Microscope objectives are characterized by two parameters: magnification
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 and numerical aperture
Numerical aperture
In optics, the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one...

. The former typically ranges from 4× to 100×, while the latter ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 to 2 mm, respectively. For high magnification applications, an oil-immersion objective or water-immersion objective has to be used. The objective is specially designed and refractive index matching oil
Index-matching material
In optics and fiber optics, an index-matching material is a substance, usually a liquid, cement , or gel, which has an index of refraction that closely approximates that of an optical element or fiber, and is used to reduce Fresnel reflection at the surface of the element.In fiber optics and...

 or water must fill the air gap between the front element and the object to allow the numerical aperture to exceed 1, and hence give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.

Photography and imaging


Camera lenses (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 aberrations. Image projector
Image projector
An image projector is an optical device that projects an image onto a surface, commonly a projection screen.Most projectors creates an image by shining a light through a small transparent image, but some newer types of projectors can project the image directly, by using lasers...

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.

Telescope


In a telescope the objective is the lens at the front end of a refractor
Refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

 or the image-forming primary mirror
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...

 of a reflecting
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...

 or catadioptric telescope. A telescope's light-gathering power and angular resolution
Angular resolution
Angular resolution, or spatial 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...

are both directly related to the diameter (or "aperture") of its objective lens or mirror. The larger the objective, the dimmer the object it can view and the more detail it can resolve.