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A triplet lens is a 
The three lenses may be cemented together, as in the Steinheil triplet or the Hastings triplet. Or a triplet may be designed with three spaced glasses, as in the 
Jewellers'
compound 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 ...
consisting of three single lenses
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 ...
. The triplet design is the simplest to give the required number of degrees of freedom
Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...
to allow the lens designer to overcome all Seidel aberrations.
The three lenses may be cemented together, as in the Steinheil triplet or the Hastings triplet. Or a triplet may be designed with three spaced glasses, as in the Cooke triplet
The Cooke triplet is a photographic lens designed and patented (patent number GB 22,607) in 1893 by Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of t ...
. The former has the advantage of higher optical throughput due to fewer air-glass interfaces, but the latter provides greater flexibility in aberration control, as the internal surfaces are not confined to have the same radii of curvature.
Jewellers' loupe
A loupe ( ) is a simple, small magnification device used to see small details more closely. They generally have higher magnification than a magnifying glass, and are designed to be held or worn close to the eye. A loupe does not have an attached h ...
s typically use a triplet lens.
See also
*Doublet (lens)
In optics, a doublet is a type of lens made up of two simple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element". With ...
*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 com ...
* Apochromatic lens
* The five Seidel aberrations
*Mangin mirror
In optics, a Mangin mirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration if certain conditions are met. This reflector was invented ...
References
Lenses 3 (number) {{optics-stub