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A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye.
1: Imaging by a lens with chromatic aberration. 2: A lens with less chromatic 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.
For a doublet consisting of two thin lenses in contact, the Abbe number of the lens materials is used to calculate the correct focal length of the lenses to ensure correction of chromatic aberration.
In optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is an approximate measure of the material's dispersion (change of refractive index versus wavelength), with high values of V indicating low dispersion.
Abbe was the first to define the term numerical aperture, as the sine of the half angle multiplied by the refractive index of the medium filling the space between the cover glass and front lens. Abbe is credited by many for discovering the resolution limit of the microscope, and the formula (published in 1873)
In optics, the Abbe sine condition is a condition that must be fulfilled by a lens or other optical system in order for it to produce sharp images of off-axis as well as on-axis objects. It was formulated by Ernst Abbe in the context of microscopes .