Biconvex lenses or convex lenses are also called converging lenses because they collect a parallel beam of light in one point, the focal point. Biconvex is a lens that is curved outwards on both sides. An aspherical lens has considerably better imaging properties than a lens with spherical radii (e.g. biconvex lens).
Aplanatic optics for the highest inspection requirements or very delicate work where very good imaging properties are necessary! An aplanatic lens system consists of 2 plano-convex lenses which are arranged with their curved (convex) side to each other and shows a sharp-edged and distortion-free image due to this arrangement. These plano-convex lenses are mainly made of glass. Aplanatic lens systems are used in magnifying glasses that are supposed to have excellent imaging properties right to the edge. Such magnifiers differ from 'standard' magnifiers with biconvex lenses in that they have significantly better imaging properties, as the distortions that usually occur at the edge are completely eliminated.
In optics, an achromat is a system of two lenses consisting of glasses with different Abbe numbers (different degrees of dispersion). The system contains a converging lens (usually made of crown glass) and a diverging lens with a smaller Abbe number and thus stronger dispersion than the converging lens (usually made of flint glass). This allows the longitudinal colour error, which every simple lens has, to be corrected for two wavelengths, i.e. the system has the same cut-off for these two colours.
An apochromat (Greek for free of colour) is an optical system, e.g. a lens, in which the colour error is corrected as far as possible. In contrast to an achromatic lens, not only the primary but also the secondary spectrum is corrected. In its original meaning, an apochromatic lens system is calculated in such a way that the cut width matches for three wavelengths (colours) in the visible range, whereas with the simpler achromat the match is required for only two wavelengths. For the remaining wavelengths, too, there is then only a very small deviation of the cut width, and the longitudinal colour error is thus very well corrected.