What are Aspherical lenses?

Many Sigma lenses make effective use of Aspherical lens elements. This technology allows one Aspherical lens element to perform the functions of two or more conventional spherical elements, thus allowing the lens to be made more compact and lightweight, while retaining high levels of performance. In the 20mm f/1.8, 24mm f/1.8, 28mm f/1.8 EX DG lenses, for example, these Aspherical elements eliminate the sagittal coma flare that accompanies large lens diameters. They effectively suppress the flare. In zoom lenses, where the wide angle range is extended - such as the 15-30 f/3.5-4.5 EX DG - distortion control can be enhanced by the use of Aspherical elements.

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