What is an Aspherical Lens Surface?
In order to achieve a high quality image even at wide open T-stops, lens designers are using new shapes for the surfaces of individual lens elements. These new shapes help eliminate optical aberrations and distortions, while keeping the lens' size and weight down. One of these shapes is the aspherical lens surface, as used in the ARRI/Zeiss Master Primes, Ultra Prime 8R, Ultra 16 lenses and Lightweight Zoom LWZ-1.
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Most lenses have spherical surfaces, that is their surfaces have a constant curvature, as if the lens surface had once been part of a sphere. Aspherical lens surfaces, in contrast, have surfaces that have a different degree of curvature at the center and the edges. The sophisticated technology for creating such lens surfaces was originally developed for lenses for computer chip manufacturing. it requires ultra-high precision in manufacturing and a complex and expensive holographic measurement process for surface accuracy verification.
Aspherical lens surfaces offer excellent correction of spherical aberration (the inability to focus all light rays from a point source onto a point on film, directly affecting resolution performance), chromatic aberration (color fringes) and geometric distortion (keeping straight lines straight). By using aspherical lens surfaces, the optical designer can create a lens that is smaller, lighter and optically better than lenses using only spherical surfaces.
An aspherical lens element of extreme curvature is known as a "radical aspherical lens".
The above illustration shows the basic principle of how spherical aberration is corrected. Spherical lens elements suffer from an inability to focus all light rays from a point source onto a point on film. Therefore additional lens elements are needed to compensate, making the lens heavier and introducing other performance issues. The further away a light ray is from the optical center, the more pronounced this aberration becomes, making it a crucial issue for fast lenses with their larger diameter elements.
