Aspherical Element
A lens element whose surface departs from a simple sphere, correcting aberrations that spherical glass cannot.
Measured in
count
In depth
A spherical surface focuses edge rays and centre rays at slightly different points, the defect called spherical aberration. An aspherical profile fixes this with one element where several would otherwise be needed, allowing smaller, lighter, wider-aperture lenses. Cheaply moulded aspherics can leave concentric tooling marks that show up as onion-ring bokeh.
Examples in practice
A compact 35 mm f/1.4 uses two aspherical elements to stay sharp wide open in a barrel a classic design could not match.