Aspherical lenses offer better correction for many common lens aberrations than regular spherical lenses. Lenses with a spherical profile are easier to grind into the correct shape, but aspherical lenses have a more complex profile that’s more difficult and more expensive to make. Many modern lenses use moulded aspherical elements instead to get round this. Lenses with aspherical elements in their design command a higher price and generally give better results.
Lens aberrations explained
No camera lens is optically perfect. In order to offer the focal lengths, zoom ranges, maximum apertures, sizes or prices that people want, the designers have to make some compromises. This means that all lenses exhibit 'aberrations' to some degree. Typically this may involve distortion, where straight lines start to appear bowed, chromatic aberration, or colour fringing, around object edges, and corner shading or vignetting, where the edges or corners of a picture are darker than the centre.
Here's a selection of topics explaining a little more about aberrations and what lens designers can do about them.
An apochromatic (APO) lens is designed to offer improved correction of chromatic aberration and spherical aberration using specialised materials and combinations of lens elements. It’s a selling point for lenses, though only indicates the lens design used and isn’t really a guarantee of good performance on its own.
A special type of distortion correction once built into DxO Optics Pro but now built into the separate DxO ViewPoint application. It fixes the distortion usually seen with wideangle lenses where objects near the edge of the frame appear disproportionately wide – it’s most obvious with human figures.
A type of distortion (lens aberration) where straight lines near the edge of the picture appear to bow inwards. It’s not as common as barrel distortion, but you do see it quite a lot with telephoto zoom lenses when the lens is set to its maximum focal length. You may not notice it with many types of subject, but it can apply distortion correction with software later anyway.