Whenever a new lens is announced, we always get a breakdown of the optical elements that make up the whole. So many elements in however many groups. It’s often followed by statistics about how many there are of different types of elements. Low dispersion, extra-low dispersion, aspherical, etc. But that last one, what does it mean?
Well, in simple terms, it’s why modern lenses exhibit much less chromatic aberration and increased edge-to-edge sharpness than lenses of the past. They’re designed to solve issues inherent in most spherical lenses used in the past, and this three-minute video from Canon explains exactly how and why they work.