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.
Designing and manufacturing aspherical lenses is much more difficult than spherical ones. The shape of spherical lenses is pretty much set in stone. They’re essentially slices cut from the outside of a sphere. They’re all based around an arc of a given radius. But aspherical ones, just by their very nature aren’t that simple. But the effort is definitely worth it.
As you can see from the illustrations above, aspherical elements have a very varied cross-section. Aspherical lenses are used to help correct misalignments in the path of light as it travels through the lens that cause different parts of the image to converge at the wrong point. Their goal is to solve the issue of softness and chromatic aberration as you move out from the centre of the lens.
I do find it perhaps a little ironic that the pursuit of perfection in lenses, particularly over the last couple of decades has led to so many older vintage lenses being popular today. Lenses that don’t have advanced aspherical elements and offer an “imperfect” view of our world by today’s standards. But there’s definitely a time for perfection when it comes to lenses – particularly when working for clients that have exact and demanding needs.