So I'm reading the "History of the Photographic Lens" book by Kingslake and he occasionally mentions "zonal aberrations". My question is... what exactly is a "zone"? Does it have to do with the object distance or something? Maybe this is a stupid question, but I haven't actually seen it spelled out anywhere. Any help would be appreciated. Thanks!
It's the off-axis parts of the lens. Zonal aberrations are ones where rays passing through the outer part of a lens are focussed to a different point than ones passing through the center.
I think I am right in saying that a 'zone' in normal pictorial lenses refers to an annular ring around the axis. In real, 'thick' lenses there is room for that ring to move about (on the front element, the aperture stop, the pupils etc) but in an idealised thin lens, which can still have spherical aberration etc, the idea is clear.
An example of a non-zonal aberration would be longitudinal colour: even rays arbitrarily close to the optic axis are focussed to different places.
... and another example on non-zonal aberration is distorsion. Even if the lens is stopped down by a conventional iris located on-axis, even if the image is acceptably sharp, distorsion remains even if all rays are forced to cross the iris very close to the axis.
A good example of a zonal aberration is coma. I remember being a student trying to show in practice the effect of coma, in fact this is almost impossible to show with a single lens element. You need at least a doublet corrected from spherical aberration (chromatic aberration being optional, you can show the effect with a broadened laser beam). Or a parabolic mirror.
Then you cut out two pieces of cardboard inner/outer circles and define an annular aperture ; then select one after another various parts of this annular aperture, or zone.
Eventually you'll se the characteritic shape of a coma spot and how it is generated by combination of separate parts of the annular zone... but if spherical aberration is there, forget it...
An example for spherical aberration. It is usual to adjust the design so that rays near the lens center and from a ring at .7 of the radius come to a common focus, this gives the best overall correction. but it isn't perfect. Rays from smaller radii, and from larger, miss the focus a bit. The varius radii are called "zones", so that the .7 zone is corrected, other zones have some error. In general, any correction that is good for only one radius, leaving some error at other radii, leaves zonal aberration.
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