This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Correction Of Chromatic Aberration. Readjustmerit of the plate for the plane of focus of the actinic rays - even though such a lens were perfectly satisfactory in other respects - is inconvenient, except when the lens is always used at the same distance from the object. It therefore becomes desirable to so construct the lens, that the actinic and visual rays will come to a focus at the same point. Figures 4 and 5 together will give us an idea of how this may be accomplished. A pair of prisms placed base to base as in Fig. 4, and a simple lens corresponding in shape, will have an inward bending and spreading tendency, bringing the violet rays closer in towards the lens. Another pair of prisms, placed apex to apex, as in Fig. 5, and a simple lens of corresponding shape, will have an opposite, outward tendency, bringing the violet rays farther out and away from the lens. The inward tendency of the one is opposed by the outward tendency of the other, and it is not very difficult to see from Fig. 4 and 5 that by combining the two lenses, choosing for them glass of proper refractive and dispersive power and giving them suitable curves, we can re-combine the yellow and the violet, the visual and the actinic rays and converge them toward the same point. This is how the optician corrects chromatic aberration in a photographic lens, and a lens thus corrected is called achromatic.
Spherical Aberration. Another inherent fault in a lens is spherical aberration, due to the spherical shape of the lens, and it may be described as the inability of a lens to bring both the central and marginal rays of the same pencil of light to a focus at the same point. Suppose that in Fig. 6 a set of rays emanating from the same point on an object enter the lens. The function of the lens then is to bring these rays together in one and the same point, to converge them to the same focus and make them all form one image point corresponding to the object point from which they all emanate. But the rays passing through the margins are bent more than those passing through the center, that is, the marginal rays are brought to a focus nearer the lens than the central rays. In Fig. 6 each pair of rays, aa, bb, cc, meets and forms an image at a different point, aa nearest to, and cc farthest away from the lens, and consequently we can get no distinct image point, for where we would get a distinct image formed by one pair, we also get, superimposed on it, the blurred images formed by the two other pairs.
Correction Of Spherical Aberration. It is evident from Fig. 6, that we can, to quite an extent, remedy the trouble, by cutting off the marginal rays. If by interposing a stop either in front of or behind the lens we cut off aa and bb we can get a distinct image formed by only the central pair cc. This, however, means a loss of much illumination and it therefore becomes incumbent upon the optician so to construct the lens, if possible, that it will converge both central and marginal rays of a light pencil to one point. It is accomplished in the same manner in which chromatic aberration is corrected - by combining two or more glasses of suitable character and suitable shape. A lens is commonly said to be aplanatic when corrected for both spherical and chromatic aberration.
It should be well understood that spherical aberration, caused by the unequal bending of individual rays (members of the same light pencil) passing through the different portions of a lens is distinctly different from chromatic aberration, caused by the separation of the different colors in each one of those individual rays. Chromatic aberration refers to the separation of the constituent parts of the ray, caused by their unequal refrangibility; spherical aberration refers to the bending of those constituent parts collectively - the bending of the ray as a whole. Thus we may presume that within each ray in Fig. 6, the actinic and visual colors are combined, but a ray as a whole suffers more bending the nearer to the edge of the lens it passes.