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.
Superiority Of Anastigmat Lenses. From what has already been said in explanation of spherical aberration, astigmatism, curvature and flatness of field, covering power and definition, the value of an anastigmat over an ordinary R. R. lens is sufficiently clear to require very little further explanation. Its superior correction for spherical aberration and astigmatism not only results in more perfect definition per se, but this definition extends over a larger part of the field with more perfect evenness. Its flatness of field and larger area of critical and even definition combined, give it a covering power vastly greater than that of the R. R. lens. With the same focal length, the anastigmat is therefore capable of giving, with the same or larger relative opening, more perfect and even definition over a considerably larger area. In addition to this, it gives more even illumination over a larger area, and its superior marginal correction extends its covering power close to the edge of the field, when the aperture is reduced.