About the word "
apochromatic", I checked in a few books that I have conveniently at hand. Here is what I found.
"Optics in Photography" (Rudolf Kingslake, page 39) says that
Secondary Spectrum
When we speak of a lens being "achromatized," we mean strictly that the spectrum has been bent back on itself, and that the images in two of the colors of the spectrum have been united at a common focus. The remaining colors, however, will in general depart slightly from the common focus of the two selected colors. Thus, an achromatic lens may show a slight residual of color in the image (secondary spectrum), which in a lens of long focus may be great enough to be significant, especially for the ultraviolet and infrared. The light in both these spectral regions comes to a focus slightly beyond the best visual or photographic focus, so tat when using infrared-sensitive film, it is advisable to move the lens forward away from the film by about 0.5% of the focal length, to secure the sharpest possible definition.
By the use of crystalline fluorite or unusual types of optical glass, however, it is possible by careful design to construct a lens in which the secondary spectrum is reduced to the point where it is completely negligible. Such lenses, used commonly in three-color process cameras, are known as apochromats.
"A History of the Photographic Lens" (Rudolf Kingslake, page 316) says
Apochromat. A lens in which three wavelengths instead of the usual two meet at a common focus.
"Modern Optical Engineering" (Warren J. Smith, fourth edition, page 428) says that
A lens in which three wavelengths are brought to a common focus is called an apochromat. Often this term also implies that the spherical aberration is corrected for two wavelengths as well. By properly balancing the glass combinations given above one can achromatize the triplet for four wavelengths; such lenses are called superachromats.
"Optical Design of Microscopes (George H. Seward) does not define apochromatic, but instead (page 129) simply uses the word in one description.
However, the plot does not display the "S shape" of an apochromat, where a vertical line indicates three corrected wavelengths. The lens is certainly an achromat; however, it is not apochromatic.
Of course the phrase "common focus" is pretty vague. But Kingslake and most of the discussions that I find on the web seem to be treating "common focus" in terms of what you would control with the focus ring, hence longitudinal (axial) color.
--Rik
