[Leica] apo definitions

[Erwin Puts <imxputs@knoware.nl>]

Mike wrote in part that there two definitions of "apochromatic" and 
cites Sidney Ray (Applied photographic Optics) as the source.There is 
a danger when citing from one source if one is not well acquainted in 
the relevant disciplines, that misunderstanding will be the result.
The one person who has studied the apochromatic corrections fully is 
Ernst Abbe, and in his writings you find three different descriptions 
of the apochromatic correction:
correction of the secondary spectrum, on axis convergence of three 
colors and higher order achromatism. The second description is the 
currently preferred one. So Mike's  "looser definition" is in fact 
the correct one. There are not in fact a stricter and a looser 
definition. There is one and only one definition of apochromatic 
correction. The background is this: if a designer talks about an 
apochromatic correction, he automatically assumes a corresponding 
apochromatic error. You can only correct something if you have a 
baseline. As example if you talk about distortion as an optical error 
or aberration, you know that the correction of this distortion has to 
be done relative to a position of no-distortion. Abbe was able to 
calculate this apochromatic error exactly and used this knowledge to 
design microscope lenses that were fully aprochromatically corrected, 
that is the apochromatic error was extremely small. Now saying that 
an error is extremely small, automatically implies that there is some 
error left in the optical system. That is true and this leftover 
error is called "residual secondary spectrum". That leftover can also 
be reduced by aiming for a correction of four different wavelengths. 
(wavelength, spectral color and spectral line are three words for the 
same thing.) That is what Zeiss does with the Super-achromat for the 
Hasselblad system.
So we can classify all optical systems according to the amount of the 
apochromatic error left in the system. This error is identical for 
enlarging lenses, microscope lenses, telescopes and photographc 
lenses. This error can be numerically calculated and so we can grade 
all optical systems as
achromats, semi-apochromats, apochromats and super-achromats.  The 
distinction between a socalled loose and strict definition of 
"apochromatic" then is not supported by theory and not by fact. Any 
lens (photographic and enlarging and what you have) can be corrected 
apochromatically to a certain level.
The problem now is that this classification is just this: a 
classification and anyone is free to use it or not. The designation 
of "APO" has attained a mythical status (as so often in photography 
without any serious understanding of what is the case). The 
definition tells us that a lens is apochromatically corrected if 
three wavelengths  are brought to the same focus. But in optical 
design we discuss errors in fractions of microns. Now if three colors 
have a focal difference of one tenth of a micron, do we then accept 
this as being "the same focus".
The level and rigor of apochromatic correction then is a gradual 
transition from achromat to super-achromat and any manufacturer may 
denote any lens that is located somewhere along this line as "APO". 
As soon as a glasstype with anomalous dispersion is used in a 
lens,the apochromatic error is potentially corrected a bit. And one 
may claim that the lens is of APO-type.
If we assume extremely small residuals of the secondary spectrum, 
then only reprolenses, microscopes and some telescopes are corrected 
aprochromatically. So "Nikon Special Optics" is right in asserting " 
no current consumer enlarging lenses are true apochromats". If they 
interpret "true" in the sense as discussed above their statement is 
correct but also a half truth! They should have added; " and so are 
most if not all of the consumer photographic lenses". But then Nikon 
has photographic lenses  with the designation 'APO" and no enlarging 
lenses with that sticker?

Erwin