[Leica] Abbe, apo and demagogy

[Erwin Puts <imxputs@knoware.nl>]

Mike,
I did assume that you were interested in enlightening this group 
about a serious topic, such as the use and definition of the "apo" 
designation. But it seems that would like to score demagogic victory 
points for an audience. Let me tell you that I am not interested in 
the type of  debate as exercised in the current political campaign. 
Your remark that Abbe died in 1905 is correct. Your inference that 
his work is therefore of no relevance for todays optical industry and 
theory, is illogic to the extreme. Fraunhofer, the one of the 
Fraunhofer lines, died in 1826, Schott in 1935. Does the moment of 
time that someone dies imply that his work automatically becomes 
irrelevant? Then you make the remark that the book of Mr Ray is the 
universally accepted standard. Universally? Well if you checked this 
claim by the people who really design lenses, you may be in for a 
rude awakening. It is simply your choice to continue believing in 
what you already think you know.
Now for the serious part. As I said in my earlier post , the 
apochromatic aberrations are defined as the deviation between the red 
and blue lines along the axis of the lens (longitudinal) and the 
deviation between blue and red lines vertically (that is in image 
height). These deviations can be calculated quite simple, if 
laboriously. Abbe could do this already as he new the properties of 
the glasstypes to calculate with and the trigonometric rules for 
refraction were known since ages. Mike, these rules were defined in 
the 18th century, and are still in use and true. I noted in my 
earlier post that the apochromatic error is caused by the dispersion 
of glass. So to correct the error you need to use glass with 
anomalous dispersion. In the case of the achromat, only the C (red) 
and F (blue) lines are focused to the same location. (that is 
Fraunhofer again).  But D (yellow) is not and g (another blue!) is 
not. Now it is simple to calculate the sum of the difference between 
the distances for D and g, relative to C/F. If we do some 
calculations (just as Abbe did) we can get results like these:
(all related to the d line)
achromat: deviation for the C line: +5 (in millimicrons), for F: +5, 
for for G: +22.
apochromat with glass types x,y,z: C=+0.9, F=+0.2, G=+5.
apochromat with glass types a,b,c: C=-0.4, F=-0.4, G=+0.4.
So while it is easy to calculate the apochromatic error, this error 
has no fixed values as these change when different combinations of 
glass are used. Now there is no industry norm that first describes 
which glasstypes you have to use to make a lens an "apo" and secondly 
describes which numerical deviations are required for such a 
designation. Now I do think that the small numerical differences 
between the two apochromats as calculated here, would disqualify one 
or the other.
The Leica guy is partly right.  If you stop down the apochromatic 
error becomes automatically smaller and if it reaches very low values 
at f/8, you have an apochromatic correction at that aperture. But 
maybe not at f/4.
These calculations and background info is not in Ray's book by the 
way. If you buy yourself an optical design program (as I did) you get 
a handbook that  informs you how to  do these calculations.
The whole topic of apo is very interesting and could be very 
informative for Luggers and other people.
The attempt to discredit a post that tries at least to be on topic 
and to provide some useful background info is not gentleman-like.

Erwin