[Leica] APO

[Erwin Puts <imxputs@knoware.nl>]

As usual there is more behind a word than many people can imagine.The
visual spectrum comprises an infinite number of wavelenghts, some of which
are designated a colour name. In optical theory important wavelenghts (the
ones the designer uses to compute his raytracing) are designated letters
and are  called Fraunhofer lines. Normally glass will exhibit chromatic
aberrations (longitudinal: along the optical axis and lateral: changing
image height). This last error (an image point is represented on film as a
series of coloured circles of different radius) give the well known colour
fringing. Normallnses are corrected for two wavelenghts (Fraunhofer lines).
mostly F (hydrogen: blue) and C (hydrogen: red). For achromatic correction
the F and C imges are equal and so all wavelenghts between these two are
also corrected. But there are many wavelenghts still uncorrected. That is
called the secondary spectrum or 'amount of residual colour'. A lens that
is apochromatically corrected, has three Fraunhofer lines (C, D and F)
where aberrations are zero. But there is still some residual colour left.
Only with 4 Fraunhofer lines a theoretically almost perfect lens can be
created. Officially only the Zeis Sonnar 5.6/250 Superachromat is of thie
latter type. In fact we have three steps of chromatic correction:
achromatic, apochromatic and superachromatic.
Now it is possible to reduce the amount of residual colour without going
the full apochromatic way, that is using special glass with apochromatic
characteristics. While the effect of the corrections looks like an
apochromatic solution, in reality the aberration graphs will show you that
only two Fraunhofer lines are being corrected. But the lateral chromatic
aberration is smaller than expected while using normal glass types (quite
often fluorite or aspherics are used). That is the way of most japanese
manufacturers. Often the good level of correction hold for some distances
and most importantly only for some apertures (mostly f/8 or smaller).
Eric's remark that the coverage of image field is also a criterion is not
correct.

It goes without saying that Leica lenses are true APO lenses with three
Fraunhofer lines corrected and a very high degree of correction of residual
chromatic aberrations and the secondary spectrum.

Erwin