[Leica] more APO-Telyt

[imxputs <imxputs@knoware.nl>]

Mike asked:
        The close-up performance of the AT holds till 1.5 meters.
        'APO' is quite often misunderstood in its practical effects. Normally 
the rays of light that enter the lens
        are bent under a different angle depending on the wavelength. So a 
designer must make a choice what
        colour he/she thinks is most important for practical photography and 
adjusts his calculations
        accordingly. Some colours than have larger blurcircles than the 
corrected colours. Officially this
        behaviour is documented as colourcircles of differing diameters. The 
result will be soft fringes at the
        edges of subject contours. A less well documented effect however is 
the fact that very small details will
        become a bit fluffy when they have the colour of the uncorrected 
spectrum. The apochromatic
        correction then ensures that even very small details are crisly 
recorded independent of its spectral
        composition. There is also some misunderstanding about colour and B&W. 
Any B&W film will record
        all colours from the object being photographed, not as colours but as 
grey shades. So if a lens is well
        corrected for colour, it is automatically well corrected for 
greyshades. And the other way around. I do
        not know of any optical design that is *better* corrected for 
greyshades than for colour. If you look at
        the parameters of optical design programs (now and optical 
calculations in earlier times) you will see
        that calculations are done with several wavelenghts (mostly five, but 
some simple designs can work
        with three colours. Leica now uses seven). Most important here is not 
the number of colours being
        used, but their relative weighting in the calculations. So forget 
about the story that a lens is designed for
        colour or B&W. All good designs are equally suited for both types.

        I will do some additional research on Mr Watsons comments. What he 
notes is crrect. None of this
        however implies automatically different optical performance. He also 
notes that the older TE design
        suffers a bit from corner softness, where the more recent TE should be 
improved. My exemplar of the
        older TE however has already impeccable behaviour in the far out 
corners. I will find mself a most
        recent TE and check it. It will take me a week to find one. I hope.

        I do understand the joy of R users that they, with the APO's 2,8/100, 
2,8/180 and 4/280 at hand, can
        easily dwell in optical nirvana. That is BTW one of the raison d'etres 
of the R-series. Sharing this joy
        with the M users is great. We M users have better wide angle and 
standard lenses and better wide
        aperture optics :-) in the range 21 to 90 mm.


        Erwin