Do APO (apochromatic) lenses make a difference? (An apochromatic lens focuses all colors equally, avoiding axial and lateral chromatic aberrations eg color fringing). See for yourself in my update to the Coastal Optics 60/4 UV-VIS-IR APO macro review. Expect more on this topic in the future using additional lenses.
Even my cherished Zeiss ZF 100/2 Makro-Planar shows the ugly magenta/green discoloration common to non-APO lenses. Shown below is an actual pixels crop. Of course, in most images the background blurs in such a manner as to mask the problem, but it is an unnatural effect with images containing high-contrast details, one the eye picks up on.
Magenta/green color fringing @ f/2
(Nikon D3 + Zeiss ZF 100/2 Makro-Planar)
In the real world our eyes don’t see these “bad trip” color fringes, so why is it acceptable in a lens? Alas, cost intervenes—the Zeiss 100/2 Makro-Planar would likely sell at 2-3X the price were it apochromatic, because achieving apochromatic performance not only requires exotic glasses (eg fluorite), but also extremely high precision during lens coating and assembly (see also my blog entries on the Leica 90/2 APO).
It is not by accident that the Leica 90/2 APO Summicron-R ASPH, 180/2.8 APO Elmarit-R and 280/4 APO lenses are so expensive—achieving APO performance is very difficult; Nikon and Canon cannot claim a single APO lens between them (though many of their lenses are very well corrected for color).