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| p.21 #10 · Official: Zeiss Otus 55mm f/1.4 APO-Distagon |
I think one strategy that has often been employed to create good bokeh is to rely on under corrected spherical aberrations (SA). It seems the old Noct Nikkor did this and many other lenses (another good example would be the Leica R 80 f/1.4). These lenses do seem to create good bokeh, IMO, but they do so at wide apertures at the expense of sharpness and especially contrast (the lenses can be sharp and contrasty stopped down, however, when SA diminishes). I like this strategy (and have several lenses that seem to use it), however, it seems you are always in a tradeoff between good sharpness and good bokeh if this is the strategy employed.
I'm not at all sure that such a tradeoff is inevitable. I can think of a few lenses that seem to be both sharp and have good bokeh. The C/Y mount Zeiss 100 f/2 for example. It seems the designers of the Otus 55 have dramatically reduced the SA this lens produces and it still seems to have good bokeh. If those observations are correct, then it would appear that they didn't rely on undercorrected SA to produce the good bokeh. IMO, that would be a good thing.
Incidentally, I think one of the reasons that aspherical lenses get a rap for having bad bokeh is not only the onion rings that they sometimes show in highlights, but also because asphericals reduce SA and if you are relying on undercorrected SA to make good bokeh the aspherical will increase sharpness at the expense of bokeh. This makes asphericals a mixed blessing. Hence another reason that a lens designer might want to go away from undercorrected SA as a means to good bokeh.