- The existing 50mm Nikkor will not autofocus with the newer range for low-budget Nikon SLRs.
- The 50mm f/1.4 design hasn't changed in two decades, and as with most lenses that were designed more than 20 years ago, the design was looking slightly outdated.
How many people buy the 50mm lens anymore? It would be interesting to see some statistics. I am sure that the majority of the 5 million Nikkors sold in the last year were mostly zoom (kit) lenses.
I want a good 50mm lens that performs well under low light conditions. It is no good having an f/1.4 lens that performs well at f/5.6 but poorly at f/1.4. I am not talking of sharpness alone. At f/1.4, most lenses handle point sources of light very badly due to coma aberration. I haven't tested the old 50mm Nikkor AF-D, but I know that even the Zeiss ZF 50mm f/1.4 doesn't handle coma very well at f/1.4.
I remember that one of Leica's main selling points was that its lenses were designed to perform well at their maximum apertures. I still remember that in a brochure they had provided comparisons of a night scene shot with the Summilux-R 50mm f/1.4 and an unspecified competitor lens.
The introduction of the Sigma 50mm f/1.4 lens was interesting because of the special attention Sigma had paid to the correction of coma aberration. Sigma claim that even at maximum aperture, this lens is very well corrected. Surprisingly, the two tests published so far (DPreview and Photozone), do not test this aspect of the performance. This goes to show that most testers do not test a lens for its primary design goals, they just apply a simplistic yard stick to judge a lens, regardless of the especial characteristics of each lens.
With the new AFS Nikkor f/1.4, Nikon are also claiming good correction of coma, but it is not clear whether this is at maximum aperture. Surprisingly, Nikon have not used an aspherical lens element in the design (they used an aspherical lens element in the famous 58mm f/1.2 Noct-Nikkor and in the 28mm f/1.4 Nikkor to correct coma). I think this has something to do with preserving a good bokeh. This is probably the same reason why the 85mm f/1.4 Nikkor doesn't have an aspherical lens element. Cost probably is no longer an issue, as aspherical lens manufacturing has improved considerably, but I could be wrong.
The lens is much smaller in size compared to the Sigma. It remains to be seen whether its maximum aperture performance is better than the Sigma. I would expect the bokeh to be better, but coma correction to be worse.