The first thing I noticed is that the lens gives very strong flare when used at 50mm/4. The flare will mostly disappear if I stop it down to 50/5.6 or change the focal length to 45mm. For quite a while, I was wondering whether this is a design issue or I just got a bad copy. Cosmetically, mine looks very clean.
Recently, when I googled about this lens, I noticed that an article about this lens
http://imaging.nikon.com/history/nikkor/46/index.htm
has been added to the series NIKKOR - The Thousand and One Nights. In this article, the author mentioned that "At the maximum telephoto position of 50mm, spherical aberration causes a slight amount of flare throughout the entire frame with shooting at maximum aperture. From design data as well, we can see that overcompensation for spherical aberration results in flare." This seems to confirm my findings on the lens. The lens is more like a F4.0-5.6 lens.
I have collected a few manual focus Nikkor lenses as a hobby and I did not actually used these lenses often (which I plan to do so in future). To me, whether a lens has the best image quality is not that important, it is more fun (especially since I am not a great photographer) to find out the character of each lens. In this sense, it is very satisfying that my own experience with this lens is confirmed by someone knows about the lens very well.
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Intersting links about this lens:
From Photography is Malaysia AI Zoom-Nikkor 25-50mm f/4
Bjørn Rørslett says (referring to Zoom-Nikkor 25-50mm f/4 AI/AIS) "Never a volume seller in its days, this long discontinued lens exemplifies the good qualities achievable by a zoom without colour aberrations. Images are sharp corner-to-corner and light fall-off is very low even wide open. It quickly attains peak sharpness at f/5.6 and the image quality holds up well when the lens is stopped down, so even f/16 delivers good results. Compared to e.g. the 20-35 Nikkor, centre sharpness is slightly lower, but corners are much better. Field curvature is moderate and the same holds for the barrel distortion. A crisp rendition extends into the depth-of-field zone to make this an excellent choice for landscape photography. In fact, this was my preferred lens against modern zooms for this very application. Compared to most zooms, the 25-50 is very resistant to flare and ghosting is kept to low levels whenever the front element is carefully cleaned. This is one of my favourite lenses for the F2, F4 and F5 cameras, and it handles nicely, too. You could complain about a rotating front end but that's just nit-picking. A great lens that is sold cheap on the second-hand market. My F2 Titan would be naked without this zoom lens.
Later development shows this even might apply to the D2X for landscape work. Thus, the venerable 25-50 is able to produce silky-smooth images when it is attached to the D2X. There is a roundness and tactility to the image that makes the 25-50 downright enchanting. You might get slightly sharper images with other lenses, but hardly more pleasing to the photographic, inner eye. The virtual lack of distortion, vignetting, and chromatic aberration helps the 25-50 to project its endearing image quality onto the imaging sensor of the D2X."
