Alternatives
Nikon 50mm ƒ/1.8D ~$120
The most obvious comparison will be with the D-version of the 50mm ƒ/1.8, at around half the price. Sharpness has been improved, especially in the corners on an FX body, where the ƒ/1.8D was fairly atrocious at fast apertures. However,
the ƒ/1.8D achieves its best sharpness by ƒ/4, where our sample of the ƒ/1.8G had to be stopped down to ƒ/8. Chromatic aberration and distortion test results were better on the ƒ/1.8D. The major advantage of the ƒ/1.8G is compatibility, as the ƒ/1.8D lens won't autofocus on modern consumer-level cameras such as the D3100, D5100 or even D40x.
Nikon 50mm ƒ/1.4G ~$440
For about twice the price, you can buy an extra two-thirds of a stop of light-gathering ability. The 50mm ƒ/1.4G and ƒ/1.8G lenses are practically identical, with only a fifty-gram weight difference (owing to the two extra lens elements). The ƒ/1.4G is indeed sharper than the ƒ/1.8G, offering better results for chromatic aberration as well; however, shot at ƒ/1.4, the ƒ/1.4G lens will exhibit signs of spherical chromatic aberration more prominently than the ƒ/1.8G (magenta-green fringing in areas of high contrast, before and behind the focus plane). Distortion is better controlled in the ƒ/1.4G, but corner shading is definitely more prominent at higher apertures.
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Conclusion
With so many choices in the 50mm category, even just from Nikon, it's hard to figure who the target audience for the 50mm ƒ/1.8G. Obviously, owners of consumer bodies such as the D3100 or D5100, which don't have mechanical autofocus screws, will gravitate to this lens. However, for a 50mm "experience", those users are better served by the 35mm ƒ/1.8, which offers a similar field of view to the traditional 50mm lens. FX camera users can use either of the D series lenses, and the odds are if you have spent the money on a D700 or a D3-level body, you're probably more interested in a ƒ/1.4G instead of a ƒ/1.8G.
Out of context, the Nikon 50mm ƒ/1.8G AF-S is a fairly solid performer; stopped down to at least ƒ/4, it offers very sharp, contrasty images with little chromatic aberration and no corner shading.
However, when you market the lens with a fast aperture, you have to imagine that people are going to use it predominantly at ƒ/1.8, and that's where the lens exhibits all of its shortcomings: it's good but not especially sharp, offering uneven performance on both FX and DX bodies. Chromatic aberration is at its most notable, and on FX bodies, corner shading is somewhat evident.