|Nikkor standard prime lenses
Standard lenses are so-called because they match the magnification of the human eye. As such they produce very natural-looking results and are ideal general-purpose models. The standard focal length for film and full-frame DSLRs is 50mm and before the widespread adoption of zoom lenses, almost every photographer owned one.
Like most fixed focal length lenses, standard 50mm models generally have a much brighter aperture (smaller f-number) than zoom lenses, allowing them to gather more light and be more flexible in low light conditions. These smaller f-numbers also allow you to more easily blur the background on portraits.
Lenses with f-numbers smaller than f2 are normally very expensive, but 50mm models are the exception. Due to their ease of construction and high manufacturing volumes, 50mm lenses with very bright apertures can be very affordable – and models with f1.8 or f1.4 apertures are most common. To put these numbers in perspective, most kit zoom lenses at 55mm have an aperture of f5.6. A lens with an f1.4 aperture can actually capture 16 times more light. This low light performance also makes them great for astro-photography.
Nikkor offers three 50mm lenses, one with an f1.8 aperture and two at f1.4. The f1.8 represents particularly good value, although the f1.4 models offer greater light-gathering power and superior construction. Note, the two older AF 50mm models require a Nikon body with a built-in AF motor in order to autofocus. This means they will not auto-focus on Nikon bodies without built-in focusing motors. So if you're using a D40, D40x or D60, they will be manual-focus only. The latest AF-S 50mm f/1.4G features its own SWM focusing motor, so will auto-focus on all Nikon bodies.
All of Nikon's DSLRs (apart from full-frame FX models like the D700 and D3(x)) have a reduction factor of 1.5x, which effectively multiply lens focal lengths by 1.5 times; this effectively turns 50mm lenses into 75mm models. So if you want 50mm standard coverage, you should consider the AF-S DX 35mm model.
That said, an equivalent of 75mm gives you short-telephoto coverage which is ideal for portrait work, especially with the small f-numbers allowing very blurred backgrounds. As such, 50mm lenses are proving very popular with DSLR enthusiasts.
For a full explanation of lens specifications and examples of coverage at different focal lengths, check out our Camera Labs Lens Buyers Guide.