Nikon Rumors is reporting — here and here — that Nikon is set to announce a 35mm f/1.8 prime lens in DX format. We’ll know soon enough whether this rumour is true, but I want to address the idea of a DX prime lens, since many commenters are suggesting it’s a super-bad stupid idea.

If you assume that the idea of a fast prime lens — i.e., a fixed-focal-length (non-zoom) lens with a fast focal ratio (usually f/2.8 or less) — is a good idea, then there are two problems with Nikon’s lineup of fast primes.

First of all, they’re all full-frame (FX) lenses. Nikon only makes four full-frame cameras right now: the D3, the D3x, the D700, which have full-frame sensors, and the F6, which is a film camera. All of Nikon’s other SLRs — the D40, the D60, the D90 and the D300, plus every single digital SLR made before the D3 came out — are DX cameras with a 1.5× crop factor. That bog-standard 50mm lens, for example, becomes a short telephoto equivalent to 75mm; you need a 28mm or 35mm lens to replicate the field of view of a standard lens. Not only that, but all of Nikon’s primes are, as a result, bigger than they need to be for DX cameras, because they cover an area bigger than the sensor; DX lenses are generally smaller and lighter than their FX counterparts.

And second, Nikon’s entry-level cameras can’t make full use of Nikon’s primes because they lack autofocus motors. With the exception of the new AF-S 50mm prime and a bunch of high-end telephotos, Nikon’s primes are AF, not AF-S, and can only be focused manually on the D40, D40x and D60. Now I’ve done this very thing, but even I’ll admit it’s a disadvantage. This means that, in terms of new cameras, the minimum camera able to autofocus all of Nikon’s prime lenses is the D90. This also means that an awful lot of Nikon’s customers with D40s and D60s can’t autofocus an awful lot of Nikon’s lenses.

Now consider the fact that Nikon sells a lot more DX cameras than FX cameras, and that they also sell a lot more D40s and D60s than anything else. Nikon can do the math as well as anyone.

From the photographer’s perspective, a DX prime lens would be a relatively inexpensive way to get an f/1.8 lens for inexpensive cameras, and at 35mm, would be functionally equivalent to a 50mm lens on a full-frame camera. It would almost certainly be cheaper than Nikon’s AF-S 50mm f/1.4 lens, which lists for more than $500 in Canada, and it would be more practical. Yes, it would probably be more expensive than the dirt-cheap 50mm f/1.8 lens, but that’s a pretty old lens. (I have both that 50mm f/1.8 and the Sigma 30mm f/1.4, and from a field-of-view perspective, 30mm is much more useful on a D90 than 50mm. The 50mm makes a dandy close-range portrait lens, though. I make heavy use of both.)