One I see often in such arguments comes down to these criteria:
- Nikon make better cameras
- Canon has a better lens selection
Today I'll sit down with evidence and look at the second argument.
Lets start with some history. First, Nikon make use of the F-Mount. This mount was developed in 1959 and has been used by Nikon since, with small adaptations to add new features (such as in-lens motors, VR, CPU chips, etc). Thus, with some exceptions, you can mount any F-Mount lens on your current DSLR bodies (the exceptions being invasive fish-eyes; it's just wise not to touch these). Another interesting fact is that Nikon made DX lenses - a smaller image circle for their ASP-C cameras. With their latest full-frame range (D700, D3, D3x), it's possible to mount the DX lenses in crop mode, meaning that your DX glass will still work (D3/D700 will drop to 5.1MP with DX lenses, D3x will drop to 10MP). They are still f-mount lenses and thus will always mount.
Canon, meanwhile, have the EF lens mount designed in 1987. This is a radical change from their FD mount to incorporate electronics such as AF and VR in the lens from the outset rather than trying to manipulate the FD mount into the current era. Thus you can't mount an FD lens on an EF camera without using adaptors, and since Canon re-designed the flange to focal plane length as well, the adaptors will need optical elements. Thus it's not overly economical to mount an FD lens on an EF camera. Canon also made a few EF-S lenses, but these were physically different enough from EF lenses to mean that you can't mount them on Canon's full-frame range, not even in a cropped mode.
Now, to the official lists. Nikon's Lens Compatibility Matrix lists all the f-mount lenses and how compatible they are with the current crop of Nikon DSLRs. The major rule is the D40/D40x/D60 will not AF with older lenses, and anything less than a D200 will not meter with really old lenses (AI/Pre-AI) either. However, all will mount (with the noted exceptions), so as long as you're prepared to do a bit of manual work, you'll be able to use any lens in that list (With a D200 onwards, you'll always be able to meter, focus might be manual if the lens is a manual focus one).
Nikon list something near 200 lenses in their matrix. This covers everything from a 6mm fisheye to the behemoth 2000mm reflex-nikkor.
Canon's EF/EF-S specification list, meanwhile, lists 61 lenses of which 4 of them are EF-S and thus will not mount or work with Canon's full frame cameras. The list goes from 14mm to 1200mm.
What does this mean?
Quite simply this means that Nikon have a larger selection of lenses in the general case. Quick examples are:
- Nikon has five fisheye lenses, Canon only has 1
- Nikon have 8 macro lenses and 2 tilt/shift Macro lenses, Canon only has 5 macro lenses, none of which are tilt/shift.
- Nikon have a selection of zoom lenses that go past 400mm (200-600mm, 360-1200mm, 1200-1700mm), the longest Canon zoom is 100-400mm
- Nikon have medical, defocus control, reflex, UV and noct lenses, Canon does not.
Being fair, all of Canon's listed lenses are AF, where as the giants such as the Nikon 1200-1700 is not (though try find an AF motor to move THAT optical assembly!). Canon also do have some more consumer friendly zoom lenses (70-200mm f4 IS comes to mind), but if you look for them, Nikon have many f4 lenses for their zoom focal lengths (and even f3.5 and f3.3 and other obscure fixed-aperture zooms)
Trying to do a more direct comparison, though, didn't quite go as planned.
The big problem is for every lens Canon has that you can find a matching Nikon for (in both aperture and focal length), there'll be some Canon lenses that are of slightly different focal lengths. For example, trying to find a matching Nikon lens for the Canon 28-90mm lens is impossible; Nikon do have 28-85mm but no 28-90mm lenses.
You can also do the inverse. What Canon lens would match the Nikon 43-86mm f3.5? or the Nikon 36-72mm f3.5? How about a match for the Canon 100-400? Nikon 200-400? Canon 17-85? Nikon 50-135? That's a small example of how you can't call one for the other.
I will say this, though, for every lens Canon has in their EF list, there is a Nikon that is comparable, with a few minor exceptions (Nikon do not have an 85mm f1.2 for example).
If you're reading this and want to argue that you can contrast, go for it. You have both lens charts. Try find a Canon lens that has no Nikon equiv, and I'll find you two Nikon lenses that have no Canon equiv. Tit-for-tat really at this point.
In conclusion we can see that the lens argument isn't as clear cut as people make it out to be. If you want ultra wide lenses, Nikon does appear to have the edge (there's 5 prime lenses between 20 and 13mm for Nikon and 1 fish eye, and two fisheye lenses that are below 10mm). If you want AF in all lenses you can buy, Canon wins hands down. If you want more long-range zooms, Nikon wins here. If you want more fast primes, Canon wins. So if you have no SLR glass and are looking at Canon VS Nikon, think about what you'll be shooting. Both offer a good selection of zoom and prime lenses, but if you're looking for something more special (fast primes? Canon. Long zooms? Nikon. Ultrawides? Nikon. Fisheyes? Nikon.) then have a look at the lists provided and select based upon that; don't listen to fanbois propping up their brand because they happen to have a camera and a few lenses.