Despite the wide range of Canon/Nikkor lenses, there are some gaps in their ranges. Which is where the third-party manufacturers tend to aim. For instance, the closest thing Canon makes to Tokina's 11-16mm f/2.8 is a 10-22mm f/3.5-4.5, whose maximum aperture was too tight for my needs. Sigma makes an autofocus 10mm f/2.8 fisheye for APS-C, while Canon makes no APS-C fisheyes I'm aware of. Then there's the Sigma 18-35mm f/1.8, which has a significantly wider aperture than any other current-production wide zooms. And the Sigma 120-300mm f/2.8, which is an interesting zoom range that would tempt the heck out of me if I had the money for it.
Incorrect, John.
Tamron,
Tokina, and
Sigma all make APS-C lenses (links to each manufacturer's lens lineup).
Performance-wise, I think you'll find the gap is narrowing. Check out these (rather tiny) blur index charts from SLR Gear comparing single samples of the $4,000 Zeiss 55mm f/1.4 Otus to the $950 Sigma 50mm f/1.4 Art, both taken with a 1Ds mkIII. Zeiss left, Sigma right. Center performance is nearly identical, but there is a difference at the edges. Still, not bad for a $3,050 savings.
The main thing that's kept me away from
some third-party lenses is autofocus speed. Particularly among their 70-200mm f/2.8 lenses, AF speed just doesn't seem to match OEM. For an action shooter, that's critical, and it's one of the main reasons I use OEM primes with ultrasonic motors. Granted, no one has come up with an empirical test for AF performance yet (I suspect Roger Cicala of Lensrentals is on that), so this is a subjective opinion. Grain of salt recommended.