While the Sigma 12-24 is smaller than the Nikon 14-24, I'd hesitate to call it small. There's a whole crop of FX UWA that are a bit smaller than the Sigma (like the Nikon 18-35, Sigma/Tamron 17-35's, and Tokina 20-35). And there are primes that a significantly smaller, like the Tokina 17, Nikon 20, and the tiny Voightlander 20.
None are as good as the Nikon 14-24, so you just have to pick your poison, and know your priorities. I've used the Sigma 12-24, Tamron 17-35, Tokina 20-35, and currently own the Nikon 14-24 and Tokina 17.
The 17 is a very nicely sized lens that is small enough to take without being a burden, places where the massize Nikkor would be (and provide a filterable option if needbe). The Sigma is the sharpest corner-to-corner (other than the Nikon) with the least distortion, but is slow aperture and unfilterable. The Tamron is light, but feels cheap and flimsy, requires stopping down to f/8 at least for sharp corners. The Tokina 20-35 is much sturdier feeling (but heavier), constant f/2.8, and sharpens up faster than the Tamron.
thanks maybe should have specified I currently have the 10-20 sigma and would want something at least equal to this on the wide end.
I have only seen the sigma 12-24 and nikon 14-24 once in person. My memory might not be right but the nikon was a big pro lens. The sigma was bigger and heavier then my 10-20 but it wasn't a huge difference like I remember the 14-24 to be
I currently have the nikon 20mm 2.8 ais but would want wider
There is the Samyang 14/2.8 that is manual focus, but affordable.
I have the Nikon AFD18/2.8. Centre is sharp, but corners are soft. Susceptible to flare. I like it because it is small enough to carry all the time. I had a 18-35 nikkor, and it was just as light but bulkier and larger. I never carried it with me, and thus never used it. I would love the 14-24 or 16-35/4, but would only use it on dedicated trips. The Tokina 17/3.5 probably has similar performance as the 18/2.8 but alittle heavier and bigger. If you were willing to compromise field of view of 18mm, and corner/flare performance the 18//2.8 might fit your needs.
I almost went for a Nikon aFD14/2.8, but I don't use 14mm as much as I would 18mm. Plus it is 2/3 the size and weight of the 14-24/2.8. I find the choices get harder for me in the FX lens selection, because I am getting sensitive to the weight and size of stuff. I just cant keep up to friends and family if I have too much gear with me!
15mm. The conversion is simply 1.5x, so, for example, a 50mm on DX is a 75, and conversely, and to get the FX equivalent of a 50mm on DX, you'd need a 33mm. (Which is why the 35mm is so popular on DX.) The crop factor isn't a true 1.5, and it differs from sensor to sensor, but it's so close that the difference in effective focal length is only going to be a mm or two.
Trying to match the 10mm of the 10-24 on FX, the 14-24 will be slightly wider, and the 16-35 will be slightly less wide, but probably so close that you'd never know the difference. Try zooming your 10-24 to 11mm and you can see how small the difference is.