I'm looking to invest in an ultra wide angle lens for landscape photography next year. I had a play with the Sigma 10-20 f/4.0-5.6 (Nikon fit) in August and I liked it a lot, so I'm looking to get one of those. I know there is an f/3.5 version of this lens too, which is more expensive. Is it worth paying the extra for the fixed aperture? Currently, brand new, the f/3.5 is £100 or so more expensive than the 4.0-5.6 (but this may change of course) and the 4.0-5.6 is easier to find second hand. Are there any other ultra wide lenses for the Nikon fit which would be worth considering? I don't really have a budget, but cheaper is better for me! Second hand is definitely an option here too! If there's nothing better than the Sigma which is affordable I suspect I'll probably end up with one of those. Thanks!