JMB, any and all lenses that you can attach to your camera will be 'multiplied by 1.6x' regardless of the manufacturer. It is an industry standard to always publish lens width in 35mm film frame equivalent measure since there are a multitude of different sensor sizes on the market and every sensor need it correction multiplier that is supposedly know to the user of a camera i.e. 1.6x for you, 2x for micro four-thirds users, 1x for us full frame shooters, etc.
For a (mostly) distortionless wide angle images your best bet is to go with the Canon EF-S 10-22mm lens
. It's focusing speed and accuracy is superb and it's guaranteed to work on all Canon APS-C cameras in the future as well. The little distortions you will have with wide angle lenses can trivially be corrected in Adobe Lightroom, Adobe Photoshop and DxO Optics Pro with their integrated lens distortion correction databases. The distortions will be mostly invisible unless you're looking for them or you have shot geometric straight lines like a piece of furniture or a building with straight walls.
Stay away from fisheye lenses since they will always distort images heavily and even with correction in software you will either have some distortion left or you will need to crop so heavily that you will lose the angle of view benefit from the fisheye lens and could get better results with a normal ultra-wide lens that keeps lines straight from the start. If 16mm isn't enough then you will need to look at full frame cameras with ~14mm ultra-ultrawide lenses or do Panorama Stiching
from multiple overlapping shots after the fact.
I got bitten with some old Sigma lenses I have for my film Canon EOS1000F/N that refuse to work on any of my Canon digital bodies (10D, 5D mark II) because Sigma & the lot have reverse engineered the Canon lens control firmware and not licensed it from Canon so aperture control does not work on my current cameras at all on those old lenses.
Sigma makes optically good quality lenses for sure and their price is lower than Canon branded ones, but I would go to a camera shop with your camera body and try the lens out physically i.e.
- go through the different shooting modes (Auto, Av, Tv, M) to make sure that their lens software version supports your camera model and does not result in lock-ups with Err99 like my 10D does
- try focusing close and far alternating between them to get a grasp of the focusing speed and find out does the focus 'search' back and forth or does it work like the Canon ultrasonic (USM) does i.e. it just whirs for a half second and stops spot on. To my understanding both Sigma and Tamron at least have developed their equivalent to USM focusing for some models of their lenses.
- if possible try focusing on dark areas to see how well the lens is able to operate in normal lighting conditions (lot less light than in a shop with fluorescent lighting) though this also measures how sensitive the cameras focusing points are. Usually the centre focusing point is a so called cross-type one and more sensitive than the rest so you will get best results with the centre point focus activated.
My neighbor bought a camera from Simply Electronics
for a great price - haven't used them myself and have considered their website a bit dodgy looking, but he got his Nikon underwater point-n-shoot in a couple of weeks without hassles so I guess they are good. Anyone alse got experiences with them?