sigma or canon 20mm wideangle

Which of these two lenses would you reccomend.

The Canon ef 20mm f2.8 usm

or the

Sigma 20mm F1.8 EX.

By just looking at them the sigma seems the likely choice as it is cheaper and has a larger aperture.

But thought I would ask to see if any one reccomends one or has some usefull info on which to choose.

It will be used on an Canon Eos 30 (film) and a Canon eos 10D.

I have heard about the compatability of some sigma lenses with eos digital cameras, but need to look into that a bit more as I have only heard about it.

Thanks

Dan

I assume you are planning on using this 20mm lens underwater?? What housing(s) are you planning on using?

If you are planning on using with an Ikelite housing you only have one choice. Canon. The sigma with a diameter of 3.5 inches will not fit in an Ikelite port.

I dont know about other brand housings but make sure you check that the sigma will fit in your housing before you consider it an option.

Also you will never use the lens at 1.8 or even 2.8 underwater because in WA underwater you usually need more DOF than 2.8 provides so the wider aperature is not really an advantage for underwater use.

As im in to surf photography I will be using a SPL water housing.

That is a good point about the dia of the lens and port size. Didnt even think about that. That will be a major factor.

Will be using lens in and out of the water as you would expect.

I had the same choice in front of me when I got my first surf housing, an aquatech for eos5.

I chose the sigma on cost mainly, but the F1.8 is good in crap light when you will still need at least 1/750th shutter speed.

My advice, and this is what I did, is take your housing and camera to your sigma distributor and check the thing physically fits. On the aquatech there is no extra space forward of the lense, that is to say the lense rim is actually touching the front port.

The Australian distributors of sigma are super helpful.

I guess the canon is a better lense, but i'm happy with the result I get.

One other thing, check for vignetting at the corners, I'm assuming its a flat port.