Canon 70-200

I'm thinking about getting a 70-200 f/4, mainly for photography but filming as well, however I'm not one to jump into a purchase, I look into every detail. I've been thinking about this lens because of the af speed, fixed aperture, and build quality, and everyone goes on about how sharp it is.

First off image quality, I saw the drtv comparison between the 70-200 2.8 canon ii and the 70-200 2.8 sigma, and the canon was a huge amount better, but there's no comparison between the canon 70-200 f2.8/2.8is/f4/f4is. I was wondering if there was as big a difference between them, and if anybody had both the 2.8 and the 4, could they take a comparison photo?
Also how is it compared to the tamron 17-50? I've seen in sample photos it looks pretty awesome, but without a direct comparison it's hard to tell.


Next af speed and accuracy, there's some videos of autofocus tests on the net, but they're all either inside when it's dark, or with the lens cap on. I was wondering how it handles outside, and could somebody post a video of it focussing 1.2-infinity, and then 3-infinity, so I can see how much of a difference it makes. Again how does this compare to the tamron 17-50?

Next the build quality, I'm selling my 50 1.8 just because its way too delicate, I just don't trust myself to ski with it without it breaking if I fall. From what I've seen on YouTube, it's at least part metal, but could somebody let me know how much of it is metal? The whole barrel? And if so is there any plastic on it.

And finally I read on an earlier thread that a tripod collar makes it more stable, and less shake in the video, is it worth it for just the f4? And how much of a difference does it really make?

Thanks