The Sigma is a really good alternative, honestly. I've been thinking about that lens since it launched but never pulled the trigger because I've also wanted to reduce the number of lenses I'm carrying when camping. I find that I'm doing landscape astrophotography mostly while camping, and replacing the Rokinon with the Sigma would drastically improve image quality, but also take up more bag space and increase my carry weight - it would work perfectly well, but it is a trade off.
Typically I've been bringing the Rokinon 14 and a Canon 16-35 f/4 (plus a 24-70 and 70-200), but if I can replace those wide lenses with one wide/fast zoom then I can potentially increase image quality and reduce the number of lenses. On the other hand, the price will definitely be an issue and the weight will likely be more than I'm already carrying, so this won't likely be a perfect fit either. Until this rumour though, I was expecting to just go for the RF15-35 and see if it stacks more efficiently, but considering that I can't really travel much at the moment, I'm not in a huge rush to solve the problem and can wait for some final details to decide what is the best fit.
Thanks for the suggestion!
You almost read my mind. I was thinking of suggesting you the16-35 2.8 to consolidate 14 and 16-35 f/4 !!
I have many UWA lenses because I got them as soon as they became available and then didn't want to part with the other ones!
Zeiss 21mm 2.8 (it has a hard stop at infinity!)
Canon 14mm 2.8 L II (that was a bargain bought used in perfect condition using many lenses I didn't want as part exchange).
Canon 16-35 4L IS (finally a travel uwa zoom with excellent IQ. I part-exchanged the old non-is 24 2.8 for that)
Canon 16-35 2.8L III (Because I wanted 2.8!!!!!!!! But it has huge vignetting)
Canon 11-24 f/4 (I used it less than I thought. And t is very big!)
Sigma 14mm 1.8
Canon RF 15-35 2.8L IS (because it has IS and I want to use it at church and museum interiors)
It has the same vignetting as the 16-35 2.8L III
So many overlaps!
I have done astrophotography mainly with 14mm 2.8L II and last year with Sigma 14mm 1.8
I liked the results (I think I had a little flare when there was the moon (not full of course) in the sky even when it wasn't visible in the frame. But when I shot at the lights at home I didn't see any flare!)
A 14 2.8 is small and can be put almost everywhere in a bag. By the way both 14mm lenses I have have much less vignetting than the 16-35 and the 15-35 zooms.
P.S By the way I can't travel either. Also there is the issue with light pollution. And the astronomik filters do not work well with fast lenses (they mention less than f/3 for FF cameras which practically limits them to f/2.8 lenses!)