Sigma 18-35mm f1.8 has been my first choice, but it's nearly 3 times the price of a Sigma 17-50mm f/2.8 here.
This is a comparison of these two, and two other fast zooms. While the 18-35 is better than all, the 17-50 isn't really bad. I will try to find the 18-35 cheaper and maybe try to find a used one but I'm not sure how will that turn out.
Here is Rokinon 14mm f/2.8 at f/2.8 and here is Sigma 17-50 f/2.8 at 17mm f/2.8, Rokinon is just awful. And here is the 18-35mm f/1.8 at 18mm f/1.8, better than Sigma 17-50 but not as much of a difference as the prior.
Where I live the Sigma 17-50 is cheaper than the Rokinon. I can't find any comparisons with the cine Rokinons but I REALLY doubt it's any better, in terms of sharpness.
I really don't think it's a good idea to pay more to an inferior lens and get fixed focal lenght. Sure the advantages of a cine lens is nice and cool but if I'm buying a prime cine I expect better optics. Rokinon is just awful in that department.
The only consolation is that these are still picture comparisons so the video performance might not be this bad, but there isn't a comparison for video that I can see it with my own eyes. So I can't just "believe" that it "shouldn't be that bad". Logical thing is to think that the difference will scale in the same proportion to the video as well. This might or might not be true, but until seeing it, it's the best thing to assume.
TL;DR: Do you think the Sigma 17-50mm f/2.8 would be a bad choice in terms of optics performance to use on BMPCC, leaving the build quality/cine features out? That's what I want to know.
I might even regret buying this just because it lacks cine features, but that must come later. Right now my only luxury is to compare optics quality. If it is better than Rokinons, which the overwhelming evidence says so, then I must choose the better optics. If I'll regret it, I'll have to regret it. Pretty sure I will also regret getting Rokinons with THAT optics performance. With the Sigma I will only regret that I don't have enough cash for decent cines, NOT Rokinon cines.