I don't know if this type of adapter exists, but I just bought a C100 and all my glass is full frame. Not a serious problem since the glass still works just fine, but my problem is mostly on the wide end. I have a 14mm that I use for doing exterior/interior establishing shots, and just places where I need more view. This was great on my 6D when I had a full frame sensor, but with the crop factor on the C100 I'm finding that it's just not quite wide enough (as well as the bottom end of my run-and-gun-lens, a 24-70.)

Is there an adapter out there that will allow my full frame lenses to step down closer to the crop factor of the C100, without losing exposure stops?

You would think that Metabones would have come up with a speed booster product for this application but it appears they have not. It might be worth a letter to them, as it certainly would seem to be a product that people would want and is in the same basic toolbox as their other format downconversion lenses.

Not quite as wide as the 14 on fullframe, I use a Tokina 11-16 f2.8 on a crop sensor. It spends most of its time on 11.

IMO it's not as sweet a solution as the Canon 14 on full frame, but it's pretty good. That Canon 14 is really a stunning lens for interior work on full frame. I've not seen the Rokinon/Samyang 14.

I too have not seen the kind of adapter you're thinking about.

Seth- I currently use the Rokinon 14mm. Sharp in the middle, fuzzy in the corners (don't care much for the type of shots I'm doing with it) and the distortion is only noticeable if there are horizontal planes running through the bottom or top 1/3 of the frame. But hey for $300, what do you want. The Tokina might do it for me at a bargain price, although I was hoping to get something that was wide enough to work on the C100 and that I could still use on my 6D.

I doubt there's going to be an adapter. The back focus distance has to be the same as on the still cameras so there isn't any physical space for a simple mechanical adapter. The only way an adapter could work is if it had optics to re-adjust the back focus distance and then it gets pricey. Metabones Speed Booster for the BMPCC is over $600 because it includes optics. You'd also have to consider the optical quality of an adapter compared to that of the Canon glass. I have a Canon 10 - 22 mm lens which works quite well. From what I hear, the 11 - 24 is a much better lens but it's also fairly expensive.

I also have the 14mm Canon and it's a great (if heavy!) piece of glass. Just for fun I've tried my 8mm Sigma fisheye on the C-100. The "distortion" makes me think of a GoPro!