So it may just be a legacy thing, but I've always wanted to get an actual S35 frame out of the Red. I like the idea of my lenses giving me the same FOV as they do on a film camera. The R1's sensor was a little too small, so with the Epic it seems like I could finally get there.
I did a bunch of research and the hardest thing was actually figuring out what, exactly, a S35 frame is. I was able to get the gate dimensions (24.9x14) but once the image is scanned, it seems the standard frame size is actually 24x13.5 (thanks to David Eubank)
I think there are two major benefits to setting up the camera this way:
1. Your lenses and your mattebox should behave as you expect, no vignetting.
2. When recording full 5K for S35, you end up recording a look-around which can be used to stabilize or repo shots later without eating into your resolution. I think this is one of the biggest advantages of shooting 5K.
After doing the math, to get the standard S35 frame size (24x13.5) the pixel resolution is 4444x2500. Seems like enough to get a good looking 4K finish.
I made this chart to be used as a reference when making custom framing guides in the camera. I wish Red would let us store more than one user frame guide!
As you can see the S35 frame barely fits into 5K 2:1. Personally I'd rather have the extra wiggle room in the recorded image, but if you need higher frame-rates...
I tweaked the full-gate frame size slightly (less than a .01%) to get a nicer pixel dimension.
pCam / David Eubank stated that the 1.85 extraction of S35 had a frame height of 12.980. My calculations gave me 12.972. I went with David's numbers =)
Anyway here is the chart if anyone is interested. Please post feedback or any errors you find.