Originally Posted by Jay-Rock

Look into getting a cropped sensor there is much more forgiveness when focusing.
If you use a higher iso and FSTOP this helps out greatly as well. You need to be super careful when shooting porn video/stills in full frame even if you know your shit it is still extremely difficult after 50mm when shooting multiple people not on the same focus plane. I never shoot full frame for sex anymore now that I know better. Full frame is awesome for single person portraits with short depth of field or "Bokeh Blur" cropped sensors are better for more than one person in the shot. If you need multiple faces in focus after 50mm and lower than F8 you subjects aren't all in focus unless the stand on the same focus plane.

If you are going to shoot full frame video go with a wider lens like the 12-24 and you will have much more in focus areas with full frame after 50mm your depth of field gets tiny.The problem with shooting wide on a full frame lens if you see more distortion and vignetting and wider focal lengths. With cropped sensors you are using the center portion of the lens so your lines are straighter and you rarely ever see any vignetting.

If you are going to go 4k don't get the A7S unless you want the same focusing issues and having to plugin an external recorder to get 4k.

The Panasonic GH4 is amazing for 4k I have tested it and recorded 1:25 min without stopping till my 64 gig card fills up. I have been using it for around 5 month and haven't even seen a hiccup with this camera. With the Panasonic lenses you can get face tracking and usable autofocus for run an gun. I do it all day for mainstream clients for run and gun and rarely does it hunt for focus. The Micro 4/3 format is amazing and your camera bag ends up weighing 1/4 of what full frame does as well.
If you are concerned about shooting in low light and want a 35mm reverse crop you can get the Metabones SpeedBooster that turns your Micro 4/3 into a full frame field of view and give you that extra full stop of light.