Calculate depth-of-field on BMPC 4K using a 50mm/1.8
You will need an app or chart for the calculation. There are apps on both iOS and Android for this. Just search depth of field calculator. 

With a 50mm lens shooting at f/1.8 you're depth of field will be very shallow; more so the closer your subject is to the camera, with your DOF becoming deeper if your subject moves away. 

Some lenses have a built in Hyper-Focal chart on the top of the lens; showing what your "Reasonably considered in Focus area is at" which changes as you change your aperture. 

If you need a less shallow depth of field, you can always stop down the lens. Also keep in mind; the 50mm 1.8 is not incredibly sharp at 1.8 to begin with. Most lenses have sweet spots in terms of tack sharp; typically about 1/3 into the f-stop range. 

The only lenses that will give you pristine tack sharp focus at apertures like 1.8, are for example the Master Primes by Arri; running $30k per prime. 

Hope this helps.