So whatever the type of render you are into (default, equirectangular, VR, ect...) you can check in the advanced options to render the Z Depth Map of the scene as a separate file
A Z Depth Map is a grayscale image where the closets object is white and the furthest object is black, this allows for some cool cinematic tricks in post production (tilt-shift, desaturate far away objecs, ect...)
See Wikipedia:https://en.wikipedia.org/wiki/Depth_map