The Z-Depth Render Element is a grayscale image that renders each pixel based on its distance from the camera in the view.
The Z-Depth Render Element provides information about each object’s distance from the camera in the current view. Each pixel representing the object is evaluated for distance individually; different pixels for the same object can have different grayscale values. In this render element, objects (or portions of objects) closest to the camera appear as white while those furthest away appear black. Objects between the furthest and nearest points are rendered in varying shades of gray. The lighter the gray, the closer an object pixel is to the camera.
The most common usage of the Z-Depth Render Element is as a control for depth of field in the composite.
Depth of field can be challenging to do correctly; using the Z-Depth Render Element in a composite is usually much faster, and allows easier and faster adjustment.