The Light > Lights Shadow sub-palette controls how the selected light casts shadows. Some of these settings only apply to Best Renders. BPR shadows are set in the Render palette.
The Shadow Intensity slider determines the strength of the shadow cast by this light.
Shadow Curve takes effect when rendering using Render:Best. The shadow curve determines how shadows fade. For example, to achieve very black, hard-edged shadows, set the shadow curve to be a straight line from the lower left to the top left, and then a straight line from the top left to the top right.
The Shadow Length slider determines the maximum length, in pixols, of the shadow cast by this light.
The ZMode Shadows button causes ZBrush to approximate the shape of objects when rendering this shadow. When 3D objects are placed on the ZBrush canvas and converted to pixols, surfaces facing away from you the viewer are no longer present. ZMode takes this into account when constructing shadows, often yielding better results. If objects are partially obstructed or not completely visible, ZMode shadows may be incomplete.
The Unified Shadows slider reduces noise artifacts from standard ZMode shadows, producing more unifed shadows and faster rendering. If the Rays slider is set to a small value, this can produce more painterly shadows. This slider is active only when the ZMode button is pressed.
The Shadow Blur Radius slider how crisp or soft the edges of the shadow are. Higher values result in softer (more blurred) shadows.
The Rays slider determines the number of light rays evaluated in determining each pixol of this shadow. Higher numbers produce more accurate shadows but increase rendering time. For maximum effectiveness, use this slider when ZMode is active.
The Aperture slider affects the sharpness of shadow edges by simulating a narrow or broad stream of light upon items. Smaller values simulate a narrower stream of light, thus producing sharper edges.