Render > Preview Shadows sub-palette
ObjShadow
Controls the intensity of the model’s real-time shadows.
DeepShadow
The Deep Shadow button will increase the intensity of the preview shadows. By having this on the shadows will have a larger gradient range from surface interseting points.
Length
Increases the scan range that ZBrush uses to create the shadow. A larger Length will lengthen and soften the shadow but will also increase computation time.
Slope
Sets the azimuth of the light used to project the shadow. All shadows fall at a 45 degree angle across the canvas. This cannot be altered. Slope controls the angle in Y at which the light casts the shadows. Zero means that the light is directly overhead of the model. A larger number means that the light is more in front of the model.
Depth
Deepens and enlarges the shadows. The effect of Depth depends on the setting for Slope; at small settings of Slope, changing Depth will have little effect.