Lighting Setup Tutorial:
(45 seconds @ 28.8 Kbps)
Step 1: Default settings, ZMode: OFF
(EDIT: ZMode:OFF is very fast, but does not allow for the creation of 'accurate' shadows. It is fine for certain illustrations where a sense of depth is desired, but not for 3D scenes where accurate shadows are desired.)
Step 2: ZMode: ON, Length: 50 is not enough...
(EDIT: Shadow Length determines 'the maximum length of the shadow,' according to the pop-up help in ZBrush. It would be more accurate to say that it determines maximum distance beyond a pixol at which a shadow will be generated. If a pixol should cast a shadow on a second pixol, but the second pixol is at a distance from the first pixol that is greater than the Shadow Length setting, no shadow will be cast.)
Step 3: Length: 150 - Some render artifacts are present.
EDIT: It has been revealed by Aurick that when Rays is set to the default of 1, and certain wierd render artifacts show up in the shadow of a complex model, this is due to one ray being insufficient for ZBrush to calculate an accurate shadow. The Rays setting needs to be increased. /EDIT
Step 4: Rays: 2, Length: 200. Increased the Rays and Length. Setting Rays to 2 results in some rendering artifacts common to Global Illumination renderers when the number of Rays are small. Increase the Rays for less spottiness or graininess on the surfaces near shadows. ZBrush's renderer will slow down as the Rays are increased, but it is still lightning fast compared to the hours some other rendering engines need to do the same number of rays.
Step 5: Rays: 25 - Increasing the rays causes the shadow to soften, while reducing the spottiness. Often the very soft shadows are desirable, but decreasing the Aperature will create a strong shadow once again.
Step 6: Aperature: 15 - stronger shadows, but still very soft.
Step 7: Aperature: 2 - strongly defined shadow, with a touch of blurring of the shadow outline as distance is increased.
Step 8: Rays: 50, Length: 500, Shadow Intensity: 100, Light Intensity: 1.0, Ambient: 0 - These are more ideal settings for a strong, accurate shadow in this situation. Increasing the rays does not greatly improve the render in this situation, but it will greatly increase render time if it is maxed out. In some scenes or light set-ups, a higher number of rays may be necessary to acheive similar results. The Shadow Aperature may need to be reduced in other situations, and certainly in the case that softer shadows are desired.
EDIT: Increasing the Length in Step 8 was to make sure that some render artifacts in the shadow were minimized. The Shadow Length refers to the distance a ray travels beyond the shadow-casting pixol to see if it hits a surface. If the distance to a surface behind a pixol is greater than the setting for Shadow Length, then the pixol will not cast a shadow. Therefore, in Step 2, the lower sphere casts a partial shadow, and the upper spheres do not cast a shadow at all on the ground plane. However, the upper spheres do cast shadows on the sphere below. The pixols in the middle of the bottom sphere were too far away from the surface behind it. Only the pixols at the lower edges of the bottom sphere were close enough to the ground plane for a shadow to be cast when the Shadow Length was set to 50. It also 'seems' that even at high settings, the center of each sphere's shadow is slightly lighter than the edges. So I increased Shadow Length to the max to minimize this effect, IF the effect is due to Shadow Length. I'm not certain of this.
To see the effect of Shadow Length, draw a shphere on a plane, turn on ZMode, and Shadow in the Render palette. Gradually increase Shadow Length and observe the shadow as it lengthens and fills in. Try a tall, thin cylinder if it is not clear to you what is happening./EDIT
This should give everyone unfamiliar with the Shadow palette a good understanding of what these settings will do, and a good base from which to begin to obtain the ideal shadow in your scenes.