Lighting: The Easiest Thing That’s Tough - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 108

use a shadow lamp with a sample value
below 8, and often push it up to the
maximum value, 16, for final renders. If
you begin to notice banding in your
shadows, raising the sample value can
help to fix it.

Soft
This controls the fuzziness of the cast
shadow. Technically, soft represents the
sampling area during map creation, but
the end result is that low values (1.0)
produce fairly sharp-edged shadows,
while higher values (8 and up) make
softer edges. When choosing a Softness value, consider how
the shadow would look if cast in the real world. Few
shadows, even those from direct sunlight on a clear day, are
perfectly sharp. Several factors control real-world shadow
sharpness, including the size of the apparent light source
(remember that only the area lamp has a “size” in Blender)
and the distance from the shadow-casting object to the
receiver. A larger light source and a larger distance both
lead to softer shadows. Note that a high Softness value with
a low Samples value will produce bad results.

Figure 5.12 Different blend values.

Bias
Many rendering computations have a fudge factor thrown
in, in case things just aren't working properly. In the case
of buffered shadows, this is called Bias . Basically, you
should try to keep the Bias value as low as possible, begin-
ning around 0.2. Make it smaller, if you can get away with
it. How will you know when it's too low? You'll begin to
see tight concentric circles in the surfaces of your render,
like the ones in Figure 5.14 . If that happens, raise your Bias
value. If the Bias value gets too high, cast shadows will
appear not to touch their casting objects, which is fine if
the object is flying, but bad if it's sitting on a surface.

Figure 5.13 My default shadow buffer settings.

Clipping
When a shadow map is being created, it has a limited range of values that it can use to represent distance.
It doesn't use the actual distances, it just puts things into a set of distance categories. If you have a lamp