Troubleshooting shadow banding when light angle on a directional light is set above 15 degrees
 
 
 

When the Light Angle of a directional light is set to a high number, shadow banding occurs.

Avoid a Light Angle of higher than 10 degrees. The Light Angle fakes a light source with a certain shape by bending shadow ray directions, thus promoting the common shadow raytracing artifacts with coarse models.

Alternatively, you can avoid this problem by using a "shape" area light (e.g. rectangle, sphere) instead of a light angle on a directional light.