The Grain Thickness parameter on the Wood Parameters rollout controls the spacing of grain. The effect of thickness is relative to the context of the object. A grain that appears unrealistically wide on a small table might be acceptable on a large overhead beam.
Decreasing Grain Thickness: Creates grain lines that are closer together. The effect can resemble the fine grain of slow-growth hardwoods. At 0, grain disappears, resulting in what can look like composition board made from sawdust.
Increasing Grain Thickness: Creates grain lines that are farther apart. The effect can resemble tropical woods that grow continuously.
Grain thickness is shown increasing with identical noise settings.
Grain Thickness = 1, 3
Radial, Axial Noise = 1 (defaults)
Grain Thickness = 7 (default), 14
Radial, Axial Noise = 1 (defaults)