OIM^TM has the answer for your sample! For many years light microscopists, scientists, and engineers have relied on the ASTM grain size as a method of characterizing microstructural scale and relating the microstructure to structural properties. Traditional optical light assessment of materials is no longer enough to characterize the elastic and plastic strain behavior of the material in application.

The material or mineralogical performance is based on so much more. Modern analytical laboratories now rely on higher tech methods of anlayzing the microstructure, ranging from SEM's, TEM's, X-ray diffraction, and now EBSD. 

Grains can be separated from subgrains by specifying a tolerance angle. Note how the appearance of the grain structure changes with tolerances of 10, 1, and 0.5 degrees.