Brother MFC 8220 - Manual (Page 175)

USING YOUR MFC WITH A MACINTOSH ® 8 - 24
Color Type
Black & White—Set the image type to Black and White for text or line
art. For photographic images, set the image type to Gray (Error
diffusion) or True Gray.
Gray (Error Diffusion)—is used for original documents with
photographic images or graphics. (Error Diffusion is a method for
creating simulated gray images by placing black dots instead of the
true gray dots in a specific matrix instead of true gray dots.)
True Gray—is used for original documents with photographic images
or graphics. This mode is more exact because it uses up to 256
shades of gray. It requires the most memory and takes the longest
transfer time.