They are classified by shape. Round rolls, and flat doesn't. This works well, but the pitch of the blanket often varies. If it is too steep, the leaves tumble into the bucket too. We put a piece of plywood under the blanket, funneling only the bottom to channel the berries into the bucket.
Experiment Try different methods of separating berries from the leaves and sticks by using wind.
A variable speed fan provides many opportunities to experiment. You will note that berries that fall a long ways will damage when they hit, especially those picked later in the year. Try rolling berries down a blanket. Try different angles. Put a piece of plywood under the blanket.