Craig Nelson demonstrates painting negative spaces in trees
Craig Nelson using an "egbert," or long filbert, to paint foliage
He demonstrates using a particular kind of brush - an "egbert" – also called a long filbert. Notice how very long and slender the bristles are? It gives a delicacy and flexibility to the brush strokes that he is particularly looking for.
Nathanael Provis makes sure you never use a fan brush again
And if I haven't already discouraged you from using a fan brush to create generic-looking foliage, here's an artist who may help you see the light... See Art Disasters 3: How to Paint Trees - artist Nathanael Provis. Be sure to watch all the way to the end - you don't want to miss his rant on fan brushes. (Note: he is using oil paints, which dry much more slowly than acrylics.)