Have you ever found it difficult reading, as if your eye sight wanders, jumping from line to line? All of a sudden you've lost track and this keeps repeating. When painting, same thing happens. I draw the lines of a wild strawberry seed. A moment later, it seems to be of, not really where I thaught I drew it. I check with my reference photo, and sure enough, its about one centimeter to the left. So I undo, erase and redraw the seed one centimeter to the right. Check again and so it goes on...