Dominator: okay, I see!
lowebo: I'm not sure what you mean, do you mean soft and sharp edges?. getting soft and sharp edges in a drawing is very important to create depth. What you have to do is just make an edge more or less broken up, if it's too sharp, make it fade more, if it's too soft, erase the blurry part.
Here's a small update with a quick(er) cast drawing I did at the National Museum in Stockholm. I went there four times and spent two to four hours each time. It turned out okay, but after all the charcoal work I've done in school, I'm really spoiled with how the charcoal handles. Pencil is so much more permanent, and charcoal is easier to move around. Ah well..