Getting a Line on Shading

In some of today's Scratches, I tried to use drawing pencils of varying softnesses, and to incorporate shading and value gradations. What I seem to have achieved is still a lot of middle tone. Perhaps I need to go 6B or softer to get darks that aren't forced...

If you'd like to see a great sketcher at work in this regard, go see Victor Ambrus' book Drawing Animals. It's a wonderful compilation of his sketches from life of animals as he observed them. We're all duffers compared to him when it comes to sketching from life, and maintaining energy in line while incorporating value and focus areas...