Thought I'd stick a post here in response to Castor as it may be of interest to others.
The 'Harry Patch' drawing was done with a 4H Staedtler; quite hard for me. I usually go with something softer; either a B or a 2B but this is mainly because my local stores don't stock the H, which I'd prefer. Even the HB is rare round these parts. You can apparently get an F (which in the scale lies between the H and the HB) although I've never seen one in the shops or met anyone who's owned one. Anyone out there seen one?
If a caricature has a large area of dark a 4H would be a struggle so I may assess the reference pic before and go with a B or a 2B if there's gonna be a lot of filling in. Anything softer than a 2B and it becomes hard to keep a sharp enough line - constant whittling with the scalpel to keep a point.
I have discovered that a pencil grade in one make can have a different softness/hardness in another. The rule so far has been the cheaper the pencil manufacturer then the softer the graphite (relatively) so careful when you buy a box!
To be honest; I'm constantly experimenting.