This pen drawing is roughly based on a photograph of Charles Darwin by Julia Margaret Cameron. The photograph is here. I was enchanted by the texture of his beard. I didn't use any pencil; I drew directly with pen and took my chances. Blue .05 Micron pen in a Canson Field Sketch book.
Both Darwin and Cameron were pioneers. I assume everyone knows of Darwin. Cameron (1815-1879) was a British aristocrat who took up photography the last 11 years of her life. She did portraits of many luminaries of her day, and her pictures influenced modern photography.