One footnote to the first of the introductory exercises is that I finally did the sums to work out the “normal” focal length for both a frame of 35mm film and for my smaller DSLR sensor and was slightly surprised to discover that:
DSLR: 23.7 squared + 15.6 squared = 805.05 or 28.4 squared: – ie the diagonal = 28.4mm
Film: 36 squared + 24 squared = 1872 or 43 squared – ie the diagonal = 43mm
So, rather than being normal, a 50mm lens is ever so slightly telephoto and the nearest I can get to a normal angle of view with a prime lens on a film camera is my Olympus Trip 35 with its 40mm fixed lens. And on my DSLR I need a 28mm lens rather than the 35mm one I’ve blithely been using for ages now.