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.
So there…
Simon Chirgwin's Learning Log 
Everyone (well most of use) gets confused with that exercise because it’s one of the original ones before the days of digital.
It’s not confusing (I’m cool with the idea that my DSLR’s sensor size means that you need to multiply focal lengths by 1.5) so much as it’s more just a slight sense of wondering when 50mm = normal become a generally accepted truth when the sums make it out as nearer 40mm. And on the other hand, if you do the calculation for Medium Format (a 6cm x 6cm frame – 36 + 36 = √72 = 8.49) gives a ‘normal’ focal length of 85mm. Which is what people still say it is.
I’m rather hoping that the course will continue to throw up things that make me think again about stuff that I think I know…