Thanks xaraa33, but it does not answer the mystery of the 50mm. While the author stresses 50mm is "normal," he also states "normal" is equal to the film diagonal, which is 43mm. The mystery is why 50mm was chosen by some unnamed optical engineer so long ago. The only answer that may explain it is based on the square root of two (1.4x). That when in the past a lens series was designed, it took focal lengths in that progression - 35mm, 50mm, and 70mm. But that does not make sense as it could have equally been 30mm, 43mm, and 60mm. The real reason may be lost in time.
Some oddities of the camera world are known. On digital cameras the aperture is usually given with a capital "F." It should be given with a small "f" as that is the symbol for focal length. Before the great digital revolution, film cameras used a 7-element numerical LCD display (just like the ones on calculators). With those LCDs, only capital "F" could be displayed. When digital cameras with monitors came along, the engineers were so used to displaying aperture with a capital "F" they continued to do so even though it would be possible to use a small "f" including the slash (/). But you never see the aperture written on a digital camera the proper way - f/5.6. So now we have aperture incorrectly displayed on digital cameras.
BTW, aperture should always be shown to two significant figures, but you will see camera manufacturers display F4 and F8 on the screen when they should be displayed f/4.0 and f/8.0. It seems like nit picking, but that zero has meaning.
Enough of camera trivia.
