Press the shutter release on a fully mechanical SLR and you set off a sequence that finishes in under a twentieth of a second, with no electricity involved anywhere in the chain. The order matters. The reflex mirror swings up and out of the light path. The automatic diaphragm, held wide open until this instant so the viewfinder stays bright, is released and springs down to the aperture you selected. Only then does the first shutter curtain begin to travel. The second curtain follows after a delay set by the speed dial. The mirror drops back, the diaphragm reopens, and the finder returns to full brightness before your eye has registered the blackout. The focal plane shutter is the clever part. Two curtains run just in front of the film, and at fast speeds the second one starts moving before the first has finished. What crosses the frame is a travelling slit, not an open window. - Curtain traverse across the 36mm gate takes roughly 10 to 16 milliseconds whatever speed is set - At 1/10...