The day after Christmas was also the traditional day on which the aristocracy distributed presents (boxes) to servants and employees  a sort of institutionalized Christmas-bonus party. The servants returned home, opened their boxes and had a second Christmas on what became known as Boxing Day.
300 mm. For a 35 mm camera with a diagonal of 43 mm, the most commonly used normal lens is 50 mm, but focal lengths between about 40 and 58 mm are also considered normal. The 50 mm focal length was chosen by Oskar Barnack, the creator of the Leica camera.
An easier way to do this is to move the decimal point two spaces to the left. The effective focal length of a 250mm lens on a digital camera is 400mm, so divide 400 by 100 to arrive at four.Step 4 Multiply this value by two to find the magnification of the lens in terms of viewfinder magnification used in binoculars or telescopes.