Quote Originally Posted by glbeas
f=focal length. F/stop is, say f/16, 1/16 the focal length of the lens. This is the size of the aperture measured from the front of the lens. This mathematical relationship allows for the same amount of light getting through for f/16 on a 1000mm lens or a 50 mm lens.
Should be lower case "f". The formula is f/(divided by) aperture diameter... where "f" represents the distance from the aperture to the focal plane ... wherever the aperture may be. Normally, f/stop markings on the lens are arbitrarily calculated from the position of the aperture when the lens is focused at infinty.

They are called "stops" because, originally they were finite holes in a plate that slid through the lens ... and there was a indexing feature ... a "shot bolt pin" that stopped the sliding plate in the proper postion.