Fractions.

      For those folks who have wondered what the "f-stop" numbers on a camera lens stand for, listen up.  The f stands for "fractional".  A lens with a wide open aperture of f-2 means that the lens' widest aperture is one half the nominal focal length.  f-8 is one eighth, f-4 is one quarter etc. On a 35mm camera a 50mm f-2 lens would have a maximum aperture opening of 25mm.  So forth and so on.

    The focal length determines the angle captured by the lens.  A 50mm lens on a 35mm camera duplicates, roughly speaking, the same magnification as the human eye.  A "normal" lens as it were.  A lower number focal length is called a "wide-angle" lens and the reverse is of course a telephoto lens.

     The real purpose of fractional stops is to equalize exposure across varying focal lengths.  An F-stop of 2 will, again roughly speaking, allow equal amounts of light to hit the film/sensor regardless of lens focal length.  Also regardless of camera size.  Exposure differences in this schema are caused by variations in how much a given lens' optical elements will affect the transmission of light.

     Note:  "35mm" refers to the diagonal width of the film used.  Digital cameras come in many different sensor sizes, which can be confusing, but despite that, f-2 is still f-2.  Cameras whose sensors are close to same size as a standard 35mm film camera are called "full-frame" cameras.

     A lens' aperture also controls what is called "depth of field", sometimes called depth of focus.  Wide apertures result in shallow focus ranges and small apertures result in wider ranges of sharp focus.  Very small apertures on small lenses can result in "diffraction" which is caused by light rays having to make too sharp a turn going past them.  35mm equivalent focal length lenses rarely go smaller than f-16 to help control this diffraction.  Larger film/sensor sizes can operate without diffraction at smaller apertures.  An 8x10 view camera lens can stop down to f-64 before diffraction becomes a problem.

     A "pinhole" camera, as the name suggests, uses a hole the size of a needle to make images without the need for optical glass.  With a pinhole camera everything is in focus, but is also relatively fuzzy.  And forget about action photos since pinhole cameras often need ten or twenty seconds of exposure time even in bright sunlight.

     Finally, 35mm film originated as a cinema standard over a hundred years ago, but a certain outfit named Leica spooled the film, sprockets and all, into small cassettes for still photography use in the mid-1920s.  That about covers it.  There will be no test later.