A photographic term that relates to halving or doubling of light. A photographic stop is either half or double the light.
In practice the area of the aperture must either be reduced by half or doubled for each stop. Sometimes used synonymously with f-number as in ‘f-stop’.
Stop Down
To decrease the diameter of the light-admitting orifice of a lens by adjustment of an iris diaphragm. (Kodak)
Step
An exposure increase or decrease, usually by a factor of 2 but may be smaller. The same as ‘stop’, except stop specifically refers to lens aperture. A patch of a step tablet used for sensitometer exposures, as in ’21-step tablet.’ (Kodak)