to bracket: in the context of photography, taking exactly the same shot with different exposure settings. It is a typical behavior of the professional or obsessive amateur photographer.
It is done when you are not absolutely sure of exposure (high contrast or unfamiliar setting), or when the picture must absolutely come out right. Bracketing is particularly advisable when shooting slide film and other stock that has little exposure latitude.
Some cameras can do the bracketing for you; this is called auto exposure bracketing or AEB.
see also: bracketing] in filmography, in yossarian's excellent Film Terms glossary.