1/30 second @ f4 is the same amount of light reaching the film as ½ @ f16 is, but not necessarily the same as 2 seconds @ f32: it turns out that equivalent exposure, reciprocity, is reliable only between 1/10,000 and ½ second. Usually. Since most of us won’t be dealing with the ultra-fast times, let’s become aware of the longer ones.
http://home.earthlink.net/~kitathome/LunarLight/moonlight_gallery/technique/reciprocity.htm is a good place to look at this because it provides a chart of some recommended corrections. This will come into play for some of us as the weather improves and the daylight extends further into the evening, and low-light exposures become a real possibility.
Another factor in all this is that, for many films, the ideal situation also involves modifying (usually shortening) the developing time to as well, to a calculable degree, and that the film that sidesteps this modification best is Acros 100.
Creative Camerawork 