mike@asgb.UUCP (Mike Rosenlof) (05/07/85)
> What is the relationship between f-stops and stops? I think I know what it is > in shutter speeds: > > delta(stops) = log2(old-speed / new-speed) > > Can anyone tell me the equivalent formula for f-stops? A description of > exactly how they are measured would be nice, too. > > Jim Heliotis delta(stops) = log2( new-stop**2 / old-stop**2 ) This is using, for example, "16" for f/16, not 1/16. The ammount of light comming in is proportional to the area of the openning, which is inversely proportional to the stop value squared. ( again 16**2, not (1/16)**2 ). An f-stop describes the effective openning of the lens's iris. For example, at f/2, the iris of a 50mm lens is effectively 25mm in diameter. Unfortunately, the effective and actual sizes aren't necessarily the same. It's as confusing to me as the optical center of the lens, which in many cases is outside the body of the lens ( the mirror of an SLR would hit the back of a 24mm lens if the optical center was really inside the lens body). I suspect actual values are measured either by transmission ( T-stops, haven't heard of THOSE in a while! ) or by some measurement on the light cone projected by the lens ( does it come from the entire rear element or just a portion ). Any optical types out there REALLY know how to answer this??? Mike Rosenlof ihnp4!sabre!\ hplabs!sdcrdcf!-bmcg!asgb!mike { ihnp4, ucbvax, allegra }!sdcsvax!/ Burroughs Advanced Systems Group Boulder, Colorado