wdr (03/24/83)
The f/number of a lens aperature is the ratio of the focal length to the aperature. The f 'stands for' focal length: by saying aperature=f/1.9, you are saying the aperature is 1/1.9 times the focal length. The amount of light collected by the lens is proportional to the area of the front element, or the square of its diameter or radius. Thus f/4, which is half as wide as f/2, collects 1/4 the light. However, the internal glass of a lens absorbs some light. The effective collection and transmission of light by a lens is dramatically below the theoretical value for 'perfect lenses' (similar to frictionless surfaces, massless pulleys, levers, and ropes, which the Physics 101 professor always talks about). Since Zoom lenses often have more total glass in them than quality single length lenses, it would not surprise me if your zoom absorbed enough extra light to drop the T number by one 'stop', thus dimming the light reaching your meter by a stop's worth. Modern Photography has had an article on this topic within the last year, which gives exact definitions of T numbers and the variance seen in lenses, including the new f/3.5-4.5 zooms, on which the effective f/stop for both exposure and depth of field will change as you zoom. Bill Ricker decvax!genradbolton!linus!security!wdr after 4/1: decvax!genrad!linus!security!wdr wdr@security.uucp security!wdr@linus.uucp allegra!linus!security!wdr