It is probbably a good time to explain what an fstop means. F stands for fraction. If you use a 100mm lens at f4 then the fstop is 1/4th of th focal length or 25mm. If you set f8 on a 50mm lens then the fstop is 1/8 the focal length or 6.25mm.
The f stop markings are computed with the lens focused at infinity. If you focus at a distance of less th 8 focal lengths away you have to start allowing for the fact that your lens's fstop markings are sufficiently different from infinity and allow for it.
i know what f stops are, been amerture or pro for over 30 yrs, just didn't know if the longer focal length would change the amount of light even though the opening was larger, you guys belong on betterphoto.com, ask any question there and you'll get forulas to figure it out, i now photo for FUN, if you can't handle that then don't read my post
even though the opening size on a longer lens is larger, in theory based on the standarization of the f/stop, the light reaching the film will be the same, hence the same shutter speed for the same exposure as a shorter lens, there are variables that come into equations based on how true the aperture is based on the manufactures tolerances, but yes, an f/stop is a f/stop across the broad range of lenses.