In another thread it became obvious that some people do not understand the difference between f-stops and t-stops. The page from wikipedia is actually pretty good on the subject and so I will reference it. http://en.wikipedia.org/wiki/F-number f-stops are calculated via a mathematical formula of focal length divided by the aperture diameter, t-stops are actually measured via light transmittance through the lens. No lens has 100% transmittance of light so a t-stop is always higher than an f-stop. Different lenses can vary a fair amount from others in how much light actually transmits through it.