I always looked at it as an increase in aperture, numerically. I'll explain. It's geometric. An aperture of f/ 1 would be twice the geometric area of an aperture of f/1.4. An aperture of f/1 would be four times the geometric area of an aperture of f/2. So, in using a tube that would extended the focal length of 2 you would be reducing the light reaching the film to one quarter the original amount. If one use, say, a 1.4 teleconverter, you would reducing the light reaching the film by one half. It would have the same light reducing affect using tubes/teleconverters as stopping down the same factor in aperture settings. The effect is in functional aperture as shutter speeds remain unaffected.