15:56 May 11, 2009 

BTW, the source text is incorrect. Of the 4 items listed as changing 'the amount of light passed through the lens to the imager ...', the only thing that always (and deliberately) has that effect is the iris, because the iris changes the aperture of the lens. The three other items may cause an apparent loss of brightness and/or contrast in the image, but do not reduce the 'amount of light' per se.

15:49 May 11, 2009 

'backfocus' (aka 'back-focal distance') is defined as the distance between the rear-most glass surface in the camera lens and the sensitive surface on which the image is focused (film emulsion or electronic sensor). In professional cameras, and in some semi-professional equipment too, the backfocus can be adjusted to ensure that the image remains properly focussed on the sensitive surface when the lens is zoomed from one extreme to the other.
Source: personal experience gained from translating professional TV camera lens specifications.