[Leica] energy flow

[Erwin Puts <imxputs@knoware.nl>]

Someone asked about the energy flow through an optical system.
To explain:
T-stop or aperture refers to the illuminance level  of the film area. 
And the front lens of a 1,2/60 is 5cm as is the front lens of a 1/50. 
The entrance pupil as this is also referred to however is not the 
measure for the energy flow or more correctly the geometrical flux 
through a system. A lens can be seen as a light pipe, that is a pipe 
on front of the camera, that lets the energy from the object pass to 
the image area. There is a very complex equation that computes the 
geometrical flux and this flux is a direct measure for the level of 
aberrations to correct. The result of this equation?
1,2/60 = 11.8
1,4/50 = 11.9
1.0/50 = 17.0

The figures tell you that the 1.2/60 and 1.4/50 have identical 
geometrical flux and that the 1/50 has a 44% higher flux and 
therefore an equal amount of addition aberration to correct.

Erwin