the exact path of light leaving a telephoto lens

[Dominique Cesari]

Now, the experimental method:

I placed the rear element of a 600 mm Nikkor at 50 mm from the focusing glass (circa 2 inches).
The measured diameter of the cone is 126 mm (circa 5 inches)

This diameter remains more or less constant as closing down the lens, the limit of the illuminated zone is only more accurate.

[Dominique Cesari]

Now, a better understanding of the question :

you mean : inside the lens

Sorry, I will not drill the back element to experiment that