"Brian Tung" wrote in message

snip

To summarize, then, scopes of the same focal *length* have the same
image scale. In contrast, scopes of the same focal *ratio* have the
same image intensity. This in turn determines how fast they will
expose a piece of film and gives rise to the terms fast (low focal
ratio, meaning more light spread out over a smaller image, so more
intense illumination) or slow (high focal ratio, meaning less light
spread out over a larger image, so less intense illumination).

snip

It is my understanding that the area of the objective primarily determines
the *amount* of light collected by a given instrument. You do not have
"more light" or "less light" available due to strictly a difference in focal
ratio, unless you want to take into account the effects of increased
scattering along the longer focal length of the higher focal ratio scope.
Understand that I am not questioning your assertion of the dependency of
image intensity upon focal ratio, only your implication of the dependency of
the amount of available light (i.e. "more light" "less light").

Richard