Scopes
transmit light. They do not "gather light" in the sense some believe they do. All scopes lose some percentage of light, none of the them transmit 100% of the light available. By using larger lenses and better coatings they can lose
less light, thereby making them brighter than other, lesser scopes, but NO conventional scope transmits a brighter image that available light. Larger, yes; clearer, yes; brighter, no.
It is impossible for any scope to "gather" light. It can only transmit existing light. And, regardless of advertising claims you may have heard, there is no riflescope made that can transmit 100% of available light....
The very best rifle scopes human beings can create will transmit to your eye—under perfect conditions—a maximum of 94.5% to 95% of available light.
http://www.schmidtbender.com/facts_light.shtml
Now if you want to argue this point, find a quote that
explicitly states that a scope can offer an image brighter than ambient from a source you think knows more about riflescopes than Schmidt & Bender and
include a link to it.
Please no more fuzzy analogies about funnels, magnifying glasses, "gathering light" or displays of ego. We've had enough of those already.
Here's some more:
Scopes don't gather light, as most people think, although the term "light gathering ability" has become accepted jargon. Scopes transmit available light through the lenses to your eye, always losing a bit in the process. The best a scope can hope to offer in light transmission is about a theoretical 98%, which only the very finest (read expensive) scopes can hope to approach. Anything above 95% is considered great, and most scopes are around 90%, give or take a bit.
http://www.opticsplanet.net/how-to-choose-riflescope.html
So what's the big deal about large objectives? Aren't they supposed to be brighter?
Indeed they are, but they are only
brighter than other scopes, not ambient light and only under certain conditions. Here's how it works:
The ratio between the objective lens and and the magnification of a scope is called the exit pupil. For example a 40mm scope at 8 power will have an exit pupil of 5mm. (40mm/8) For a given size of objective lens, a scope with higher magnification will have a smaller exit pupil.
Depending on how bright it is, the diameter of the human pupil varies. If the exit pupil is larger or equal to the diameter of the eye's pupil looking through the scope both scopes will appear to be similar, since the eye's pupil will act as a bottleneck on the amount of light that gets in. OTOH, if it's relatively dark and the pupil is wide open (~7mm), then a scope with an exit pupil less than 7mm will be the bottleneck and appear darker than one that has an exit pupil at or above 7mm.
There's a good writeup on exit pupil here:
http://en.wikipedia.org/wiki/Exit_pupil
Finally, thanks to the experts at
http://www.opticstalk.com/forums.html who helped me remember exactly how all this works.