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.
As light enters the objective end of the scope, before it reaches your eye it passes through several lenses. Each lens absorbs a small quantity of light. Residual reflection from the individual lenses will also prevent a certain amount of light from passing through the scope. In addition, undesired reflections within the metal tube can hinder the quality of the viewed image and the transmission of light.
Each lens has two surfaces. Thus, the total number of lenses within a scope (a variable-power scope can have between seven and ten) is multiplied by two, then multiplied by 0.25% to determine the amount of light lost in the transmission. Simple multiplication is not accurate, however, as each succeeding lens progressively reduces the total amount of transmitted light. It is a favorite technique of some scope manufacturers to claim light transmission values of nearly 100%. Of course, they’re measuring the first objective lens only, conveniently forgetting about the other eight or nine!
Any higher transmission levels are physically impossible to achieve with current technology, and claims to the contrary are to be discounted. What does light transmission mean in practical terms? An average scope may transmit 85% or so, and inferior scopes substantially less. The human eye can distinguish transmission differences of 3% or more. Consequently, there is a very real difference in what you can see through a superior scope versus run-of-the-mill optics.
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. There are but a handful of scope companies remaining that produce optics approaching these levels, Schmidt & Bender being one of them.
Under hunting conditions, when you might be trying to distinguish a target at absolute last light, these differences can be critical. It can determine whether you bag your game or whether you have long since called it a day.