Scopes and Light Transmission: Myth and Truth
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.
What Happens Before Light Reaches Your Eye?
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.
However, this is not accurate or completely true because each succeeding lens reduces the total amount of successfully transmitted light. When some scope manufacturers claim their scopes have light transmission values close to 100%, they are probably measuring just the first objective lens and conveniently leaving out the rest!
So, What Does This Mean?
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 around 85%, 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.