Originally Posted by Code4
All else being equal a 42mm scope at 6x will transmit the same light as a 50mm at 7x. No scope 'gathers' light as such. I would submit that other factors such as lens glass quality and lens coatings will have a bigger impact than lens size.
You 'get what you pay for' with diminishing returns I'm afraid. You can pay twice the price for a scope with only a 10% increase in quality. For some of us, it is worth it.
Emphasis mine. That's exactly what scopes do. They gather and focus light on a specific point so that it may be viewed.
The objective diameter is directly proportional to the amount of light which may be gathered by the scope and focused at that point.
As I said "all other things being equal" the larger diameter gives you more light to work with.