While it is true that a larger objective lens transmits more light it is only part of the story. A scope is going to throw a specific diameter "circle" of light (exit pupil diameter) depending on the magnification and the diameter of the objective lens.

A human eye will only dilate so large. If a scope is creating an exit pupil larger than what your eye is dilated to then you eye can't process the light that the scope is transmitting. Conversely if a scope is creating an exit pupil that is smaller than the diameter of your eye then the image will appear dim in comparison.

A human eye will dilate to approximately 5mm in diameter during low light hunting conditions... smaller in bright light but we aren't worried about bright conditions. In order to figure out the appropriate objective lens diameter for a given magnification you simply divide the objective lens diameter by the magnification. The answer is the diameter of light that the scope is creating where it is coming in contact with your eye. A scope that creates an exit pupil that is larger than 5mm isn't going to appear any brighter in low light than a scope that creates a 5mm exit pupil.

For example an 8X56 isn't going to appear any brighter than an 8X40 because the 56mm scope is creating an exit pupil that's larger (7mm) than what the human eye can absorb.

Obviously this will vary slightly from person to person and young to old but 5mm is a good rule of thumb.



AS far as 30mm scopes go. They don't make a scope brighter. The amount of light being transmitted through the scope determined by the objective lens. What they do (assuming they actually have larger lenses inside the erector tube) is give the person behind the scope a more forgiving eye position front to back and side to side compared to a scope that has smaller internal lenses. Most 30mm scopes have the same size lenses inside the tube as do the 1" tubed scopes. Typically a 30mm tube simply gives the shooter a bigger range of adjustment.



What is going to make a real difference in image quality is the quality of glass (this starts with the purity of the raw sand), quality of the grinding and polishing of that glass, and the lens coatings applied to each lens surface. Generally speaking the more you pay for a scope the higher the quality the lenses and coatings are going to be.