Having a large objective lens will allow better light transmission, even during daylight hours, benefiting all hunters, but in particular hunters whose eye sight is not as crisp as it used to be.

All other things being equal, the bigger a riflescope's objective lens, the more light it gathers and the brighter the target will appear. Until about ten years ago, 40mm objective lenses were considered big on any scope, and were found primarily on high-magnification target/varmint scopes. Then came a top-of-the-line 50mm variable-power riflescope with conventional hunting magnification. Overnight, large-objective scopes were everywhere. Today, all scope makers' catalogs include 50mm (or even 56mm) objective lenses.

For low-light hunters, the extra brightness of a large-lens scope is a true benefit, as the rest of the scope's optical system can utilize the larger lens' capabilities. Brightness is critical, particularly for hunting sights, since game is most often available to the hunter's eye in the dim light of early dawn or late dusk.

Of course, scopes with big-diameter objectives generally cost more than smaller-objective scopes, which is fine if you get the increased brightness you seek. Unfortunately, many large-objective scopes do not actually offer a significant brightness benefit compared to scopes with smaller objectives.

Manufacturers, including some well-known names, are marketing large-objective scopes with internally stopped-down light transmission that only utilize a part of the objective lens' full aperture. In technical terms, this is called reducing the entrance pupil, or restricting the effective aperture.

Big Lens Lowdown
Optical technicalities are not normally of much interest to hunters, but in this case they can have a substantial impact on a scope-buyer's wallet. Here's the optical mechanics involved, and a simple test to find out if a scope is utilizing its full objective aperture. The first thing to understand is exit pupil and relative brightness at a scope's highest available magnification.

The exit pupil of a scope is merely the diameter of the visible image that exits the eyepiece lens. It's calculated by dividing the entrance pupil (the diameter of the portion of objective lens that actually collects light) by the scope's magnification. A scope's relative brightness is the square of the exit pupil--as the exit pupil increases, brightness increases exponentially.

If the exit pupil is as large or larger than the diameter of the pupil of your eye, the image you see will be essentially as bright as you would see without the scope. Your pupillary diameter varies from about 2.5mm in bright sunlight to about 7mm in total darkness. For daylight use, an exit pupil as small as 2.5mm is enough for a good look at potential quarry. However, in dim light, an exit pupil of 2.5mm is dimming out, because the pupil of your eye has opened and is seeking more light than the scope can provide. The same holds true for the relative brightness factor: Below a value of 6.25mm (2.5mm exit pupil squared), the scope fades out in dim light.