Stop Buying Scopes with Big Objectives to Gather Light

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It’s a long way from San Antonio to Marathon, Texas, and as Trijicon’s director of marketing Josh Lyall and I made that trip on the way to an elk hunt, we had plenty of time to discuss optics. I asked him the question that I ask everyone who works with optics: What is the biggest misconception about rifle scopes?

“The biggest misconception is that larger objective lenses gather more light,” Layell said immediately. “We still have hunters that look at a scope but decide not to buy it because they want a larger objective lens to gather the last rays of light late in the day.”

The bigger objective/more light gathering misconception is oft repeated and widespread. I don’t know who started the rumor or who perpetuated it, but it’s simply false.

Engineers who study the physics of light tell us so. But somewhere some hunter is shopping for scopes right now and seeking out the optics with the largest objective lens so he can gather those last shreds of light each day during hunting season.

Light Gathering Notion

First, let’s put to bed the notion of scopes “gathering light” like some sort of optical vacuum cleaner sucking up photos of light that get stuck in the atmosphere. Scopes don’t actively gather light. Instead, they transmit light, and some scopes do this better than others. The number and arrangement of lenses in the scope impacts light transmission. Lens coatings greatly affect light transmission. Both play a role in how well you see an object in front of you in low light.

In addition to the objective and ocular lenses, your scope will also have additional lenses inside the scope tube. In some cases, there may be up to ten lenses in each scope. Each of these lenses has its own purpose in providing a clear image, and each one also reduces the amount of light that reaches your eye. Light is reflected or absorbed by these lenses, so unless you’re looking through an empty toilet paper roll, there is no such thing as 100% light transmission.

So, what elements of optical design do impact low-light visibility? Primarily, they are lens quality and lens coatings. If you start with high-quality, precision-ground glass from a reliable source, you’re halfway to a fantastic low-light optic. Apply lens coatings that help us see more clearly to those premium glass lenses, and you have yourself a good low-light optic—no matter how large the objective diameter.

To really appreciate low-light performance, you need a pile of scope at hand in very low light. I’ve been fortunate enough to be in just such a situation, and once you peer at objects through a variety of different optics, you quickly realize that scope objective lens diameter is not that important. My Trijicon and Leupold optics, both of which have 44mm objective lenses, offer excellent light transmission. Two of my best low-light scopes are my Trijicon 1-4x24 and a Swarovski Z3 3-9x36. Both of those optics were outstanding in low light, so clearly large objectives are not critical for low light performance.

Truth Matters

If I’ve offended anyone by abusing the myth that large objectives equal superb low-light performance, I apologize, but you deserve to know the truth. When you approach an optics expert and tell them you want a large objective for low light conditions, they probably won’t call you a fool to your face (they hear the large objective/low light performance theory far too often to become irritated), but they will immediately know that you aren’t fully aware of how your scope works. However, large objective lenses do have a very real and measurable advantage for hunters, and it has to do with exit pupil and eye box.

The human eye accepts light through the pupil. Our pupils change size in accordance with the amount of light available to us; in bright light, our eyes range from 2mm to 4mm in size, and in darker conditions, the pupil dilates (grows) up to 8mm. This is important information because it relates directly to the exit pupil of your scope. Exit pupil is the size of image-forming light that exits the scope, and it is calculated by dividing objective diameter by magnification. A fixed 4x scope with a 36mm objective has a 9mm exit pupil.

A larger objective, then, increases exit pupil on a given power. This is likely the genesis of the “larger objectives are better in low light” theory. But a 4x32 scope offers the same size exit pupil (8mm) as a scope at 7x with a 56mm objective. And larger scopes offer a wider eye box at higher magnification, too. Eye box is the visible area in the scope. If you’re misaligned with the center of the scope the black ring around the outer edge is more pronounced and may even obscure the target. With larger objective lenses and bigger eye boxes, this blacked-out area will be smaller, and that can be crucial when shooting.

I lost a Coues deer once because I didn’t have a large enough eye box. We were hunting in Mexico, and a big buck appeared on the opposite hillside. He was working his way quickly toward the crest of the hill at about 400 yards, and when I flopped down into the prone position with my scope at high power, the eye box on that 44mm scope was so small that I couldn’t get a clear view of the deer. I was lying at an odd angle and couldn’t get fully behind the scope, and by the time I realized I had eye box issues and turned the scope down to 10x to increase the eye box, the buck reached the top of the mountain and disappeared from my life forever.

Relief from Eye Relief

Would I have killed that buck with a scope that had a 56mm objective lens? Maybe. My odds would certainly have been better than with my 44mm scope. I was using a quality optic with multi-coated lenses which cut down glare (I was facing directly into the setting sun when I saw the deer) and could see the deer clearly, but there was simply no chance to get the scope centered on the buck with such a small eye box at high magnification.

Large 52 and 56mm lenses don’t gather more light simply because no scope “gathers” light, and they don’t automatically perform better in low-light conditions than scopes with smaller objectives. But large objectives do offer more forgiving eye boxes, so they certainly do have their advantages. You just need to know what you’re really getting by going to a scope with a larger objective.

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