The Truth About Riflescope Brightness (And How to Pick the Best Hunting Scope for Low Light)
There’s far more to scope brightness than meets the eye. Here’s what you need to know
Bigger! Brighter! Nearly 110 percent light transmission! Sees in the dark!
Few things in hunting optics are more hyped and misunderstood than riflescope brightness. Every shooter wants a bright, bright view. And every scope maker promises one. But the truth is:
- Riflescope brightness is not as big a deal as advertising makes it out to be.
- Differences in brightness from scope to scope are often undetectable to users.
- Some features credited with brightness don’t even contribute.
- Some high-end scopes aren’t as bright as some costing half that.
Before we dive into details, let’s agree that a bright scope is useful. Who wants to peer through a dim scope at murky images? When we put that magnified view over a deer or bear, we want to clearly see not only the subject, but our aiming reticle hovering over it. It’s important to know what “ingredients” provide that in a bright riflescope.
- Big objective lens—Here’s something everyone understands. The bigger that front lens, the more light it lets in. If you want to see what you’re aiming at in low, low light, get a giant objective lens! The bigger the better. Maybe.
- Big main tube—Forget those antiquated 1-inch main tubes. You need 30mm at a minimum and 34mm is even better because anything smaller restricts the light passing through, right. Uh, no.
- Lens coatings—Anti-reflection coatings, to be precise. Do these really make a difference? Oh yes!
- Light gathering ability—Oh yeah, this is the big one. A scope that really gathers light is the brightest, right? Wrong.
Half truths and misconceptions are what’s wrong with the four points above. Let’s address them individually.
Is Bigger Better?
Yes, a big objective lens does contribute to scope brightness by letting more light into the instrument. But bigger is not always better because a big window does not stand alone. The image (light) has to pass through the main tube and a bunch of interior lenses, sometimes as many as ten! In addition, some scopes include a “stop” ring. This is a ring of opaque material, usually aluminum, that acts as an image sharpening device. It’s akin to the f-stop diaphragm in a camera lens. When reflected light passes through a small hole, the image appears sharper because fringe light rays bent severely near the lens’ edges get removed. This is why virtually all photographic lenses are sharper at f/11 than f/5.6. You can see the same effect by looking through a toilet paper roll or your fingers curled together to make twin tubes.
Maintaining a sharp image is as important as raw brightness. A sharp image at any brightness level looks “brighter” than a softly focused image. Resolution is a critical ingredient for making images seen through a scope appear brighter.
Main Tube Mysteries Exposed
As for the wider main tube contributing to brightness, forget it. Doesn’t work that way. Regardless how much light enters the objective “window.” The higher the magnification, the less light that gets out. The amount of light that exits the scope at the eyepiece depends on the power level. Both objective lens and magnification work together to create the beam of light that exits the eyepiece. It is called the Exit Pupil (EP.) You can see it by holding a scope at arms length and pointed it at a bright surface. The little circle of light you see in the eyepiece lens is the EP. If you have a variable power scope, turn the power ring and watch the EP enlarge and shrink as powers goes down and up. The lower the power, the larger the EP and the more light that exits the scope.
You can determine EP diameter mathematically by dividing the diameter of the objective lens (in millimeters) by the power setting. A 50mm at 10X would give you a 5mm EP. A 56mm at 10X would yield a 5.6mm EP. Not a huge increase. A 32mm at 10X would project a 3.2mm EP. That’s getting pretty small and will make for a noticeably “less bright” scope.
While EP matters, extremely large EP does not because its effectiveness is limited by our own pupils. The human pupil opens (dilates) to about 7mm max. But this doesn’t happen until it’s nearly dark. In full daylight our pupils shrink to about 2.5mm. On cloudy days they might be opened to around 4mm. Place a 5mm diameter beam of light in front of your 4mm pupil and the extra 1mm rim just bounces off your iris. So why bother with a 50mm objective?
The magic hour for hunting is often the last few minutes of legal shooting light a half hour or more after sunset. Our pupils might then dilate to 6mm. They could use all of the 5mm EP and 1mm more. So step up to a big 56mm objective and what do you get? A 5.6mm EP. Not quite the 6mm you could fully use. You’ll need a 60mm objective at 10X to get the full 6mm EP. And if you want maximum brightness at the last minute or two of legal shooting light, you might need a 7mm EP scope. To get that, of course, you’d need a 70mm objective at 10X.
I don’t think they make hunting scopes with 70mm objectives. An easy, inexpensive (actually, free!) alternative is to TURN THE POWER DOWN. A 50mm objective at 7X yields a 7.1mm EP. You get all the light your pupils can take in! Of course, you no longer have the magnifying power of 10X, but do you really need it? There are diminishing returns at play in hunting. As light dims, all kinds of ethical conundrums pop up. Will you be able to detect a hit or miss? Is there a branch in the way you can’t see? Is there another deer behind the one you’re targeting? Will you have any light for follow up shots? Generally, the farther away your quarry in low light, the more problematic the potential for a clean, effective shot regardless the sight picture.
Back to Those Main Tube Myths
Now on to that bigger main tube riflescope brightness claim. It’s not true. EP is determined by objective diameter divided by magnification. Main tube diameter does not enter into the equation unless it’s so small that the interior lenses it harbors (the erector lenses, parallax focus lenses, power zoom lenses, etc.) are smaller than the 7mm EP. In a 1-inch scope, most of these erector tube lenses are about 12.5mm in diameter, a good 5mm larger than our useable 7mm EP.
So what good are larger main tubes? Those with correspondingly larger erector lenses can contribute to better resolution simply because larger lenses are inherently sharper than smaller. Larger tubes also leave more room for turret adjustments, but only if erector tubes are not also enlarged.
Can Coats Really Improve Brightness?
Next we come to something that really makes a difference in riflescope brightness: anti-reflection coatings. These are almost magical because they can nearly double scope brightness without adding bulk or weight. The microscopic coatings of rare earth minerals reduce reflection loss via wave phase cancellation. If this sounds confusing, that’s because it is.
The important thing to know is that raw glass loses about 4 percent of the light that hits it and another 4 percent that exits. It is the air-to-glass transition that does it. Many scopes have 6 to 10 lenses with twice that many air-to-glass surfaces. With uncoated lenses you can easily lose more than half the light that enters the scope.
One layer of anti-reflective coating can cut this loss in half. Another coating cuts it in half again and so on. The best, multiple layers can reduce reflection loss to as little as 0.02 percent, according to some claims. Alas, a buyer can’t determine how many layers are actually used or how much light is actually transmitted. Some scopes claim 85%, 90%, even 95%. What I find interesting is that I can’t always detect these differences. Too subtle.
So how can anyone choose the brightest scope? By comparing the view from one to another. Do this in low light, especially at sunset looking into a dark shadow with the sun actually shining on the lens (in other words, you can sort of recreate how Outdoor Life does it’s low-light optics testing). The less orange flare and glare obscuring your view and the more detail you can see in the shadows, the better the anti-reflection coatings are working.
More Isn’t Always More
What’s surprising about this test is that it often reveals less expensive scopes looking brighter and sharper than some more expensive scopes. Sort of a what you see is what you get test.
What all this brightness testing doesn’t identify is scope durability, precision and repeatability, traits that, to my way of thinking, are as important as or more important than brightness. I don’t need the world’s brightest view if the reticle can’t maintain zero. And I don’t need bright if the image is fuzzy. Give me a balance of bright, dead sharp, consistent zero shot to shot, and in-field durability.
A final point about “gathering light.” While I think most of us understand what is meant by this, I fear others may have a misconception. A scope can accept light, but not gather it in the sense that it increases. It’s not like filling a basket with flowers. Night vision instruments can increase available light, but riflescopes can only transmit some of it. When I read about a scope’s light gathering qualities, I’m skeptical. I think the advertiser is claiming the instrument somehow sucks up more light than is out there to create a brighter than ambient light image. Impossible. But… we’ve all looked through a scope at 6X to 15X and noticed how distant objects appeared much brighter. If that isn’t gathering and increasing brightness, what is it? An optical illusion.
Enlarged objects seen through a scope appear brighter the same way any object appears brighter in dim light when you move closer to it. The mysterious black blob at 100 yards resolves into a blackened stump instead of a bear at 10 yards. Well, a 10X scope makes the same stump appear to be just 10 yards away, thus it looks brighter because more of it is covering a greater percentage of your retina.
Don’t chastise yourself if you’ve been confused about riflescope brightness. Understanding light is part and parcel or Einsteinian physics. Light is weird stuff, both particle and wave, pure energy, but just a small slice of the electromagnetic spectrum that travels faster than any other known thing in the universe. I’m just glad we have today’s grand selection of riflescopes that can so effectively handle it.