OST HUNTERS rate riflescopes almost entirely by optical quality, usually called "brightness," though it's actually a combination of brightness and sharpness. Both factors affect how much detail we can see to help us aim.
Most hunters also think the quality of what they call "glass" is the major factor in scope brightness, but glass and coatings are only one factor among several. In fact, we can brighten our view of the world simply by looking through a tube without
The next time the roll of paper towels in your kitchen runs out, take the cardboard tube and hold it to your aiming eye and look around: The view will be noticeably brighter, especially if you stand outside your garage's open door and look
inside. This is because the tube blocks out any light except the light passing through the tube. Our eye's not bombarded by the sunlight from the sky, or reflecting off the shiny silver pickup in our driveway.
While still looking through the tube, move it father from your eye. The view will become dimmer, due to both sky and reflected light from the pickup. Something of the same effect occurs when looking through a riflescope, but we'll look
again at that effect later. Right now let's look at glass.
Contrary to popular belief, "German" glass isn't the finest in the world. While much of the science of modern optics arose in Germany in the 19th century, the glass itself can be made by any competent factory today and even "German" glass often
Many hunters equate fine optical glass with the Schott company, for decades closely associated with Zeiss, but Schott has over 40 factories around the world, including one in China. Not all specialize in optical glass, but those that do make glass
just as good as any coming out of German factories, and perhaps better, because some countries (including China) aren't bound by the very strict environmental laws of modern Germany. They can use lead in their optical glass, and a tiny amount of lead
makes glass denser, increasing the ability of lenses to bend light.
Most optics companies don't even make their own lenses anymore, instead buying lenses ground and coated to their specifications from specialty firms around the world, then test the delivered lenses to make sure they meet their specifications.
The most important function of lens coatings is reducing the amount of light lost due to reflection from lens surfaces. A bare lens reflects up to half the available light, but a very thin coating of certain "rare earths" allows the light to pass
through the coating. Essentially, most of the light that bounces off the glass "rebounces" off the coatings and back through the lens.
This is where optics companies come up with the percentage of light transmission through a scope. They know how much light passes through each lens surface, so can calculate the percentage of overall light transmission through the number of lenses
in a scope. This is why fixed-magnification scopes normally have higher light transmission than variable scopes, which use extra lenses in their erector systems to increase or decrease magnification.
Multi-coated lenses pass more light along to our eye than single-coated lenses, because each layer of coating is designed for specific wave-lengths of light. Coatings for certain wave-lengths of light can enhance our ability to see detail in dim
light, even though the overall light transmission may be lower in that particular scope.
Dim light tends to filter out certain colors of the spectrum. Ever notice how the world appears bluer in dim light? That's because reds and yellows aren't as visible when the sun starts to drop, due to light having to pass through more
of the earth's atmosphere, which acts as a filter. Lens coatings designed to transmit more of those reddish colors can provide more detail to most eyes, even though overall light transmission is lower than in another comparable scope.
Notice the "comparable." Magnification also plays an important role in apparent brightness, especially in dim light. In effect, extra magnification is like walking closer to an object: While out on an evening stroll, we may not be
able to tell whether a vehicle parked a block away is a pickup or minivan, but as we walk closer the differences becomes clear.
Magnification does the same thing, so comparing the "brightness" of a 2-7x scope to a 3.5-10x scope isn't realistic and once again, sheer light transmission is a secondary factor. Most modern multi-coated scopes don't vary much more
than 5-percent in light transmission, which doesn't overcome a 25-percent difference in magnification.
However, extra magnification doesn't do us any good if the exit pupil is smaller than our eye can use. The exit pupil is that dot of light seen in the rear lens when holding a scope out at arm's length, and its diameter is determined by the
diameter of the objective (front) lens divided by magnification. As an example, a 6x scope with a 42mm objective has a 7mm exit pupil.
According to common wisdom, a 7mm exit pupil is all the human eye can use, because that's as wide as the pupil of our eye expands in dim light. An exit pupil over 7mm is wasted, because the light doesn't enter our eye. However, a 7mm
exit pupil supposedly helps only young eyes, since as we grow older our eyes grow less flexible: The pupil won't expand as much even in very dim light, one reason older people can't see as well at night.
Eyes, however, differ as much as humans. The pupils of a few people's eyes will expand to 8mm, and some older eyes will expand more than 5mm. I'm in my early 60's and even in light just bright enough to see a ruler, my pupils still
expand to 6mm.
When a scope's exit pupil is much below 5mm in diameter, contrast and color start to diminish. As an example, the exit pupil of a typical 3-10x40 scope is 4mm with the scope on 10x. This is why 50mm or larger objective lenses
supposedly "gather" more light than smaller objectives. In reality, they simply provide a 5mm-plus exit pupil at somewhat higher magnifications. Magnification provides most of the additional detail, but having a sufficiently large exit
pupil definitely helps.
Some hunters still believe 30mm scope tubes allow more light transmission than 1-inch tubes. This was a myth created around 1990 by the ad agency for a certain European scope company. One of the ad guys may have even been the
originator of the comparison to a 30mm scope tube and a water funnel: A higher volume of water can flow through a larger-diameter funnel, so logically a 30mm tube allows more light to flow through a scope.
The problem with this "logical" analogy is that light doesn't flow through a scope's tube. Instead it flows through the lenses, which are subject to the same optical laws regardless of the tube diameter. The spout of the
optical "funnel" is not the scope's tube diameter, but the exit pupil.
A 30mm tube restricts the amount of light passing through a scope only at relatively low magnifications, below about 6x. With a typical objective lens of 40mm diameter this means the exit pupil at 5x is 8mm, larger than 99.9% of human eyes can
use anyway. (This effect can actually be compared to a water funnel, since it's like trying to pour water through an 8mm funnel into a bottle with a 7mm neck. The extra millimeter of water isn't going to enter the bottleneck.)
When a variable scope gets turned to any magnification from 6x up, the exit pupil size is controlled not by the main tube of the scope, but by the lens system. If you don't believe this, measure the exit pupil of a 30mm variable at different
magnifications. This is easily done by holding the scope at least a foot away from your face, then holding a metric ruler across the eyepiece. You'll find the exit pupil measures exactly what it should according to the
objective/magnification formula. So no, 30mm tubes do not increase a scope's brightness.
Two other factors also increase the apparent brightness of the image. First, in better scopes the interior is baffled, or painted matte black, to reduce interior reflections. These reflections interfere with the image, exactly like
shining a bright light on a computer screen.
Second, eye relief and ocular (rear) lens diameter also affect contrast. Remember our experiment with the paper towel tube? Well, longer eye relief means our eye is farther from the scope, so more extraneous light can enter our eye, and a
smaller ocular lens creates some of the same effect. (This is also why the view through binoculars appears brighter than the view through a scope of the same magnification and objective-lens diameter: Our eye is less than half an inch from the
ocular lens surrounded by an eyecup.)
Everything in optics is a trade-off. For many years, European scopes usually had shorter eye relief and larger ocular lenses, because legal hunting hours in Europe often extend into what Americans would call "night." The shorter eye relief
and larger ocular lenses reduced the amount of extraneous light entering the shooter's eye, making the apparent image brighter.
But the shorter eye relief increased the chance of the scope whacking the shooter in the eyebrow. This didn't matter as much in Europe as North America, because most European big game isn't as large as American elk and bigger bears, and even
European moose are smaller than most North American moose. As a result, most Europeans don't use the hard-kicking magnum cartridges often used here, so shorter eye relief wasn't a big deal.
Another interesting aspect of modern scopes is that the etched reticles now favored by many hunters result in a slightly dimmer view. They're etched on glass, so introduce two more air-to-glass surfaces inside a scope, and illuminating reticle
electronically dims the view even, thanks to more light bouncing around inside the scope. This is one reason many older European scopes used heavy wire reticles, often a combination of posts.
All of these factors affect the apparent brightness of scopes, and one may cancel another out. Very high-quality glass and coatings are definitely a factor, but a more "affordable" scope with lesser glass but higher magnification can easily
result in seeing more detail. Lens coatings designed to transmit certain colors can also have the same effect, even though absolute light transmission is lower, though exactly how well we see through a particular scope depends on how well our own
eyes see various colors, even if we aren't technically color-blind. Objective and ocular lens diameter also play a role, along with eye relief, interior reflection reduction, and even the particular reticle.
The one certainty of scope brightness is the high percentage of hunters who rate scopes by how well they help us see in dim light, rather than recoil resistance or adjustment repeatability. But that's another story.