Several factors influence the amount of light that passes through an optic to your eye.
First light and last light are the magic times for most North American big game hunting. Being able to see your target in low light conditions well enough to make an ethical shot is mandatory. It can mean the difference between standing around the pickup drinking beer while admiring a buck in the bed or going home with the pickup bed empty.
The light transmission of your rifle scope is a huge factor in the outcome of this scenario. For this blog post we’re talking in terms of rifle scopes, but it’s much the same for binoculars and spotting scopes.
What is Light Transmission?
In short, light transmission is when light passes through the front of the rifle scope to your eye. Pretty simple right? Yes, and no. Not all light passes through the scope equally. There are several critical factors affecting the light transmission and the perceived brightness of your scope including the lens materials and design, coatings, light management, and the magnification and objective diameter. Again, simple, right? Yeah, not so much.
Light transmission is measured as a percentage of light that enters the scope through the objective lens and exits through the eye piece to your eye. How this percentage is measured can vary, but in general we’re talking about the percentage of light visible to the human eye that makes it into and out of the scope. Spoiler alert, it’s not 100% of the light the enters the scope. We’ll cover that in a minute. In the meantime, it’s a great segue for our next topic.
Mythconceptions
This is a great time to dispel some myths and misconceptions about light transmission.
In order of the frequency that I’ve heard these incorrections, here are three myths to dispel.
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Scopes gather light. Wrong! Please never say that again and we can be friends. “Gathering” implies that a scope actively collects light for use. That ain’t happening. The scope’s objective lens, and the rest of the lenses, transmit or pass along the light they come in contact with. That’s it. There’s no amassing, accumulating and certainly no amplifying. They transmit; and nothing more. Again, no lens transmits 100% of the light it comes in contact with.
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My scope’s large tube is better for transmitting light. We’re of course referring to the scope tube diameter. But here’s the thing, in general, the scope tube diameter has nothing to do with light transmission. Instead, scope tube diameter governs how much windage and elevation adjustment a scope has. That’s why most scopes for long range shooting have 34mm tubes for dialing impressive elevation correction. See, size doesn’t matter…
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A large diameter objective automatically transmits more light. Well, that’s sort of true, depending on whether you’re talking the volume of light, meaning the size of the exit pupil, or if you’re talking about the percentage of light making it through the scope. While we’re talking about the objective lens, let’s talk about its effect on the exit pupil which will partly determine how much light a scope seems to be transmitting and illustrates the difference between the volume of light and percentage of light.
Understanding Exit Pupil
The exit pupil is the diameter of the focused beam of light that exits your rifle scope (or binocular or spotting scope) that that reaches your eye. The size of this beam partly determines how bright an image looks through your scope because you want the exit pupil to be the same size or larger than the size of your eye’s pupil, which of course changes with ambient light conditions.
Unless you have Sauron’s All Seeing Eye, your pupil dilates to roughly 7mm in low light conditions. Your scope will appear brightest to you when the exit pupil is 7mm or larger. Any larger and you can’t make use of the extra light being transmitted, any less and you aren’t using as much as you could.
The exit pupil size is simple to calculate, just take the objective diameter and divide by the magnification. Example: 50mm Objective/10x Magnification = 5mm Exit Pupil.
That’s it. So, a scope with a 50mm objective lens, set at 10x, would have an exit pupil of 5mm. But if that scope were set to 7x, you would have an exit pupil of 7.14mm which would maximize the volume of usable light for your eye. This is why large objectives are known for transmitting more light, it’s because they appear brighter at the higher magnification settings many hunters like to use. But that’s not the light transmission we’re talking about because the percentage of
light making it through the scope isn’t affected by the size of the exit pupil.
Light ‘em Up
The biggest factor in a scope’s light transmission is the lenses. Each lens in the optical system loses a percentage of light that passes through. Even if it’s just one percent, a scope with 7 lenses, which is common, could at best have a 93% light transmission if everything were perfect. It’s not usually that high. Most lenses lose more than one percent. The lens design and the lens coatings affect light transmission. This is where things like “high-definition glass” and “premium lens coatings” come in. Better lens designs and materials transmit more light than inferior materials and designs. Lens coatings help the light pass through the lens rather than reflect off the surface.
Other factors that affect light transmission include light management and glare reduction, lens placement, etc. But in short, the lens’s ability to transmit light, in conjunction with the number of lenses, is the biggest factor in light transmission as measure as a percentage that reaches your eye. Of course, this does not automatically mean that the image viewed through the scope is crystal-clear, in focus, etc., but those topics are for another post.
What It Means for You
While it doesn’t tell the whole story, light transmission is critical when considering scopes for low-light performance. The most crystal-clear image is useless if it’s too dark to see the needed detail in the critical minutes before sunrise and sunset. You can maximize your scope’s low-light performance by keeping exit pupil in mind. In low-light conditions, you’ll want your 3-18x50mm scope’s magnification dialed down to a lower power like 7-10x to have more usable light entering your eye.
Being mindful of the manufacturer’s quality of glass and lens coatings will also keep you on track to maximizing your scope’s light transmission.