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Gun Talk: The Advent of Optics

Understanding the Factors of Scope Selection

by Brad Fenson

Jack O’Connor was North America’s original gun writer, and I believe much of his work is still very relevant today. His unique adventures and knowledge of firearms and ammunition gave him the label of an expert in most circles and his writing was instrumental in changing the way hunters looked at specific weapons, calibres and cartridges.

O’Connor was always seeking out new adventures, and one of his favorite outlets was hunting for deer in the western states. In the early days he would shoot a rifle with open sights and try to push up a deer that could be shot on the run. It was the ultimate challenge, and his ability to connect on running game with open sights inspired sportsmen to master their marksmanship.

It is rare in this day and age to find a hunter who still uses  open sights. Optics have helped to fine-tune most people’s shooting ability by narrowing the field of adjustment and human error. We have rifles and calibres that can shoot farther and flatter than ever before, but it is our optics that allow us to take advantage of this craftsmanship and technology. Choosing a modern firearm to purchase is relatively easy for most people, but when it comes to optics I find that most individuals  fail to understand best value in their decisions.

I’ve been fortunate enough to have experts explain optics to me and to see scopes in production. These experiences that have helped me discern differences in quality and informed me on making the necessary assessments for sound purchasing decisions. Consider the following: one of the most popular scopes sold  in Canada has a 50-millimetre objective lens. Hunters look at the big end of their scopes and think that that bigger the lens the more light it transmits, meaning the better they can see the target. But in reality the 50-millimetre objective costs significantly more and doesn’t offer much of an advantage that will be usable to most people.

The way a scope collects light and transmits an image is important to understand, as the exit light through the scope to the pupil is the real determining factor in how much benefit can be obtained from a large objective. The pupils of most adults can dilate 4 to 5 millimetres in diameter. There is a simple formula where you divide the size of the objective lens by the magnification setting of your scope to determine the light transmission the eye can take advantage of (a value known as the exit pupil). If we are using a 2 to 10 power scope with a 50-millimetre objective our exit pupil is calculated as follows: 50mm / 10 X (maximum magnification) = 5mm. This means our pupil can take full advantage of the magnification, as the exit pupil isn’t too small. On the bottom end of the magnification there is plenty of room for our eye to see a full image: 50mm / 2 X (minimum magnification) = 25mm. A wider exit pupil means it is easy for shooters to see an image and to keep in focus no matter where they are behind the scope. The narrow exit pupil at the higher magnification requires a very small and specific viewing opportunity where the shooter has to remain perfectly centered behind the scope in order to continually see a full image.

It may seem like I’m rambling a bit at this point but there is an important message. The same scope with a 40 mm objective offers the same exit pupil when set on 8 power: 40 mm / 8 X = 5mm. What this means is that consumers purchasing the 50-millimetre objective lens are only getting a benefit at 9 and 10 power settings with the bigger scope. The question that needs to be asked is whether or not the extra weight and expense are worth it for such a narrow window of advantage?

We should be more concerned with light transmission, and most of that comes through different lens coatings. I’ve seen how glass is cut, polished, coated and finished, and I have to say that it is a major production. The final product is what provides the clarity in terms of what good glass and craftsmanship is really all about.

Jack O’Connor would likely be in awe of the optics available to sportsmen in modern times. Thinking back on my own relatively short hunting career, having started with a Weaver K4 and Redfield Wide Angle variable scope, the optics I have and use today are so superior that it is almost unbelievable. Remember to spend as much time understanding your next scope purchase as you do with firearms, and you’ll be amazed at what you are able to achieve at the range and in the field.

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