Who reads a glossary? Those are usually at the back of long books, and you never want to admit that you go there to learn some obscure word or terminology. We’ll try to handle this glossary thing a little differently and define important concepts in regular English. Besides, if you’re going to buy and use optics, then understanding these concepts is kinda important. I promise we’ll do our best to make it fun and interesting.
Here, we’ll try to cover the definition of various scope features and functions in such a way as to help you decide what features you need, and therefore what type of scope to choose, based on the type of shooting you do and the style of gun you have. For example, if your shooting world is all about fast targeting at short ranges, things like parallax aren’t going to be an issue for you, but field of view most certainly will. Will the size of a scopes objective lens make a difference in your actual shooting performance? That depends. Read on and we’ll cover those types of issues.
Because last time we covered some of the basic issues with choosing a scope, we thought it good to go through all of the different specs you’ll find, and how they relate to performance and price.
Objective lenses and exit pupil
The objective lens is the (usually) larger opening at the front of a scope. There’s a partial myth about objective lenses that won’t ever seem to die. The belief is that the larger the objective, the more light that can be let in. Like most myths, there is some truth to that. The falsehood is that there is a limit. A six-foot diameter objective lens would certainly seem to let in a lot more light, but the problem is that your eye wouldn’t be able to take advantage of that. Besides, that would be really hard to mount on anything smaller than an M777 Howitzer. Here’s why.
Choosing the right optic for the job is a careful balancing act. After deciding on fixed or variable power, you have to match the other supporting design factors to make sure everything works together. To understand issues related to lens size and magnification, it’s easiest to start right at your eyeball. Your pupil is the limiting factor for the size of “light beam” that can enter your eye. In most situations, the perfect diameter of the beam of light coming out of the scope eyepiece is 5mm to 7mm. Knowing that, we can work backward to optimize the other features of the optic.
Think of exit pupil as the size of the cone of light coming out of the scope. It varies in size based on the size of the objective (front) lens and the level of magnification. Fortunately, the formula is easy. Just divide the objective diameter by the scope magnification level. So, a scope with a 40mm objective lens and 10x magnification has an exit pupil of 4mm. Crank the power down to 4x, and the exit pupil grows to 10mm. This is why a bigger objective lens is only better to a point. For our 10x scope, an imaginary objective lens of 100mm would yield an exit pupil of 10mm – more than our eye can see anyway, so there is no “extra light coming in” benefit to that huge objective lens. The larger light cone is there, but we can’t process it.
With all this said, larger magnification scopes will often have larger objective lenses, but bigger isn’t always better. To muddy the water even more, you also have to account for the quality of the glass. Imagine two scopes, identical to each other with the exception of objective lens size and glass quality. It’s entirely possible that the scope with the smaller objective lens, but better glass, will show you a brighter image than the one with a giant objective lens, but lesser quality glass.
We’ll get into this concept in more detail later in the series, but for now, just remember that objective lens size and magnification are a balancing act that has to be compatible with your eyeball.