So why would you want or need a very high magnification scope? There are two related reasons. You might need to see and hit a reasonably-sized target at very long range, or you might need to be able to see and hit a much smaller target at shorter ranges. For example, if your job calls for spotting and engaging a man-sized target at 1,200 yards, then a zero magnification red dot or low power scope is going to greatly reduce your odds of a successful first round hit. Similarly, if you’re on a prairie dog expedition, your target is only a few inches wide, so even at ranges of 200 to 500 yards you’ll want some extra magnification to spot your target and place your shot.
Understanding exit pupil
We’re going to get into a bit of math here, but I promise that it’s really, really simple and understanding the concept will take a whole lot of mystery out of features and function of magnified optics.
First, it’s important to understand that your eye can only deal with so much light, no matter how big the lenses on the scope are. Larger lenses and tubes can have a positive effect, but it’s not so simple as “bigger allows more light to your eye, so that’s a good thing.” Your eye is limited by the size of the intake valve, otherwise known as the pupil. Think of the pupil as a hole through which light has to travel. Well, actually, that’s exactly what it is. The larger the hole, the more can go through. However, your pupil is only in the 4mm to 7mm diameter range depending on light conditions. In lower light, or if you’re on mind altering drugs, the pupil gets bigger, maybe to 6 or 7mm. In daylight, it shrinks because all that brightness is too much for your eye to deal with. The point of all this discussion about your eyeball is to illustrate the fact that no matter how big a “tube” of light your scope spits out the back, your eye can only deal with a light beam smaller than about 7mm in diameter, again depending on ambient light conditions. The size of the light tube going into your eyeball is called the exit pupil, and understanding how its size changes is a big deal when choosing a scope. It’s also a big deal in the field when you decide how much magnification to use. Let’s talk about that next.
With a variable power scope like the Burris XTR II 2-10x42mm I’m going to use for this example, the exit pupil changes based on two factors: the diameter of the objective lens (the big one up front) and the current magnification level. Here’s where the easy math comes in. Just divide the objective lens size (in millimeters) by the magnification level and you get the size of the exit pupil, also in millimeters. This particular scope has a 42mm objective lens, so when I am using it at 2x power, then the exit pupil is 21mm – far more than I need. That’s what makes the picture so clear and forgiving. If my eye moves around a bit relative to the eyepiece, I can still see a clear picture. If I crank the power up to 10x, then the exit pupil is 4.2mm. In daylight conditions, that’s pretty good, and the tube of light will pretty much fill up the size of my eye pupil. If I look through it in the dark, then my pupil will expand to 6 or 7mm, so I have less light coming into my eye than I can use. I might be better off lowering the magnification to a lower level to get a larger tube of light hitting my eye in that situation. For example, if I reduce magnification to 6x, then the exit pupil will be 7mm (42mm / 6).
So, technically speaking, a larger objective lens can (sort of) transmit more light, but it’s not quite that simple. The real purpose is to transmit more light at higher magnification levels. At lower magnification, a huge objective lens doesn’t really help you because it outputs a larger cone of light than your eye can see. I have another scope on my desk, a Burris Veracity 4-20x50mm. As the name implies, the objective lens is bigger, 50mm, so at a magnification of 10x, I see a 5mm exit pupil (50mm / 10). At 20x magnification, the exit pupil is only 2.5mm, so I want to be using that power setting only in bright daylight conditions when my eyeball pupil is small.
The net-net of all this optics gobbledygook is that you can start to evaluate the type of optic you’ll need for your shooting requirements. If you’re going to be shooting in low light, say at dawn and dusk in a hunting application, make sure that you buy a scope where the objective lens size and magnification levels you’ll be using offer an exit pupil in the 6mm range. If you do that, you’ll have the clearest possible picture through your scope. If your shooting is mostly in bright daylight, then you can go to higher magnification levels, because it will be OK to have an exit pupil in the 4mm range. Above all else, remember, that what’s most important is that, assuming good lens quality, the objective lens and magnification are well matched. Bigger isn’t necessarily better.
With that said, also remember that the exit pupil math is only one factor of evaluation. We talked about it at length here to illustrate the importance of matching the scope features to the capability of your eye in various conditions. But you can’t just select a scope based on good exit pupil math. If the lenses aren’t high quality, then nothing else really matters. We’ll get to that more later in the article.