We are at the time of the year when hunters start to take a close look at their gear and decide what needs an upgrade, and what can make it another year or two. For those of you thinking about upgrading your optics this year there are two basic optics principles that you really need to understand to help make a choice that won’t leave you regretting your decision. Let’s talk magnification and exit pupil, so you can leave the store and head to the field with the right optics for how you hunt.
The simple truth is that more often than not people purchase optics with more magnification than they need, and it ends up causing problems. While it’s true the extra magnification brings whatever you are viewing in closer, in also magnifies other things, like movement which can cause issues.
The first thing most anyone will notices is that if you compare how steady the image looks, a lower power will always win out. Use a pair of 6X binos and you will think you’re a rock-solid optics guru. Throw up a pair of 15X to your face and you’ll probably be seasick in less than a minute. While those deer you’re looking at are 15X closer, every heartbeat is magnified 15X as well. So is every sway of the tree you’re stand is hung in. Every adjustment you make to your scope to get on target. It’s all magnified, and often, that causes more frustration than it solves.
Clarity also becomes harder to achieve the higher the magnification goes. As the optical system increases in magnification it gets harder to keep things like spherical aberration (rounded distortion at the edges), chromatic aberration (color or contract issues), and light management all get more difficult to manage as the magnification increases. The target’s size may increase, but you may not actually end up with a better view of what you’re looking at. Not only that, but eye fatigue increases as the magnification goes up as well.
There’s a reason most binoculars are offered in 8X or 10X and the most popular scope chassis on earth maxes out at 9X. They offer the most balance between, “enough magnification to get a better look,” and “not so much magnification as to cause issues.” As a general rule for binoculars, 8X really is enough for most hunters that aren’t living out west, where glassing for longer durations is the rule not the exception. You may choose to go to 10X, but remaining steady, especially in a tree stand, can start to be a real problem above 8X.
Exit pupil is directly tied to magnification, so I wanted to get magnification out of the way first. In the simplest terms, exit pupil is defined as the column of light that makes it through the optic and to your eye. This determines what you see through the optic. It is measured in millimeters. To determine the exit pupil, divide your objective lens diameter by your magnification. On a pair of 8X32 binoculars, the exit pupil is 4mm. On a 3-9X40 scope, the exit pupil is a range, from 13.3mm at 3X, to 4.4mm on 9X. Simple enough, right?
So why does exit pupil matter? Because if the column of light that makes it to your eye isn’t large enough for your eye to properly focus, the perceived image quality will be poor, regardless of the optic itself. All the coating or fancy lenses in the world can’t make up for a lack of information delivered to the eye.
The pupil of your eye (the black center part) constricts when ambient light conditions are bright, and opens in low-light conditions to receive as much light as possible. Your eye only sees things through the pupil. So, if ambient light conditions are bright, and your pupil is constricted, exit pupil isn’t as much of a consideration. But as light fades, and your pupil dilates to let more light in, the opportunity arises for issues.
The generally used figure for a dilated pupil in low-light is 4mm. Your goal, for hunting optics should always be to keep your exit pupil at 4mm or larger, or in the case of scopes, at least have a setting you can use that delivers 4mm.
Again, we can see why the 3-9X40 is such a popular scope, and why most 8X and 10X binoculars (with the exception of compact 10X32 options) are so popular, throughout their range of use, they maintain a 4mm exit pupil, so the hunter’s eye always gets the light it needs to see clearly.
This is especially critical as it relates to scopes. As scopes often come in far higher magnifications than binoculars, they are more prone to exit pupil issues. You may think that 6-24x50 scope is going to help you seal the deal at long range, but if the shot opportunity comes at first or last light you may have serious issues seeing your target, as your exit pupil on 24X is barely above 2mm! That’s fine for shooting prairie dogs in the afternoon, as ambient light conditions are good, but at dawn or dusk, not so much.
This may leave you thinking your scope is defective or not the quality you’d hoped for, but again, no amount of coatings, or the name on the side of the scope can make up for your eye needing more light than the setup can deliver.
At the end of the day, the optic you choose is your choice, but if you aren’t making magnification and exit pupil two of the primary factors when you sort through all the optics options you have, you’re likely going to end up frustrated or downright disappointed by your decision.