Choosing a scope
How to Choose Scope Magnification
Quick answer
Exit pupil equals objective diameter divided by magnification, so a 44mm objective gives a 2.75mm exit pupil at 16x but 5.5mm at 8x, and field of view roughly halves every time magnification doubles. Most shooters need less magnification than they think; the Vortex Diamondback 3-9x40 Riflescope, Dead-Hold BDC Reticle at $249.00 covers the range that actually gets used.
Magnification is the spec buyers argue about most and understand least, because the top number on the box is genuinely a poor guide to what the scope does in the field. Two numbers explain almost everything that actually matters: exit pupil, which decides how bright the image stays as you turn the ring up, and field of view, which decides how much of the world you can still see while you do it.
Both relationships are exact division, not opinion. Exit pupil equals the objective diameter divided by the magnification, and field of view drops by roughly half every time magnification doubles. Once those two numbers are in front of you, the correct amount of magnification for a given objective size stops being a guess.
How to pick the right magnification range for how you actually shoot
- Determine the realistic engagement distances for your use. A rifle used mostly under 150 yards in cover needs a very different top end than one used at 400 yards and beyond across open country. Write down the actual distance range, not the longest shot you can imagine taking.
- Determine the top magnification you will genuinely dial to in the field. This is separate from the number printed on the scope. A 6-24x scope that spends its working life between 10x and 16x should be evaluated at that real range, not at 24x.
- Compute exit pupil at that real magnification for the objective size you are considering. Divide objective diameter in millimeters by the magnification from the previous step. A 44mm objective at 16x gives 2.75mm; the same 44mm at 8x gives 5.5mm, twice as bright by this measure alone.
- Compare that exit pupil to your own use case. A daylight bench session tolerates a small exit pupil without issue. A dawn or dusk hunt rewards a larger one, since a dark-adapted human pupil can open considerably wider than daylight conditions require, though this varies with age and light level.
- Check the field of view cost of chasing extra top-end magnification. Field of view roughly halves every time magnification doubles. Going from 8x to 16x on the same scope costs about half your field of view for the same objective, which matters more than most buyers expect when tracking a moving target.
- Note where mirage takes over from the optic. Past roughly 20x, atmospheric mirage on a warm day commonly limits resolution before the scope's own glass does. Paying for magnification beyond that point buys real image quality only in cool, still conditions, and buys shimmer the rest of the time.
- Choose the objective and magnification pairing, then size rings to the finished scope. Once the range is settled, confirm ring height against the actual objective diameter you chose rather than an earlier guess. See the ring height guide for the specific numbers.
How much light do you actually give up at the top of the range?
The 4-16x44 configuration is common enough to use as a worked example, and the arithmetic is identical for any objective and magnification pair.
A 44mm objective drops from a 5.5mm exit pupil at 8x to just 2.75mm at 16x, and to only 2.2mm at the very top of a 4-16x range, which is the real cost of dialing all the way up.
Sourced figure
| Magnification | Exit pupil (44mm objective) |
|---|---|
| 4x | 11.0 mm |
| 8x | 5.5 mm |
| 12x | 3.7 mm |
| 16x | 2.75 mm |
| 20x | 2.2 mm |
Exit pupil above roughly 7mm is wasted on most human eyes, since a dark-adapted pupil rarely opens that wide. The bottom of this table, not the top, is where usable brightness actually runs out.
Why past 20x usually buys you less than the box implies
A scope's resolving power is fixed by its optics, but the atmosphere between the scope and the target is not, and warm-day mirage is the more common limit at long magnification. Heat rising off the ground bends light enough to blur fine detail well before most quality glass reaches the edge of what it can physically resolve, and turning the power ring higher only magnifies that shimmer along with the target.
This is a real reason the honest recommendation, for most shooters most of the time, is less magnification than the marketing implies. A 6-24x50 scope such as the Vortex Diamondback Tactical 6-24x50 First Focal Plane Riflescope is a genuinely useful configuration, but its practical working range on most days sits well under 24x, and buying a scope specifically for magnification you will rarely get clean use out of is paying for shimmer more often than it is paying for detail.
None of this means high magnification is a bad purchase. A long, cool morning with still air lets a 24x or higher setting genuinely resolve detail a lower power cannot, and for a shooter who regularly works at distances where that detail matters, the extra top end earns its keep on those specific mornings. The point is narrower: buying the highest available magnification as a general-purpose default, rather than for the specific conditions where it pays off, spends money and field of view on a capability that sits unused far more often than it gets used.
Sources
- Exact division: exit pupil = objective diameter / magnification
- Owner-review consensus on mirage limiting long-range resolution above roughly 20x
Frequently asked questions
What is exit pupil and why does it matter?
Exit pupil is the diameter of the shaft of light leaving the eyepiece, calculated by dividing the objective diameter by the magnification. A larger exit pupil generally means a brighter, easier-to-use image, especially in low light, while a small exit pupil can look dim even through excellent glass simply because less light is reaching your eye.
Why does my scope look dim at high magnification when it looked fine at low power?
Because exit pupil shrinks as magnification rises for a fixed objective diameter. A 44mm objective gives 5.5mm at 8x but only 2.75mm at 16x, and the smaller number lets noticeably less light reach your eye. The glass has not changed; the math dividing objective by magnification has.
Do I need a 50mm or 56mm objective for hunting?
Only if your real use case involves low light at higher magnification, since a bigger objective mainly buys back exit pupil that a smaller objective loses at the top of its range. If you mostly shoot at moderate power in daylight, a 40 or 44mm objective on a scope such as the Vortex Diamondback 3-9x40 Riflescope, Dead-Hold BDC Reticle is not giving up anything you will notice.
What magnification do I actually need inside 200 yards?
Most shooting inside 200 yards is comfortable on a 3-9x or similar range, and the top of that range already provides more resolution than most shooters use in practice. Buying well beyond that for a shorter-range use case usually adds cost, weight and a smaller field of view without a matching benefit.
Does more magnification always help at long range?
No, past roughly 20x atmospheric mirage on a warm day commonly limits what you can resolve before the optic's own glass does. Extra magnification beyond that point magnifies the shimmer along with the target, and the practical benefit shrinks considerably compared to what the number on the turret suggests.
How much field of view do I lose going from 9x to 18x?
Roughly half, since field of view drops by about half every time magnification doubles. This is a real tradeoff when tracking a moving target or reacquiring one quickly, which is part of why choosing the lowest magnification that comfortably does the job usually beats defaulting to the top of the dial.
Researched, not professional advice. This page is compiled from published manufacturer specifications, published optical and ballistic formulas, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are shooter convention rather than sourced numbers, and they are labelled that way wherever they appear. Ballistic figures come from a point mass model using the ballistic coefficient the maker publishes, so treat them as a starting point and confirm them on paper at a measured distance. Confirm your firearm is unloaded with the action open before you mount, level, torque or bore sight anything. Verify a zero only on a supervised range or a safe, legal backstop, know what lies beyond your target, and follow the law where you hunt and shoot, including rules on land access and permitted cartridges.