How to Choose a Rifle Scope
What scope numbers like 3-9x40 mean, objective size, exit pupil and brightness, eye relief, first vs. second focal plane reticles, parallax adjustment, and what to check about mounting and sighting in, following Wikipedia's telescopic sight article and Alaska Fish and Game.
A scope is only as useful as how well it suits your rifle, your eyes and the distances you hunt. This guide explains the specifications on a scope, what they mean in practice, and what to check about mounting and sighting in, following Wikipedia’s telescopic sight article and the Alaska Department of Fish and Game (ADF&G). We have not tested scopes against one another and prices change, so we name no models; compare current ranges on the makers’ own sites.
Reading the numbers
Wikipedia explains that scopes are described by magnification and objective lens diameter: “10×50” means 10× magnification with a 50 mm objective lens. A variable scope gives a range, so “3-9×40” means 3× to 9× with a 40 mm objective.
- Objective lens: larger objectives gather more light, giving a larger exit pupil and a brighter image.
- Exit pupil: the objective diameter divided by the magnification (a 40 mm objective at 4× gives 10 mm; at 9×, about 4.4 mm). Wikipedia notes the brightest image comes when the exit pupil matches your fully dilated pupil, about 7 mm in a young, dark-adapted eye and less with age; by the formula, a lower magnification gives a larger exit pupil.
- Eye relief: the distance behind the eyepiece where your eye sees the full image; typical scopes range from about 25 mm to over 100 mm, and scopes for handguns and scout rifles need much longer eye relief (Wikipedia). Check the eye relief against your rifle’s recoil and your shooting position.
Reticle: first or second focal plane
On a variable scope, Wikipedia explains, a first focal plane reticle grows and shrinks with the image as you change magnification, so its markings stay correct at any power; a second focal plane reticle looks the same size as the image grows and shrinks, so it doesn’t stay true to magnification changes. Wikipedia notes most modern variable scopes are second focal plane unless stated otherwise. If you plan to use holdover marks, check which plane the reticle is in and, for a second focal plane scope, at what magnification the maker rates the marks. Our zero and holdover tool helps you work out holds for your load.
Parallax adjustment
Some scopes have a parallax adjustment, on the objective bell (“AO”) or as a side focus wheel, to bring the target and reticle into the same plane at a given distance. Wikipedia notes many low-power variable scopes, used mostly at close and medium range, have none.
Durability, tracking and warranty
Look at what the maker states about waterproofing and fog-proofing, how its adjustments are rated (click value), and the warranty. Your scope’s manual gives the click value you’ll need when sighting in.
Mounting and sighting in
ADF&G’s first step in sighting in is checking that the screws on the action, scope mounts and rings are tight, because a loose screw ruins accuracy in the field. See our scope mounting guide and how to sight in your rifle, and match the setup to the big game you hunt and the distances you expect.
Sources
- Wikipedia, Telescopic sight. Read 28 October 2026.
- Alaska Department of Fish and Game, Sighting-in your rifle. Read 28 October 2026.
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