A scope described as a 3-9x40 looks like code, but every number in it describes something you can picture. Learn the pattern once and the rest of the spec sheet reads itself.
The core: magnification and objective
In 3-9x40, the 3-9x is the magnification range — the image appears three to nine times larger than life — and the 40 is the objective lens diameter in millimeters. A single number before the x, as in 4x32, means a fixed-power scope that does not zoom.
The ratio between the two ends of the magnification range is the erector ratio, and it tells you something about the scope. A 3-9x is a 3x ratio, which is easy to build well. A 5-25x is a 5x ratio and a 1-10x is a 10x ratio; wide ratios are harder to make without compromising edge sharpness, eye box or weight, which is why a wide-ratio scope at a low price usually gives something up elsewhere.
Bigger magnification narrows your field of view and amplifies both mirage and your own wobble. A bigger objective gathers more light but adds size, weight and ring height. Both decisions are covered in how to choose a rifle scope.
Tube diameter
Listed separately as 1 inch, 30mm or 34mm, the tube diameter is the main body section that your rings clamp to. A wider tube does not by itself make a brighter image — that is a common myth — but it leaves more room for the erector assembly to travel, which means more total elevation and windage adjustment. That is why long-range scopes trend toward 34mm and 36mm tubes.
Practically, the tube diameter decides which rings fit. Measure or check the specification; do not assume from the objective size.
Field of view, eye relief and exit pupil
Field of view is usually given in feet at 100 yards, often at both ends of the magnification range. A 1-6x LPVO might show 100 feet at 1x and 18 feet at 6x. More is better for finding and tracking; it falls as magnification rises.
Eye relief is how far behind the eyepiece your eye must sit to see the full image, typically 3.5 to 4 inches. More is better on hard-recoiling rifles and on scout-style mounting. Related and rarely listed is the eye box — how forgiving the scope is about your head position — which is one of the clearest differences between cheap and expensive glass and one you can only judge by looking.
Exit pupil is objective divided by magnification, in millimeters. It predicts low-light performance: a 40mm at 4x gives 10mm, far more than your eye can use, while the same scope at 9x gives 4.4mm, which is about right for dusk.
Reticle, focal plane and units
The reticle name — Duplex, BDC, Mil-Dot, MOA hash, or a tree pattern — tells you the aiming system; see understanding reticles. Whether it sits in the first or second focal plane decides whether its holdover marks stay true across the magnification range: see FFP vs SFP.
Turret values appear as something like "0.1 MIL/click" or "1/4 MOA/click", and the reticle will be in one of the same two units. Match them. A MIL reticle with MOA turrets is a conversion waiting to go wrong — see MOA vs MIL.
You will also see total adjustment range, often as "100 MOA elevation" or "28 MIL". Half of that is theoretically available above your zero, and in practice less, which is why dialing to long range sometimes needs a canted base.
Parallax and the rest of the sheet
"Parallax: fixed 100 yd" or "Side focus 20 yd to infinity" tells you whether the scope can be focused for the target distance — important above about 12x and explained in what is parallax.
Beyond that, look for nitrogen or argon purging and O-ring sealing (fog and waterproofing), the presence of a zero stop if you intend to dial, illumination and whether it has a daylight-bright setting, and weight, which is easy to ignore on a spec sheet and impossible to ignore on a mountain.
Putting it together
Now a line like "1-6x24 FFP MIL, 30mm, illuminated, 3.7in eye relief" reads as a complete description: a low-power variable for close work that still reaches 6x, a 24mm objective because at these magnifications nothing larger is needed, a first-focal-plane MIL reticle so the holds are true at any power, a standard tube, and daylight illumination so it works like a red dot at 1x.
Or "5-25x56 FFP, 34mm, 0.1 MIL, zero stop, side focus": a precision scope built to be dialed, with the travel and the parallax control to support it.
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