A scope like a 3-9x40 looks like code, but every number describes something you can picture. Learn the pattern and the spec sheet reads itself.
The core: magnification × objective
In 3-9x40, the 3-9x is the magnification range — from 3 times to 9 times life size — and the 40 is the objective lens diameter in millimeters. A single number before the x (like 4x32) means fixed power. Bigger magnification narrows your field of view; a bigger objective gathers more light but adds size and weight, as covered in how to choose a rifle scope.
Tube diameter
Usually listed separately as 1 inch, 30mm or 34mm, the tube diameter affects internal adjustment range and which rings you need. A larger tube generally allows more elevation travel for long range.
Reticle, focal plane and eye relief
The reticle name (Duplex, BDC, MIL, MOA) tells you the aiming pattern; whether it is first or second focal plane decides if holdovers stay true across magnifications — see FFP vs SFP. Eye relief is how far your eye sits from the scope for a full image — more matters on hard-recoiling rifles.
Put it to use
Now "1-6x24 FFP MIL" or "5-25x56" tells you the whole story at a glance. Match the units on your reticle and turrets — read MOA vs MIL — and browse rifle scopes or LPVOs with confidence.
