What a scope actually does
A riflescope is a precision optical instrument with one job: present a clear, magnified image of the target with a reticle accurately superimposed over it. Understanding its components tells you what can go wrong and how to fix it — and why cheap scopes fail where quality ones don't.
The key elements are: the objective lens (front, gathers light), the erector assembly (the adjustment mechanism inside the main tube), the main tube itself (holds it all together, carries adjustments), the ocular lens (rear, where your eye goes), and the turrets (external adjustment dials).
MIL vs MOA — pick one and own it
Turrets and reticles are graduated in one of two angular units. 1 MIL (milliradian) subtends 3.6 inches at 100 yards; 1 MOA (minute of angle) subtends 1.047 inches. Both are angles, neither is more precise than the other, and the choice is about matching the people and data you shoot with rather than about optics quality.
⛔ The one rule that matters at this level: reticle and turrets must be in the same system. A MIL reticle over MOA turrets turns every single correction into a unit conversion, performed under time pressure, in the field. Buy MIL/MIL or MOA/MOA and never mix them.
The units themselves — how they convert, how to turn a measured miss into clicks, and how to choose between them — get a lesson of their own, linked below.
First Focal Plane (FFP) vs Second Focal Plane (SFP)
First Focal Plane (FFP): the reticle sits in front of the magnification lens stack. As magnification increases, the reticle appears larger on the image — and crucially, the angular subtensions (the spacing between marks) stay accurate at any power setting. Dial 4× or 16×; the 1-MIL hash still subtends exactly 1 MIL on target.
Second Focal Plane (SFP): the reticle sits behind the magnification stack and stays the same size visually regardless of magnification. The subtensions are only accurate at one specific magnification — usually maximum power, sometimes marked on the scope. At other powers, you must scale your holds accordingly.
For ranging and shooting holds at variable magnification, FFP eliminates mistakes. For a dedicated low-power or fixed-power setup where you always shoot at one magnification, SFP is fine. For most precision rifle work, FFP is the practical choice.
Turret direction conventions
On virtually every modern riflescope: UP on the elevation turret raises the bullet's point of impact. RIGHT on the windage turret moves impact right. This is so universal that any exception will be printed explicitly in the manual.
When in doubt — do a box drill. Fire a shot, dial 10 clicks up and observe where impact moves, then dial 10 right and observe, then reverse each. Impact should move in a predictable box. This verifies tracking and direction simultaneously.
Zero stop: a physical stop built into some precision turrets that prevents the turret from going below your zero. Makes returning to zero in the dark or under stress fast and certain. If your scope has one, use it.