How To Zero A Rifle Scope or sight

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Rifle Scopes Glossary of Terms

Descriptions of rifle scopes can be a little confusing at times, especially if you're new to scopes and shooting. So I thought I'd do up a little rifle scope glossary of terms

Starting with the basics (I get asked this a lot on ebay)

What does 4x20 mean?
  • 4 - The first number means the magnification. In this example it is 4x normal eyesight.
  • 20 - Means the diameter of the viewing lens (not the eyepiece) in millimetres.
    • The tech term for this is "Objective Lens Size"
  • 4x20 - Therefore is 4x magnification seen through a 20mm wide view lens.

And what about 3-9x40 then?

  • This means the scope has variable "zoom" capability.
  • In this case it ranges between 3 and 9x magnification though a 40mm wide view lens (Objective lens)
  • Unless otherwise stated the numbers are whole numbers, so on a 3-9x there will be a 3, 4, 5, 6 ,7, 8 and 9x position but not fractions or some sort of granular 3.75x type of thing.
  • As an aside, 3-9x40 is the most popular general purpose scope size. If you can afford one get one. (oh I sell them by the way ;-)

You can see the dial here on this 3-9x40.

3-9x40 variable rifle scope

The more techie terms now -

Exit Pupil: The size of the column of light that leaves the eyepiece of a scope.

  • The larger the exit pupil, the brighter the image.
  • To determine the size, divide the objective lens diameter by the power (a 4x40 model has an exit pupil of 10mm).

This is why big mag scopes tend to have big lenses e.g., 8x56 and why you don't get 3-9x20s cos they'd be rubbish

An 8x56 scope. Big Objective lens!

Eye Relief:

  • The distance a scope can be held away from the eye and still present the full field of view.
  • Unlike cartoons you don't have the eyepiece pressed up against your eye and geta  balck eye from recoil!

Field of View (F.O.V.): The side-to-side measurement of the circular viewing field or subject area.

  • It is defined by the width in feet or meters of the area visible at 100 yards or meters.
  • A wide field of view makes it easier to spot game and track moving targets.
  • Generally, the higher the magnification, the narrower the field of view.
  • Which is why variable magnification scopes are so handy. Spot the target at low mag and. Lock on and zoom in.

Windage & Elevation - The 2 turrets mid way down the scope body that are a key part of zeroing a scope.

When you fit a rifle scope they are up and to the right. See the scope fitting page for info on this side of things.

Windage - The one on the right.

  • As the name suggests is to do with wind.
  • It adjusts the crosshair minutely left and right.
  • And for in the field adjustments for wind strength where, for example a high crosswind will make long range shots veer slighty.

Elevation - The top on. Up and down basically.

  • Again critical for getting zero and in the field where a strong head or tail wind effects a pellets lateral trajectory

An experienced shooter will use the these turrets to make a situational zero adjustments. If you're not experienced don't worry. Just be aware and experiment as that's how you become experienced!


  • A condition that occurs when the image of the target is not focused precisely on the reticule plane.
  • Parallax is visible as an apparent movement between the reticule and the target when the shooter moves his head or, in extreme cases, as an out-of-focus image.
  • Some scopes, higher prices ones typically, have parallax correction built in so that this focus problem does not occur.

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