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Scopes Explained - A Basic Guide

Scopes Explained - A Basic Guide

This guide may be helpful to newcomers to the sport of Airgunning, with regard to the basic specification nomenclature of scopes.

Many people can become confused by the vast array of numbers and letters which accompany the manufacturers name of a scope, and which explain what properties it has to offer the user. Hopefully this guide will help unravel this mystery, and help you gain a better understanding of what it all means.


Scope models come entitled with an arrangement of numbers, for example a 4x40, 3-9x40IR, or a 4-16x50PX, or whatever.

The first number (or numbers) before the 'X' relate to the level of magnification the scope will provide, so a 4x will increased the viewed image the user sees by four times. If that number was 6x, then the subject would look six times bigger than it actually was.

If the scope gives two numbers, it means the magnification is adjustable anywhere between the two extremes, so a 3-9x can adjust the image as low as three, or as high as up to nine times magnification. Likewise a 4-16 (or whatever) is variable between four to sixteen times the actual size of the image. This property makes the scope known as a 'zoom' scope.

The number that comes after the 'X' is the diameter in millimetres of the objective lens (furthest from your eye), so a x32 has a 32mm front lens, a x40 has a 40mm front lens, a x50 a 50mm lens, and so on.

If a scope has either PA, PX or AO in it's title (eg, 4-16x50PA), it means that the scope is 'Parallax Adjustable', or has an 'Adjustable Objective', and it has the ability for the image of the target to be precisely focused upon, regardless of what range that might be, so it will always provide a pin-sharp picture of what you are looking at.

Adjustment is made by one of two ways, either by an adjustable front (objective lens) collar, or by a 'sidewheel' (sometimes prefixed SW) mounted on the turret saddle.

Parallax adjustable scopes are sometimes referred to as 'Rangefinder' scopes, as they also provide the side-effect of allowing you to read off the range on the focusing collar/wheel, once your intended target is in sharp focus.

Scopes without PA, PX or AO ability are only perfectly focused at a given fixed distance, usually 30 or 100 yards at the factory, though it is possible to re-adjust them yourself, to whatever fixed distance you desire, but it will always remain focused perfectly for that range only, unless you chose to re-adjust it again (not quickly done).

Moving onto the turrets, which are used to 'zero' the scope of the rifle/pistol to the point of aim/impact, so the pellet actually goes to precisely where you are aiming. These are sited on the saddle (in the centre of the scope), one on the top, and one on the righthand side of the scope when viewed from the rear (ocular) lens.

The top turret controls the up/down movement of the shot (elevation), and the side turret controls the side to side placement of the shot (windage).
Some turrets have covers to protect the settings from the elements, and from being knocked out of position.

Some turrets however, are exposed, and are generally known as BDC (Bullet Drop Compensating), or 'target' turrets, and again, this acronym is sometimes added into the scope title, eg 5-20x50PA BDC. Some scopes have lockable BDC turrets, as non-lockable ones can be very easily inadvertently knocked, and so ruining the zero.

Another thing which is sometimes added to the specs, is what type of 'reticle' a scope has. A reticle is another name for the 'crosshairs', or the aiming marks, that are viewed through the scope, the most common being 30/30 and Mil-Dot.
There are many other different types of reticle, far too many to go into here, so I might explain those in a future article.

One thing I will add now though, is some scopes have an IR prefix. This means the reticle has the ability to be illuminated (IR = Illuminated Reticle) to enable shooting at night, or in dark conditions, when a normal reticle would not be visible.

The 'body tube' of scopes come in two common sizes, 1" or 30mm.
30mm is said to allow more light into the shooters eye, and so provide a brighter sight picture, but this is debatable, and again, a subject for future discussion.

So, having gone through the 'basics' of scope specifications, I hope that has been helpful in creating a good foundation of your understanding of them, so when you next see a 4-16x50PA-IR-Mil-Dot, you will know exactly what it all means.
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