A telescopic sight has a fairly straightforward job to do, but the terminology can be a bit daunting. Mike Morton explains the basic features to help simplify scope selection
Today’s telescopic sights are precision instruments, often chock-full of exciting features. At its most basic level, however, a scope has just one very simple role to play: it helps you point the barrel of your airgun in a particular direction. It doesn’t matter what type of shooting you’re doing, or what type of airgun you’re using, the principle is the same: a scope is a type of sighting system that ensures the pellet will fly through the air in a known trajectory and hit your target.
A telescopic sight – which can also be referred to as an ‘optic’ or a ‘telly’ – is just one type of sighting system available to the shooter. Let’s briefly look at the others so we can understand the differences.
Open sights, also known as ‘iron sights’ or ‘irons’, feature a foresight at or near the front of the barrel and a rear sight closer to the shooter’s eye. The system works by the shooter aligning a front element – usually a small post – inside a rectangular or V-shaped cut-out in the rear sight. When these two elements are correctly aligned, the barrel will be pointing in a very specific, controlled direction – this is the key to any sighting system.
Aperture sights are similar to open sights, but instead of a notch, use a fully enclosed element for the rear sight. This is typically shaped as a circle, but other shapes, such as squares or diamonds, can be used instead. The rear aperture sight, which is sometimes called a dioptic sight, is aligned with a front element that is specifically designed to work in conjunction with it. The most common combination is to have a circular dioptic sight and a circular front sight, the human eye and brain being naturally good at placing a circle within a circle. Aperture sights are an extremely accurate sighting system, and can even be found in some types of long-range fullbore target shooting.
Lasers project a beam towards the target. They can be used to assist rangefinding, but may be used as a basic sighting system in their own right. They are especially useful where POA and POI coincide and rapid fire is important – for example when you shoot multi-shot pistols at short, set distances.
The final sighting element to consider is the red dot sight. While a laser projects a dot onto a target, a red dot sight presents the shooter with a holographic sight picture contained within the optic. The shooter then looks through the sight and places the dot over the target. The red dot sight is the only type of sighting system apart from the telescopic sight that may include some sort of magnification, but usually only to around three times (3x).
We’ve now identified the key difference between the scope and the various other sighting systems – magnification. Optically enlarging your target helps you see it more clearly and in more detail, with a typical scope offering up to nine, 10, 12 or 16 times magnification, while some specialist scopes can magnify an object up to 80 times larger than when viewed with the naked eye. Magnification means you can establish point of aim – and therefore point of impact – with great precision.
A telescopic sight shares numerous features with a conventional telescope, the most obvious of these being its external lenses. At the front is the objective lens, which transmits light through the scope. The lens at the rear of the scope is the ocular lens: this is the one that’s closest to the shooter’s eye when they are looking through the device.
The front of the scope is usually flared to accommodate the objective lens; this portion of the scope is called the objective bell. Similarly, the rear of the scope sometimes widens to house the ocular lens, and this wider portion is the ocular bell. The objective bell and ocular bell are connected via a cylinder known as the ‘scope body’ or ‘scope tube’. While both the objective bell and ocular bell can be wider than the scope body, it’s also possible to have objective or ocular lenses that are small enough to fit inside the main dimensions of the scope tube.
The scope body contains an assembly called the erector tube which holds the magnification lenses and the reticle, which is also known as the ‘reticule’ or ‘crosshairs’. ‘Reticle’ is the generally preferred term today, as the word ‘reticule’ not only refers to a sighting system, but is also a type of ladies’ bag dating back to the Regency period! The term ‘crosshairs’ relates to the fact that early reticles were made from crossed fine wires, fibres or hairs. Nowadays most reticles are created by using engraved lines or embedded fibres.
Some scopes are of fixed magnification, modern ones of this type typically being used for Hunter Field Target competition shooting. In this discipline, magnification can’t be adjusted once shooting has started, and so fixed-mag HFT scopes will be constructed with just one level of optimal magnification for the task in hand, such as 10 times.
The terms ‘objective’ and ‘ocular’ can be confusing – which is not great when you are trying to buy a set of protective scope lens covers, for example, and need to know exactly which ones you’re talking about. However, there’s a really easy way to remember which is which. The front lens – the objective lens – is the one that’s closest to the object you’re aiming at. The word ‘ocular’, on the other hand, means anything to do with the eye, and the rear lens – the ocular lens – is the one that’s nearest to your eye.
Most general scopes for airgun use do feature a range of magnification, however, and the magnification lenses inside the erector tube will move when the scope is being adjusted, going forwards towards the objective lens when the shooter is increasing magnification, and moving back towards the ocular lens when magnification is being lowered.
Whenever you use a sighting system, you are looking at a specific point on your intended target. This is your point of aim (POA). For a rifle zeroed at 25 yards and shot at the same distance, for example, you would align the sight with the exact point on the target you want to hit.
Where the pellet actually hits is its point of impact (POI). In this case, a rifle that’s been correctly zeroed at 25 yards will – all other factors remaining equal – just require the shooter to sight the gun at the target and take the shot.
In this instance, the point of aim and the point of impact are the same. If the gun is shot at distances other than the set zero, however, the POA and the POI will not necessarily coincide.
ALLOW US TO EX-PLANE…
For decades, the majority of scopes used by airgun shooters had a second focal plane, but first focal plane scopes are becoming more and more popular nowadays. The terms refer to the location of the reticle in relation to the magnification lenses inside the erector tube. Where the reticle is positioned will affect the way shooters see the image of their target as they adjust magnification.
SECOND FOCAL PLANE
It may seem the wrong way round, but let’s look at second focal plane scopes first, as these are still the most numerous type of scope – although that could well change over the next few years.
With an SFP scope, the reticle is positioned behind the magnification lens. This means the size of the reticle will stay the same when magnification is increased or decreased, while the size of the image being viewed will expand or contract accordingly.
FIRST FOCAL PLANE
With an FFP scope, the reticle will be mounted in front of the magnification lens. This time, when the level of magnification is altered, both the size of the reticle and the size of the target image will change, but remain in proportion to one another.
These distinctions are important if you use holdover or holdunder to hit your target at varying ranges. In the case of a second focal plane scope, your set of aimpoints will only be accurate at one particular level of magnification, while for a first focal plane scope the aimpoints will always remain true, regardless of the level of magnification you’ve selected. FFP scopes are therefore simpler to use if you change magnification.
Any scope offering variable magnification will have a control called the magnification ring, which is also known as the ‘power ring’, to adjust magnification to the desired level. The ring is typically located at the front of the ocular bell. The magnification ring should be clearly marked and, like any scope control, should turn smoothly.
Some second focal plane scopes may have a certain power level highlighted in a different colour, typically red. This feature is usually found on scopes using a reticle graduated in milliradians, a unit which is commonly abbreviated to ‘mils’ or ‘mrads’. A mil as seen in the reticle of a SFP scope will only be ‘true’ at one particular magnification.
OCULAR FOCUS RING
The reticle needs to be in focus for the shooter, regardless of the target that’s being viewed, the distance away from the target or the level of magnification that’s been selected. Our eyes are all different, and we need to use the ocular focus ring to ensure the reticle is crisp. If you look through a fellow shooter’s scope, the chances are that the reticle won’t appear to be perfectly in focus for you. This is just one reason why a scope should be set up for an individual shooter rather than being shared.
You may need to unscrew a locking ring before you can make any adjustments. Once the focus ring has been set, the locking ring can then be secured, meaning the ocular focus ring can’t be moved by mistake. If no locking ring is present, the ocular focus ring is said to be a ‘fast focus ring’; this is both quicker and easier to adjust, but because it can’t be locked in place it may get turned without you knowing it, in which case you’ll need to reset it to suit your shooting eye.
Setting the ocular focus ring is an important task that needs to be carried out at least once for every scope a shooter uses…
1. Ensure that the rifle is unloaded and not cocked, and that you are able to point it in a safe direction.
2. Find a neutral ‘target’ to look at, such as a cloud, overcast sky, or painted wall. Never look directly at the sun, as you may damage your eyes. The aim is to focus the reticle, so you need to minimise any background optical ‘clutter’.
3. Close your eyes, shoulder the rifle and open your shooting eye. Is the reticle perfectly in focus? If not, adjust the ocular focus ring and repeat the process.
4. Don’t adjust the ring while in the aim, as you may get a false reading. Our eyes and brain are good at ‘filling in the blanks’, and if you look too long at an out-of-focus reticle, it will start to appear sharper than it really is as your brain starts to compensate. It’s better to make a small adjustment, close your eyes, shoulder the rifle and open your shooting eye. The way you view the reticle in the first couple of seconds is its true level of focus for your shooting eye.
SCOPE TUBE DIAMETERS
Scope tubes come in two main diameters: 1in and 30mm. For many years the 1in tube was the norm, but a larger tube gives the manufacturer more room to fit bigger internal lenses, internal parts, or a combination of the two. It’s probably true to say that 30mm scope tubes are now more common than 1in tubes. Wider diameters are available, such as the 34mm tube used by scopes such as the new Discovery HD 3-18×50 FFP SS.
If you’re in the market for a new scope, the diameter of the tube should not be a major purchasing consideration. Optical clarity, the type of reticle, other features and price will be of more importance than the diameter of the tube. Tube size will be of vital importance, however, when it’s time to choose a set of mounts to fit the scope to your rifle.
Some scopes have no parallax adjustment but will be set to be parallax-free at a certain distance – usually 100 yards, which is of little use to airgun shooters. Others, meanwhile, can be adjusted to eliminate parallax error. The adjustment control can take the form of a ring around the objective bell, in which case the scope is said to have an adjustable objective or AO.
Alternatively, the parallax control can be a turret on the left-hand side of the scope tube. Scopes with this feature are said to have a side parallax control or be side focusing.
The relationship between parallax and focus can be confusing. The parallax adjustment control allows the shooter to get the target and the reticle aligned on the same focal plane. By happy coincidence, when you correctly adjust parallax, you will naturally be making the target appear clearer and more in focus; so for most shooters, knowing that you need to adjust the parallax control until the target image is clear is all you need to use the scope successfully, eliminating parallax error into the bargain.
Parallax controls, whether they’re AO or the side parallax type, will usually be graduated in either yards or metres. These graduations are helpful to an extent, but adjusting the control until the target image snaps into focus is of more importance than trying to adjust them by relying on the distance markings.
WINDAGE AND ELEVATION
Shots can be adjusted for windage and elevation by turning the turrets mounted on the top (elevation) and right-hand-side (windage) of the scope tube. Windage means adjusting shot placement in the horizontal plane, while elevation is the vertical. The direction you need to turn a turret to adjust your shot left to right or up and down is usually marked with an arrow; but if no marking is present, a general rule is to turn the windage turret anti-clockwise to move a shot right, while turning the elevation turret anti-clockwise will bring a shot up.
Turrets are typically graduated in units marked as 1/4 minute of angle or 1/8 minute of angle. Graduations in subtensions of a milliradian are also available, and sometimes the controls are simply marked 1/4in. This means one click will move a shot 1/4in at 100 yards – which equates to four clicks at 25 yards. So if you want to move a shot a full 1in at 25 yards you’ll need to adjust by 16 clicks.
Don’t let yourself get confused by all of these calculations. Pellets are cheap, and there’s no need to get into the maths if you don’t want to. Just shoot paper targets and make repeated adjustments until you’re happy when zeroing.
Some scopes will have an illuminated reticle – a battery-powered feature that will light up the reticle in varying intensities, usually in red, but sometimes in green or blue. Some illuminated reticles offer the option to switch between two colours. Illuminated reticles are typically used to highlight the ret in the shooter’s eye in low-light conditions, but they can also be useful in bright sunshine, especially when you are trying to place your otherwise black reticle over a target that’s in dark shadow. Scopes offering this feature may have a dedicated turret for illumination, or it may be built into an existing control such as the side parallax turret.
KNOBS OR TURRETS?
Traditional hunting scopes, which are intended to be adjusted just once then left alone, have low-profile knobs; while target scopes, which might constantly have to be adjusted for windage and elevation, have high-profile knobs called turrets.
Nowadays the distinction between these types has blurred somewhat, with these controls typically being referred to as ‘hunter’ turrets (low-profile) or ‘target’ turrets (high-profile). Hunter turrets are often covered by a protective cap that must be removed before they can be adjusted; while target turrets usually have no cover, allowing for quicker adjustment, and may or may not have some sort of locking feature.
THE NUMBERS GAME
If you read the description of a scope, you will encounter a set of numbers that appears to be some kind of cryptic code, but in fact is giving you all sorts of useful information about the sight. Let’s take the example of a scope that’s listed as being 3-12×40 SF IR. The ‘3-12’ part refers to the magnification range of the scope.
The ‘40’ refers to the size of the objective lens in millimetres. This is important because the size of the objective lens will dictate the size of the objective bell, and you’ll need mounts of the correct height to ensure the objective bell will sit above the barrel of your rifle instead of touching it. ‘SF’ stands for ‘side focus’, meaning parallax adjustment is carried out using a turret mounted on the side of the scope tube, while ‘IR’ means the scope has an illuminated reticle. The crucial data missing is the scope tube diameter.
CHOOSING A SCOPE
Understanding the features a particular scope has to offer will hopefully arm you with the knowledge to make an informed choice when you come to buy one. The advice to spend as much as you can afford on a scope still holds true, but advances in manufacturing techniques mean fewer genuinely poor-quality scopes are on sale today, and it’s never been easier to get hold of a good, dependable optic for less financial outlay. When you draw up a shortlist, think about the type of shooting you do, and whether the features offered by a particular scope fit your style of shooting.
The final piece of advice is to try before you buy. Most retailers will let you look through a scope on the premises, but what you really need to do is use the scope in real-world conditions, so ask around among your friends and visit clubs to see if anyone can let you have a go with theirs. You can have the ‘best’ rifle on the market – but if you’re not pointing it at exactly the right spot, you won’t hit a thing!
Once you’ve bought a scope, you’ll need to find some good-quality mounts to attach it to your rifle; but there are a number of factors to consider before you walk away with a set…
1. What type of clamping system does your rifle have? Does it have a regular dovetail rail, or a Weaver/Picatinny rail?
2. If it’s a dovetail, what size rail is it? Dovetail rails are usually either 11 or 13mm wide. Sportsmatch, for example, makes two widths of dovetail mount: the wider is a better fit for some BSA and older Weihrauch rails.
3. What size of scope tube do you have? Is it 1in, 30mm or even one of the more exotic sizes such as 34mm?
4. What size of objective lens does your scope have? This can be anywhere from 24 to 60mm – you’ll need mounts that can accommodate this size.