Optics terminology – keeping things simple
Optics terminology can be a bit daunting, so Mike Morton explains the basic features of a telescopic sight to help simplify scope selection
A modern telescopic sight is a precision instrument, full of helpful features, but at its most basic level a scope has one simple role to play – it helps us point our airgun in a particular direction. So navigating through optics terminology can be a little daunting.
It doesn’t matter what type of shooting we’re doing or what type of airgun we’re using, the principle is the same: a scope is a type of sighting system that will help ensure a pellet will fly through the air on a known trajectory and hit the intended target.
A telescopic sight, which can also be referred to as a ‘scope’ or an ‘optic’, is just one type of sighting system available to the shooter. Let’s take a quick look at the other kinds to understand the differences and break down the optics terminology.
Other types of sight
Open sights, which are also known as ’iron sights’, 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 U- 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, and this is the key to any sighting system.
Aperture sights are similar to open sights, but instead of a notch, they use a fully enclosed element for the rear sight. This is typically a circle, but other shapes, such as squares or diamonds, can be used. The rear aperture sight, called a dioptre sight if the aperture is less than 1.2mm in diameter, is aligned with a front element specifically designed to work in conjunction with it.
The most common combination is to match a circular rear sight with a circular front sight, the human eye and brain being naturally good at placing a circle within a circle. Aperture sights are extremely accurate sighting systems, and can even be found in some types of long-range full-bore target shooting.
Lasers project a beam towards the target. They can be used to assist with rangefinding, but may also be used as a basic sighting system in their own right, these being especially useful where point of aim (POA) and point of impact (POI) coincide and rapid fire is important, for example when shooting multi-shot airguns over short, set distances.
The final sighting element to consider is the dot, which is also known as a reflex sight. While a laser projects a dot onto a target, a dot sight presents the shooter with a sight picture that is contained within the optic.
The shooter then looks through the sight and places the dot over the target. The dot sight is the only type of sighting system apart from the telescopic sight that may include some sort of magnification, but usually only up to around three times (3x).
Two types of plane
The majority of scopes used by airgun shooters are second focal plane, but first focal plane scopes are becoming popular. 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 their target.
Second focal plane
It may seem the wrong way round, but let’s look at second focal plane scopes first as this is still the most numerous type of scope.
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 is 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 distances. In the case of a second focal plane scope, your set of aim points will only be accurate at one particular level of magnification, while for a first focal plane scope the aim points will always remain true, regardless of the level of magnification you’ve selected. FFP scopes are therefore simpler to use if you change magnification as you only need to learn one set of aim points.
FFP – where size matters
As magnification increases with a first focal plane scope, the size of the reticle also increases, along with the size of the object being viewed
Magnification & lenses
We’ve now identified the key difference between the telescopic sight and the various other sighting systems, this being magnification. Optically enlarging our target helps us to see it more clearly and in more detail, with a typical scope offering around nine or 12 times magnification, while some specialist scopes can magnify an object even larger, some up to 80 times larger than when viewed with the naked eye. Magnification means we can establish point of aim, and therefore point of impact, with great precision.
A telescopic sight shares numerous features with a conventional telescope, hence its name, 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 being the one that’s closest to the shooter’s eye when they are looking through the device.
The terms ‘objective’ and ‘ocular’ can be confusing, which is not ideal when you are trying to buy individual protective scope lens covers for example, and need to know exactly which lens 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.
The front of the scope is usually flared in order to accommodate the objective lens, and this portion of the scope is called the objective bell., due to its bell-like shape.
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 the 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, meaning there are no flared bells at either end.
The scope body contains an internal assembly called the erector tube that holds the magnification lenses and the reticle, which is also sometimes referred to as the crosshairs. The term ‘crosshairs’ relates to the fact that early reticles were made from crossed fine wires, fibres or hairs, although nowadays most reticles are created by using engraved lines or embedded fibres.
Some scopes are of fixed magnification, with 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 eight or 10 times.
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.
Mounting up
Once you’ve bought a scope you’ll need 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.
- What type of clamping system does your rifle have? Does it have a regular dovetail rail or a Picatinny rail?
- If it’s a dovetail, what size rail? Dovetail rails are usually 11mm or 13mm wide. Sportmatch, for example, makes two widths of dovetail mount, the wider being a better fit for some BSA and older Weihrauch rails.
- What size scope tube do you have? Is it 1in, 30mm or even one of the more exotic sizes such as 34mm?
- What size objective lens does your scope have? This could be anywhere from 24mm to 60mm – you’ll need mounts that can accommodate this size.
Magnification ring
Any scope offering variable magnification will have a control called the magnification ring, sometimes also referred to 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, turn smoothly.
Some second focal plane scopes may have a certain power level that is highlighted in a different colour, often red. This feature is usually found on scopes that use a reticle that’s been graduated in milliradians, a unit which is commonly abbreviated to ‘mils’ or ‘mrads’. A mil as seen in the reticle of an SFP scope will only be ‘true’ at one particular level of magnification.
Magnify the view
The magnification ring on this Zeiss Conquest V4 4-16×44 is easy to locate and operate, and is smooth when it’s being rotated
POA & POI
Point of aim
Whenever we use a sighting system, we are looking at a specific point on our intended target. This is our point of aim (POA). For a rifle zeroed at 25 yards and being shot at the same distance for example, we would want to align the sight with the exact point on the target that we want to hit.
Point of impact
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. Here, point of aim and point of impact are the same. If the gun is shot at distances other than the set zero, POA and POI will not necessarily coincide.
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 selected. Our eyes are all different, and we need to use the ocular focus ring to ensure that the reticle is crisp. If you look through another shooter’s scope, the chances are that the reticle won’t appear to be perfectly sharp for your eyesight. This is just one reason why a scope should be set up for an individual shooter rather than being set up by a gun shop or a friend.
You may need to unscrew a locking ring before you can make any adjustments. Once the ocular focus ring has been set, the locking ring can then be secured, meaning the 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.
Focusing the reticle
The ocular focus ring on this Zeiss Conquest V4 is the fast focus type, which makes it quicker and easier to set
The ocular focus ring
Setting the ocular focus ring is an important task that needs to be carried out at least once for every scope a shooter uses.
- Ensure the rifle is unloaded, is not cocked and you are able to point it in a safe direction.
- 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 we need to minimise any background optical clutter.
- 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 until it is.
- Don’t adjust the ring while you’re 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 our brains start to compensate. It’s better to make a small adjustment, then close your eyes, shoulder the rifle and open your shooting eye. The way that 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. I’d say that the 30mm scope tubes are now more common than 1in tubes. Wider diameters are also available, such as 34mm, but the vast majority are now 30mm.
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.
Tube dimensions
Knobs or turrets?
Which way’s up? This elevation turret is clearly marked to let you know exactly which way to turn it to adjust for fall of shot
Traditional hunting scopes, which are intended to be adjusted once then left alone, have low-profile knobs, while target scopes, which might constantly be adjusted for windage and elevation when dialling in a shot, have high-profile knobs called turrets.
But nowadays the distinction has blurred, with these controls typically being referred to as ‘hunter’ turrets (low) or ‘target’ turrets (high). Hunter turrets are often covered by a protective cap which 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.
Parallax adjustment
Some scopes have no parallax adjustment but will be set to be parallax-free at a certain distance, usually at 100 yards, which is of little use to airgun shooters. Others, however, 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’. The parallax control can be a turret on the left-hand of the saddle. 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, therefore for most shooters, knowing that they need to adjust the parallax control until the target image is clear is all they need to do to use the scope successfully, eliminating parallax error into the bargain.
Parallax controls, whether 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 manufacturer’s distance markings.
It’s easier to adjust for parallax using a side control while in the aim, and requires less physical movement compared with reaching over the ocular bell to operate an AO control
Windage & 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 saddle.
Windage means adjusting shot placement in the horizontal plane, while elevation is the vertical. The direction that 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 subtensions of a milliradian, as 1/4minute of angle or more rarely 1/8 minute of angle. Sometimes the controls will simply be marked ¼in. This means one click will move a shot ¼in at 100 yards, which equates to four clicks at 25 yards. So if you wanted to move a shot a full 1in at 25 yards you’d need to adjust by 16 clicks.
Don’t let yourself get confused by all of this terminology. Pellets are cheap, especially compared with rimfire or centrefire cartridges, and there’s no need to get into the maths of shooting if you don’t want to. Just shoot at paper targets and make repeated adjustments until you’re happy when you are zeroing your rifle and pellet combination.
Illuminated reticle
Some scopes will have an illuminated reticle, a battery-powered feature which will light up the reticle in varying intensities of colour, usually in red, but sometimes they come in green or blue.
Some illuminated reticles offer the option to switch between two colours. Illuminated reticles are typically used to highlight the reticle in the shooter’s eye in low-light conditions, but they can 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, typically located on the ocular bell, or the control may be built into the side parallax turret on the left-hand side of the saddle.
Illuminated reticles
Scopes with an illuminated reticle will offer the shooter the means to switch the feature on and off and alter the level of brightness
Crunching the numbers
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 telescopic sight. Let’s take the example of a scope that’s listed as being a 3-12×40 SF IR. The ‘3-12’ part refers to the magnification range of the scope, the minimum being 3x and the maximum being 12x.
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 the parallax adjustment is carried out using a turret mounted on the left-hand side of the saddle, while ‘IR’ means the scope has an illuminated reticle. The one piece of crucial data missing in this descriptive formula is the diameter of the scope tube.
Choosing a scope
Understanding the features a particular scope has to offer will hopefully arm you with the knowledge needed 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 of potential purchases, think about the type of shooting that you do, and whether the features offered by one particular scope fit in with your style of shooting.
The final piece of advice is harder to achieve in practice: try before you buy. Most retailers will let you look through a scope and operate the controls 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 is willing to let you have a go with theirs.
Do remember that you can have the ‘best’ rifle on the market, but if you’re not pointing it at exactly the right spot then you won’t hit a thing!
