Adjustable Objective (AO) Scopes: Parallax, Side Focus and Field Target
The abbreviation AO frequently appears in the specifications of scopes designed for air rifles, target shooting and disciplines such as Field Target. AO stands for Adjustable Objective.
Its main purpose is not simply to bring the target into sharper focus. An adjustable objective allows the optical system to be set for different target distances in order to minimise parallax error and bring the target image into the same optical plane as the reticle as accurately as possible.
This becomes particularly important when shooting at varying distances, using high magnification or aiming at small targets. These conditions are common in air rifle shooting, Field Target, Hunter Field Target, precision shooting and metallic silhouette disciplines.
An AO scope and a scope with Side Focus essentially address the same optical problem. The main difference is the location of the adjustment and how easily the shooter can operate it from the shooting position.
What does AO mean on a scope?
AO means Adjustable Objective. On an AO scope, the front section of the objective can be rotated to adjust the optical system for different target distances.
This adjustment changes where the target image is formed within the optical system. The objective is to bring that image as close as possible to the reticle plane and thereby minimise parallax error.
For this reason, describing AO as an objective-mounted parallax adjustment is technically more accurate than simply calling it front focus.
What is parallax in a scope?
Parallax occurs when the image of the target and the reticle are not formed in exactly the same optical plane.
If the shooter's eye moves slightly away from the optical axis, the reticle may then appear to move across the target even though the rifle itself has not moved.
This apparent movement can produce an aiming error. Its effects become particularly relevant with small targets, short distances and high magnification.
For a deeper explanation, read our complete guide to scope parallax and how to adjust it correctly.
The eyepiece and parallax adjustment do different jobs
This distinction is fundamental.
The dioptre adjustment on the eyepiece is used to make the reticle appear sharply defined to the individual shooter.
AO and Side Focus, on the other hand, act on the optical system associated with the target image and parallax at a particular distance.
If the reticle appears blurred, the eyepiece dioptre should normally be checked first. If the reticle appears to move across the target when the eye is shifted slightly behind the scope, the parallax adjustment should be checked.
How does an Adjustable Objective work?
On an AO scope, the shooter rotates the adjustment ring around the objective until the optical system is correctly set for the target distance.
Many manufacturers mark approximate distances around the objective ring. These markings can be useful references, but they should not automatically be treated as exact range measurements.
Optical tolerances, magnification, eyesight, lighting conditions, temperature and the design of the scope itself can all mean that the optimum position does not correspond exactly to the distance printed on the objective.
How to check parallax correctly
- Place the rifle or air rifle in a safe and stable position.
- Adjust the eyepiece first so that the reticle appears sharply defined.
- Aim at a target at a known distance.
- Rotate the Adjustable Objective gradually until the target image is clearly defined.
- Without moving the rifle, shift your eye slightly from side to side behind the scope.
- Observe whether the reticle appears to move across the target.
- Fine-tune the AO until this apparent movement is minimised.
A sharp target image will often occur close to the correct setting, but the definitive test for parallax is whether relative movement remains between the target and reticle when the eye position changes slightly.
AO vs Side Focus: what is the difference?
Adjustable Objective and Side Focus systems perform essentially the same optical function: they allow the shooter to adjust the scope for parallax at different distances.
The practical difference is primarily the location of the control.
With AO, the adjustment is made at the front of the scope around the objective. With Side Focus, a control is normally positioned on the left-hand side of the central turret housing.
Advantages of Adjustable Objective
- Relatively simple and proven design.
- Very common on air rifle scopes.
- Can provide parallax adjustment at short distances.
- Often offers excellent value for money.
- Widely available on scopes intended for recreational and target shooting.
Disadvantages of Adjustable Objective
- The shooter has to reach forwards to the objective to make an adjustment.
- It can be less ergonomic from certain shooting positions.
- Very fine adjustments can be less convenient.
- Distance markings may be more difficult to read while maintaining the shooting position.
Advantages of Side Focus
- The adjustment is much closer to the shooter's natural hand position.
- Fine adjustments can be made more easily without reaching forwards.
- It is particularly convenient when distances change frequently.
- Large side wheels can provide much finer spacing between distance references.
- It is particularly common on specialised Field Target scopes.
Is AO or Side Focus better?
Neither system is universally better.
For recreational shooting, training and many air rifles, a good AO scope can provide exactly the adjustment required at an attractive price.
When the shooter needs to change parallax frequently, distinguish small differences in distance or make very fine adjustments while maintaining a stable shooting position, Side Focus is generally more ergonomic.
This helps explain why AO is so common on entry-level and mid-range air rifle scopes, while advanced Field Target optics frequently use side parallax adjustment with a large side wheel.
Why is Adjustable Objective particularly useful on air rifles?
Air rifles are frequently used at considerably shorter distances than centrefire hunting rifles.
Some scopes designed primarily around centrefire applications have their parallax set at a fixed distance that may not be ideal when shooting at 10, 15, 20, 25 or 30 metres.
An Adjustable Objective or Side Focus system allows the scope to be set for the actual distances commonly encountered in air rifle shooting.
For this reason, our range of air rifle scopes includes models with adjustable parallax for recreational shooting, precision shooting and Field Target applications.
Adjustable Objective and Field Target
Field Target is one of the disciplines in which parallax adjustment has become particularly sophisticated.
Competitors shoot at knock-down metal targets placed at different distances and incorporating hit zones of different sizes. Accurate range estimation is extremely important because the trajectory of an airgun pellet changes significantly across the distances encountered on a Field Target course.
In this environment, the scope does more than provide an aiming reference. A sufficiently precise parallax system can also become an optical tool for estimating target distance.
Using parallax for rangefinding in Field Target
With a high-magnification scope, depth of field can become sufficiently shallow for relatively small changes in target distance to produce noticeable differences in focus.
The shooter adjusts the parallax until the target reaches the required focus and compares the position of the adjustment with a range scale previously calibrated at known distances.
This is why large side wheels are so characteristic of specialised Field Target scopes. Increasing the diameter of the wheel provides more physical space between range markings and allows finer adjustment.
This does not turn the scope into an electronic laser rangefinder. It is an optical method of range estimation that must be calibrated by the shooter using the individual scope under known conditions.
AO, Side Focus and high magnification in Field Target
High magnification is valuable in Field Target because it can make small hit zones easier to resolve and make changes in focus more apparent when estimating distance.
Higher magnification also has consequences. Movement becomes more noticeable, the field of view becomes narrower, eye position becomes more critical and depth of field generally decreases.
A Field Target scope should therefore be understood as a complete system involving magnification, reticle, parallax adjustment, turrets, optical quality and ergonomics.
Carbin offers airgun optics from manufacturers closely associated with this sector, including Gamo, Norica and Nikko Stirling.
Hunter Field Target is different from Field Target
Field Target and Hunter Field Target (HFT) share some obvious characteristics, including air rifles and knock-down targets, but the way the scope is used during competition is very different.
Under international WHFTA rules, the rifle and scope must be configured before starting the course and adjustments to the rifle or scope are not permitted once the course has begun.
This means that relevant settings such as parallax and magnification must be established beforehand.
The optical challenge is therefore different. Rather than continuously using the parallax system to estimate the range of each target, the HFT shooter needs a configuration that remains usable across a useful range of distances without readjusting the scope.
Depth of field in Hunter Field Target
Depth of field becomes particularly important in HFT because targets at different distances must remain sufficiently usable without changing the parallax setting during the course.
A very critical focus transition that may be desirable in Field Target for distinguishing small differences in distance is therefore not automatically ideal for Hunter Field Target.
In HFT, a practical compromise between magnification, parallax setting and depth of field can be more valuable.
Does an AO scope make sense for Hunter Field Target?
Yes. The fact that the setting cannot be continually changed during the course does not make an Adjustable Objective useless.
Before competition, the shooter can test different settings and establish the parallax distance that provides the most useful compromise for the chosen magnification, expected target distances and shooting technique.
Once the course has begun, the settings must remain in accordance with the applicable competition rules.
Adjustable Objective and metallic silhouette shooting
A correctly adjusted parallax system can also be valuable when shooting at metallic silhouettes.
Depending on the discipline, silhouettes have defined dimensions and are positioned at established distances. Small targets viewed at higher magnification can make precise optical setup increasingly important.
If residual parallax remains, even a small change in eye position can make the reticle appear to move across the silhouette.
An AO scope can therefore be particularly useful when the same air rifle is used to practise on small targets at several distances.
The ideal configuration will always depend on the rules of the particular discipline, target distances, shooting positions and whether optical adjustments are permitted during competition.
Parallax and different shooting positions
Parallax becomes particularly relevant when the shooter's head position behind the scope cannot be repeated perfectly from shot to shot.
From a benchrest position, maintaining a consistent eye position is relatively straightforward. Field Target, Hunter Field Target and silhouette disciplines can involve kneeling, standing or other positions determined by the course.
If residual parallax is present, a small change in eye position can produce apparent movement of the reticle even though the rifle remains stationary.
Correct parallax adjustment and consistent head position should therefore be considered together.
Gamo AO scopes
Gamo offers a broad range of Adjustable Objective scopes developed around air rifle shooting.
Typical configurations include 4x32 AO, 3-9x40 AO, 4-12x44 AO and 6-24x50 AO.
These configurations cover everything from recreational airgun shooting to higher-magnification applications where small targets and Field Target-style shooting become more relevant.
Norica Adjustable Objective scopes
Norica also uses Adjustable Objective systems on several scopes intended for air rifles.
Configurations such as 3-9x40 AO and 4x32 AO demonstrate why adjustable parallax is not restricted to highly specialised competition scopes.
At the relatively short and medium distances typical of airgun shooting, the ability to adapt parallax can be useful even with comparatively simple magnification ranges.
Nikko Stirling air rifle scopes
Nikko Stirling scopes have a particularly strong connection with airgun and target shooting.
Ranges such as Panamax AO use an Adjustable Objective, while other more specialised models employ different parallax adjustment systems according to their intended application.
This provides options for recreational shooting, training, Hunter Field Target, Field Target and other precision disciplines.
What magnification is suitable for an AO scope?
The presence of AO does not determine how much magnification a shooter needs.
A 3-9x40 AO can be an extremely versatile configuration for recreational shooting and precision work at moderate distances.
A 4-12x44 AO provides greater ability to resolve small targets.
Configurations such as 6-24x50 AO and higher magnification scopes move further into the world of precision shooting and Field Target, where magnification can be used both for aiming and for analysing focus during optical range estimation.
The correct choice should depend on the discipline, actual shooting distances and target size rather than the assumption that more magnification automatically means more accuracy.
Are the distance markings on an Adjustable Objective accurate?
They should be treated as references rather than as the equivalent of a rangefinder.
The numbers printed around the objective may correspond closely to the actual target distance, but they are not necessarily exact for every individual scope and every set of conditions.
For recreational shooting, the factory markings may provide more than enough guidance. For Field Target and other demanding applications, shooters commonly verify and calibrate their own references at accurately known distances.
Can parallax adjustment be used to estimate distance?
Yes, with a suitable scope it can be used for optical range estimation.
This is particularly effective with high-magnification scopes and sensitive Side Focus systems.
The principle is to associate a particular focus or parallax position with a distance that has previously been measured and calibrated.
The more clearly the shooter can distinguish the transition between focused and unfocused images, and the finer the adjustment scale, the easier it becomes to differentiate between nearby distances.
This is one of the main reasons why large side wheels have become so closely associated with Field Target optics.
AO, reticles and range estimation
The reticle is another part of the same optical and ballistic system.
Once the distance has been estimated, the shooter needs to know the correction required for the pellet trajectory.
Depending on the discipline, that correction can be applied using holdover references in the reticle or, where permitted, through the elevation turret.
Mil-Dot and other reticles with defined angular references provide additional aiming points that can be associated with different ranges.
The parallax system helps establish the correct optical relationship between target and reticle. The reticle and turrets can then be used to apply the required trajectory correction.
Adjustable Objective or fixed parallax for an air rifle?
A fixed-parallax scope can work perfectly well when it is normally used around the distance for which it was designed and the required level of precision does not demand additional adjustment.
AO provides greater flexibility when distances vary considerably, when shooting at short range or when small targets make parallax control more important.
For straightforward plinking at a relatively constant distance, fixed parallax may be entirely adequate. For training at several distances, small targets, Field Target or more technical airgun shooting, adjustable parallax becomes considerably more valuable.
Common mistakes with AO scopes
Using AO only to make the image look sharper
Image sharpness is a useful reference, but the technical objective is to minimise parallax error.
Trying to focus the reticle with the AO
The reticle should be focused using the dioptre adjustment on the eyepiece.
Trusting the printed distance markings completely
The markings are references and should be verified when high precision is required.
Thinking AO and Side Focus perform different optical functions
Both systems provide adjustable parallax. The principal differences are mechanical layout and ergonomics.
Believing that more magnification eliminates parallax
Increasing magnification does not correct parallax. In fact, its effects can become more noticeable at higher magnification.
Adjusting the scope during an HFT course
Under rule sets such as WHFTA, the relevant rifle and scope settings must be established before starting the course and cannot be adjusted once the course has begun.
Checklist for choosing an Adjustable Objective scope
- Define the actual distances at which you intend to shoot.
- Check the minimum distance at which the parallax system can be adjusted.
- Decide whether the simplicity of AO or the ergonomics of Side Focus better suit your needs.
- Choose magnification according to target size and shooting discipline.
- Pay particular attention to short-range parallax adjustment for air rifles.
- For Field Target, consider focus sensitivity and the ability to calibrate range accurately.
- For HFT, look for a useful fixed setup and practical depth of field.
- Choose a reticle appropriate for the corrections required by your discipline.
- For spring-piston air rifles, check that the scope is suitable for the recoil characteristics of the rifle.
- Consider the scope as part of a complete system involving the air rifle, pellets, mounts, optics and shooting discipline.
Which AO scope should you choose?
For beginners and recreational shooting, configurations such as 4x32 AO or 3-9x40 AO can provide a straightforward and versatile solution.
For shooters working with smaller targets at varying distances, a 4-12x44 AO offers additional magnification and target definition.
For Field Target and advanced precision shooting, higher magnification and more finely adjustable parallax systems become increasingly useful.
You can compare our selection of air rifle scopes with adjustable parallax, including optics from Gamo, Norica, Nikko Stirling and other manufacturers specialising in airgun and target optics.
AO as part of the complete optical system
An Adjustable Objective should never be considered in isolation.
The performance of a scope depends on the interaction between magnification, parallax adjustment, reticle, optical quality, eye position, turrets, mounts and the air rifle itself.
In Field Target, Hunter Field Target and shooting at small metallic targets, understanding how these elements interact is far more important than simply choosing the scope with the highest magnification.
To understand the optical principles behind the system in greater depth, continue with our guide to scope parallax and how to adjust it, or explore our selection of air rifle scopes.
