How Does an Optical System Work? Lenses, Prisms, Eyepiece and Exit Pupil
When we look through a riflescope, binoculars, a spotting scope or a rangefinder, we are using an optical system designed to collect, transmit and control light before presenting a usable image to the human eye.
Understanding how this system works makes specifications such as objective lens diameter, magnification, field of view, exit pupil, light transmission, parallax, focal plane and eye relief much easier to interpret.
The important point is that optical quality does not depend on one isolated specification. It comes from the interaction between glass, lenses, prisms or an erector system, coatings, mechanical construction and ergonomics.
You can explore these different systems in our main optics for hunting and sports shooting category.
How does an optical instrument work?
An optical instrument collects light reflected from the object being observed and guides it through a series of optical elements until an image suitable for the human eye is produced.
Riflescopes, binoculars and spotting scopes use different internal architectures, but they share several fundamental components:
- Objective lens or objective lens group.
- Internal lens groups.
- Prisms where the design requires them.
- Image-erecting systems.
- Internal apertures and stops that control the light path.
- Eyepiece.
- Optical coatings.
- A mechanical structure that keeps all optical elements correctly aligned.
A riflescope also includes specific components such as the reticle, erector system, magnification ring, elevation and windage turrets and, on some models, an adjustable parallax system.
The objective lens: where light enters the system
The objective is the optical group at the end of the instrument facing the object being observed.
Its job is to collect light from the scene and form an image within the optical system.
In specifications such as 1-6x24, 2-12x50 or 3-12x56, the final number indicates the nominal objective lens diameter in millimetres.
A 2-12x50 riflescope therefore has a 50 mm objective, whereas a 1-6x24 uses a 24 mm objective.
A larger objective provides the potential to accept a larger bundle of light, but this does not mean that a 56 mm scope will always appear brighter or perform better than a 42 or 50 mm model.
The final result also depends on magnification, exit pupil, light transmission, glass quality, lens coatings, optical design, ambient conditions and the diameter of the observer's own pupil.
Lenses do more than magnify: they must control aberrations
A modern optical instrument does not rely on a single lens. It uses several elements and lens groups designed to work together.
Simple lenses can introduce different forms of optical aberration. These include chromatic aberration, spherical aberration, distortion, field curvature and loss of definition towards the edge of the image.
Manufacturers therefore combine different glass types, lens shapes and optical designs to control these effects.
Optical quality cannot be judged simply by counting the number of lenses. What matters is how those lenses are designed, manufactured, centred, aligned and coated.
What happens to the image after it passes through the objective?
The objective initially forms an image inside the optical system. That image then needs to be processed before it reaches the eye correctly oriented and at the required magnification.
The way this is achieved depends on the type of instrument.
| Instrument | Main system | Function |
|---|---|---|
| Binoculars | Prism system | Erect the image and fold the optical path into a compact body |
| Spotting scope | Prisms or equivalent erecting system | Present a correctly oriented terrestrial image |
| Riflescope | Lens-based erector system | Erect the image and participate in magnification |
Prisms in binoculars
Binoculars for hunting and observation use prisms both to erect the image and to fold the optical path within a manageable body.
The two classic architectures are Porro prism and roof prism binoculars.
Porro prisms
Porro binoculars generally have an offset shape, with the objective lenses positioned farther apart than the eyepieces.
This design can provide excellent three-dimensional perception and very high optical performance with a relatively efficient optical architecture.
Roof prisms
In roof-prism binoculars, the objective lenses and eyepieces can be positioned almost along the same axis, allowing a slimmer and more compact body.
The design places greater demands on manufacturing precision and coatings. Phase correction and high-quality reflective coatings can be particularly important for maintaining resolution and contrast.
Neither architecture is automatically superior. Prism quality, optical design, coatings and manufacturing execution matter far more than the name of the prism system alone.
The erector system inside a riflescope
Inside a riflescope, the image created by the objective must be correctly oriented before it reaches the shooter.
This is achieved by an internal group of optical elements generally known as the erector system.
In a variable-power riflescope, the erector assembly also forms part of the system responsible for changing magnification.
When the magnification ring is rotated, the relationship between internal lens groups changes so that the target appears at a different angular size.
What does magnification actually mean?
Magnification describes how much larger an object appears compared with viewing it with the unaided eye.
A 2-12x50 riflescope can operate between 2x and 12x magnification.
Increasing magnification changes much more than the apparent size of the target.
As magnification increases:
- a smaller portion of the scene is normally visible;
- the exit pupil becomes smaller for the same objective diameter;
- movement appears more pronounced;
- eye position can become more critical;
- finer detail may become visible if the optics and conditions allow it.
More magnification therefore does not automatically mean a better scope or greater accuracy.
Field of view: how much of the scene can we see?
Field of view describes the amount of the surrounding scene that is visible at one time.
For riflescopes it is commonly specified in metres at 100 metres or feet at 100 yards. For binoculars and spotting scopes, it is commonly stated in metres at 1,000 metres or feet at 1,000 yards.
A wide field of view is particularly valuable for finding and tracking animals, whereas higher magnification generally reduces the amount of terrain visible at once.
This is why a 1-6x24 driven-hunting scope can have a very wide field of view at low magnification despite its relatively small 24 mm objective.
The eyepiece: the interface between the optics and the eye
The eyepiece is the optical assembly through which the observer looks.
Its task is to present the internally formed image to the eye under the correct viewing conditions.
It also influences important aspects of visual ergonomics, including eye relief, ease of eye positioning, perceived field of view and tolerance to small changes in eye position.
Two instruments with similar magnification and objective diameter can therefore feel very different in actual use.
The dioptre adjustment
A riflescope eyepiece normally includes a dioptre adjustment used to make the reticle appear sharply defined to the individual shooter.
This should not be confused with parallax adjustment.
The dioptre setting adjusts the reticle to the shooter's eye. Parallax adjustment changes the optical relationship between the target image and the reticle plane.
Confusing the two is one of the most common mistakes when setting up a riflescope.
Eye relief
Eye relief is the distance between the eye and the eyepiece from which the full image can be seen correctly.
In a riflescope it has both optical and ergonomic importance.
The scope should be positioned so that the shooter sees the complete field of view from a natural shooting position while maintaining an appropriate distance between the eye and eyepiece.
Eye relief and longitudinal scope positioning are therefore closely connected.
What do internal apertures and stops do?
Optical systems include various internal apertures and stops that control the path of light through the instrument.
These should not necessarily be imagined as the adjustable iris found in a photographic lens.
Fixed apertures and field stops can define the useful optical field, reduce stray light and prevent unwanted rays from degrading contrast and image definition.
The ability to control stray light effectively is an important part of high-quality optical design.
Exit pupil
If binoculars are held slightly away from the eyes and viewed from behind, small bright circles can be seen in the eyepieces. These are the exit pupils.
The exit pupil describes the diameter of the bundle of light leaving the eyepiece and reaching the observer's eye.
It can be estimated by dividing the effective objective diameter by the magnification.
| Configuration | Calculation | Exit pupil |
|---|---|---|
| 8x42 | 42 / 8 | 5.25 mm |
| 10x42 | 42 / 10 | 4.2 mm |
| 10x50 | 50 / 10 | 5 mm |
| 2-12x50 at 5x | 50 / 5 | 10 mm theoretical |
| 2-12x50 at 10x | 50 / 10 | 5 mm |
| 3-12x56 at 12x | 56 / 12 | 4.67 mm |
This mathematical relationship is extremely useful for understanding optical systems, although real instruments may contain mechanical or optical restrictions that reduce the effectively usable aperture.
A larger exit pupil does not automatically mean higher optical quality
Exit pupil is fundamentally a geometrical relationship between effective objective diameter and magnification.
It is not a direct measure of glass quality.
Two 8x42 binoculars both have a nominal exit pupil of approximately 5.25 mm, yet they may differ significantly in resolution, contrast, light transmission, chromatic correction and edge performance.
Exit pupil therefore tells us about the size of the light bundle presented to the eye, but it does not tell us how good the complete optical system is.
What can the shape of the exit pupil tell us?
Looking at the exit pupil can provide useful information about the geometrical illumination of an optical system.
Ideally, it should appear clearly defined without conspicuous clipping or major asymmetry.
However, its shape or brightness alone should not be used to classify an instrument as high or low quality.
A proper optical assessment must also consider resolution, contrast, aberrations, transmission, reflection control, colour fidelity, edge sharpness and ergonomics.
Light transmission: not all incoming light reaches the eye
No real optical system transmits 100% of the light entering it.
Every optical surface can reflect or absorb a small amount of light.
Overall light transmission therefore depends on the quality of the glass, the number and design of optical elements and, particularly, the coatings applied to the surfaces.
Two instruments with the same objective diameter and magnification can perform very differently at dawn or dusk if their transmission and contrast characteristics are different.
Anti-reflective lens coatings
Whenever light crosses a boundary between air and glass, some reflection can occur.
Anti-reflective coatings are designed to reduce these losses and improve both transmission and contrast.
In complex optical systems, light may pass through many glass-to-air surfaces, making the quality of these coatings particularly important.
Premium optics manufacturers develop sophisticated multi-layer coatings to control reflections, optimise transmission, maintain natural colour rendering and protect exposed lens surfaces.
Terms such as coated, multi-coated and fully multi-coated should therefore always be considered within the context of the complete optical design rather than treated as automatic guarantees of quality.
Contrast, resolution and sharpness are not the same thing
An image can appear bright while still lacking fine detail.
Resolution describes the ability of an optical system to distinguish small details that are close together.
Contrast describes how effectively differences between bright and dark areas, or between similarly coloured features, are reproduced.
Perceived sharpness results from several interacting factors, including resolution, contrast, focus and the control of aberrations.
In real hunting conditions, strong contrast can be just as important as high transmission when trying to distinguish an animal from vegetation or other low-contrast backgrounds.
The reticle: a fundamental difference in a riflescope
Binoculars and spotting scopes are primarily observation instruments. A riflescope also contains an internal aiming reference: the reticle.
The position of the reticle within the optical system determines whether the scope uses a first focal plane or second focal plane design.
First focal plane
In an FFP riflescope, the apparent size of the reticle changes together with the target image as magnification is adjusted.
Angular subtensions therefore remain valid throughout the magnification range.
Second focal plane
In an SFP riflescope, the reticle remains approximately the same apparent size while the target image changes with magnification.
If the reticle contains ranging or holdover references, their stated subtensions are valid at the calibration magnification specified by the manufacturer.
You can explore the subject in greater depth in our guide to first focal plane versus second focal plane scopes.
Elevation and windage turrets
The adjustment turrets change the internal position of the aiming system so that the point of impact can be moved relative to the point of aim.
The elevation turret controls vertical correction.
The windage turret controls horizontal correction.
Click values are commonly expressed in angular units such as MOA or MIL/MRAD.
These units do not describe the quality of the scope. They describe the angular value of each adjustment.
Parallax adjustment
Some riflescopes allow the optical system to be adjusted to minimise parallax at different distances.
This can be achieved using a side parallax control, often called Side Focus, or an Adjustable Objective.
Its technical purpose is not simply to make the target look sharper. The aim is to place the target image and reticle in the correct optical relationship so that the reticle does not appear to move across the target when the shooter's eye changes position slightly.
For a full explanation, read our guide to adjusting parallax on a riflescope.
The main tube of a riflescope
The main tube provides the mechanical structure around many of the internal components and also gives the scope rings their mounting surface.
Common tube diameters include 1 inch, 30 mm and 34 mm, with larger diameters used on some specialised optics.
A larger tube does not automatically mean that more light enters the scope.
Tube diameter has greater implications for internal adjustment range, mechanical architecture, robustness and compatibility with rings and mounts.
Mechanical construction is part of optical performance
Excellent glass cannot deliver its full potential if the optical elements are not kept precisely aligned.
The chassis, lens cells, erector system, turrets and moving mechanisms all have to operate within tight tolerances.
In a riflescope, the complete structure must also withstand mechanical stress without losing zero or changing the position of internal components.
Optical quality and mechanical quality therefore cannot be completely separated.
Mounts are part of the complete system
Even an excellent riflescope cannot perform properly if it is mounted badly.
Scope rings and mounts must hold the optic securely, correctly aligned and at an appropriate height and distance from the shooter.
Mounting height influences head position, cheek weld and line of sight.
The internal diameter of the rings must correspond exactly to the scope tube diameter.
This relationship is covered in detail in our definitive guide to hunting scope mounts and rings.
Riflescope, binoculars and spotting scope compared
| Characteristic | Riflescope | Binoculars | Spotting scope |
|---|---|---|---|
| Main purpose | Aiming and observation | Searching and observation | Detailed long-range observation |
| Optical channels | One | Two | One |
| Image erection | Erector lens system | Prisms | Prisms or equivalent system |
| Reticle | Yes | Normally no | Normally no |
| Adjustment turrets | Yes | No | No |
| Typical magnification | Variable according to use | Often 8x or 10x for hunting | Usually higher and often zoom |
| Field of view | Critical for target acquisition | Critical for searching | Narrows progressively at high magnification |
Why are binoculars so effective for finding animals?
Binoculars use both eyes, making extended observation more natural and comfortable.
Configurations such as 8x42 and 10x42 offer a particularly useful balance between magnification, field of view, stability, exit pupil, size and weight.
They are therefore ideal for initially locating animals before moving to higher-magnification observation when required.
You can compare different designs in our range of binoculars for hunting and wildlife observation.
What is a spotting scope for?
A spotting scope allows substantially higher magnification than is normally practical with handheld hunting binoculars.
It becomes particularly useful once an animal has already been located and finer details need to be assessed at longer distance.
The trade-off is normally a narrower field of view, greater sensitivity to vibration and a stronger need for stable support.
Binoculars and spotting scopes should therefore be considered complementary observation tools rather than direct alternatives.
Where does the rangefinder fit into the optical system?
A hunting rangefinder adds a measurement that the human eye is relatively poor at estimating accurately: the actual distance to the target.
Some modern systems combine binocular observation and laser ranging in the same instrument.
This allows searching, observing and measuring to be integrated without repeatedly changing between different optical devices.
What actually determines the quality of an optical instrument?
No single specification can define overall optical quality.
The final result depends on the interaction between many factors:
- Quality and homogeneity of the optical glass.
- Design of the lens groups.
- Control of optical aberrations.
- Anti-reflective coatings.
- Light transmission.
- Contrast.
- Resolution.
- Control of stray light and internal reflections.
- Edge sharpness.
- Colour fidelity.
- Mechanical precision.
- Eyepiece ergonomics.
- Field of view.
- The relationship between exit pupil and the observer's eye.
This is why an optic should not be chosen simply because it has a large objective lens, very high magnification or one impressive transmission figure.
Common mistakes when interpreting an optical system
Assuming a larger objective automatically means a brighter image
Objective diameter provides potential, but actual performance also depends on magnification, exit pupil, transmission, coatings, glass quality and observing conditions.
Assuming more lenses mean better optics
The number of optical elements is not a quality rating. What matters is how effectively they are designed and made to work together.
Assuming all optical systems use prisms
Prisms are fundamental in binoculars and many terrestrial observation systems. Riflescopes use a different internal architecture based primarily on lens groups and an erector system.
Confusing exit pupil with light transmission
Exit pupil is primarily a geometrical relationship between effective aperture and magnification. Transmission describes how much of the incoming light actually passes through the system.
Confusing dioptre adjustment with parallax
The dioptre adjustment makes the reticle sharp for the shooter's eye. Parallax adjustment controls the optical relationship between the target image and the reticle plane.
Assuming more magnification always produces a better image
Magnification can only enlarge the information that the optical system is capable of resolving. Increasing magnification cannot turn mediocre optics into high-quality optics.
Checklist for assessing an optical system
- Define exactly what the instrument will be used for.
- Choose only the magnification range that is genuinely useful.
- Consider objective diameter together with magnification rather than in isolation.
- Compare field of view.
- Consider exit pupil at the magnifications you will actually use.
- Assess resolution and contrast under realistic conditions.
- Check image quality towards the edges.
- Assess reflections and performance against difficult light.
- Check eyepiece ergonomics, eye positioning and ease of focus.
- For riflescopes, also evaluate the reticle, focal plane, turrets and parallax adjustment.
- Consider mechanical quality and compatibility with mounts.
- Always assess the instrument as a complete system rather than as a list of isolated specifications.
Optical performance comes from a system, not a list of numbers
A high-quality optical instrument results from the balance between the objective, lens groups, prisms or erector system, eyepiece, coatings, mechanical structure and ergonomics.
Objective diameter influences the available aperture. Magnification changes how the image is presented and affects exit pupil. Field of view determines how much context remains visible. Transmission and contrast influence how much useful visual information survives in difficult conditions.
In a riflescope we must also consider the reticle, focal plane, turrets, parallax adjustment, eye relief and mounting system.
The correct question is therefore not simply which optic has the most magnification or the largest objective. It is which combination of characteristics works best for the hunting method, shooting distance, lighting conditions and individual user.
Build your understanding of optics as a complete system
This article provides the technical foundation for many of the other subjects in our optics cluster. Continue with our guides to first and second focal plane riflescopes, riflescope parallax adjustment and scope mounts and rings.
To compare the different optical systems directly, start from our complete hunting and sports shooting optics range, then explore binoculars and rangefinders according to the intended use.
