What happens to light between entering a riflescope and reaching your eye? Understanding that journey makes it much easier to compare specifications and choose optics for hunting, wildlife observation or target shooting.
This guide explains how objective lenses, prisms, eyepieces and other components work together. It also shows why a riflescope, binoculars, spotting scope, monocular and laser rangefinder serve different purposes. You can explore these instruments in our hunting and shooting optics collection.
The light path in brief: The objective lens gathers light and forms an image. Other optical elements correct its orientation or change its magnification. The eyepiece presents the resulting image to your eye. Image quality depends on how well the entire system works together.
How does an optical instrument work?
An optical instrument collects light reflected by the scene and directs it through precisely positioned components to produce an image you can observe. The internal design varies with the instrument. Binoculars commonly use prisms to turn the image upright and keep the body compact. A variable-power riflescope uses an internal erector assembly, while a laser rangefinder adds electronic distance measurement.
The objective lens: where the light path begins
The objective lens faces the subject. It collects incoming light and forms an image inside the instrument. In a specification such as 8×42, the first number is the magnification and the second is the nominal objective lens diameter in millimetres. The same principle applies to a 3–12×56 riflescope: it offers variable magnification from 3× to 12× and has a 56 mm objective.
A larger objective can collect more light, but usually adds size and weight. Objective diameter alone cannot tell you how bright, sharp or contrast-rich the image will appear.
Why use more than one lens?
A simple lens can introduce colour fringing, distortion and softness towards the edge of the image. Optical designers combine lens elements, glass types and coatings to control these effects. The quality of the design and manufacture matters more than the number of lenses listed in a specification.
Why do binoculars use prisms?
An objective lens initially forms an inverted image. In most binoculars, prisms turn it upright and fold the light path so the instrument remains practical to carry.
Porro prisms
Porro-prism binoculars have the familiar stepped shape, with the objectives offset from the eyepieces. This arrangement can provide a pronounced sense of depth. As with any binocular, the image quality still depends on the execution of the complete optical design.
Roof prisms
Roof-prism binoculars place the objective and eyepiece approximately in line, allowing a slimmer body. Prism manufacture and coatings are particularly important to the contrast and light transmission of this design.
What happens inside a riflescope?
A riflescope brings the target image and a reticle together for aiming. In a variable-power scope, an internal erector assembly presents the image the right way up and changes magnification as the power ring is turned. Depending on the design, the reticle is positioned in either the first or second focal plane.
First focal plane reticles
With a first focal plane (FFP) reticle, the reticle appears to grow and shrink with the target image as magnification changes. Where the reticle includes appropriately designed measuring or holdover marks, their angular values remain consistent throughout the magnification range.
Second focal plane reticles
A second focal plane (SFP) reticle appears to stay the same size when magnification changes. Any angular measurements or holdover marks generally correspond to their stated values only at the magnification specified by the manufacturer. Our guide to FFP and SFP riflescopes explains the choice in more detail.
Magnification and field of view
Magnification describes how large a distant subject appears compared with viewing it with the unaided eye. Higher magnification can reveal more detail, but it usually narrows the field of view and makes movement of the instrument more noticeable.
A wide field of view and low starting magnification can be useful for driven hunting and locating a moving subject. Higher magnification can help when examining distant details, provided the image remains steady and atmospheric conditions allow it. Choose a range that suits the task rather than selecting the highest figure available.
The eyepiece, dioptre adjustment and eye relief
The eyepiece is the lens group nearest your eye. It allows you to view the image produced inside the instrument. A comfortable, clear view also depends on positioning the eye correctly.
- Dioptre adjustment: adapts the eyepiece to your eyesight. On a riflescope, it is primarily used to make the reticle appear sharp.
- Eye relief: is the distance between your eye and the eyepiece at which you can see the full field of view. Sufficient eye relief is particularly important on a recoiling rifle.
- Eyecups: help position the eyes behind binoculars or spotting scopes. The appropriate setting may differ for people who wear glasses.
What is exit pupil?
The exit pupil is the diameter of the beam of light leaving the eyepiece. At a given magnification, its approximate size is easy to calculate:
Exit pupil = objective lens diameter ÷ magnification
| Optic | Calculation | Approximate exit pupil |
|---|---|---|
| 8×42 binoculars | 42 ÷ 8 | 5.25 mm |
| 10×42 binoculars | 42 ÷ 10 | 4.2 mm |
| 10×50 binoculars | 50 ÷ 10 | 5 mm |
| 3–12×56 riflescope set to 12× | 56 ÷ 12 | About 4.7 mm |
A larger exit pupil can make eye placement more forgiving and may be useful in low light. It does not guarantee a brighter-looking image: transmission, contrast, magnification, viewing conditions and the size of your own pupil also matter. On a variable-power riflescope, exit pupil changes as you adjust magnification.
Light transmission, coatings and contrast
Light transmission is the proportion of incoming light that passes through the optical system. Some light is reflected at glass surfaces. Anti-reflective coatings help reduce these losses, while internal control of stray light helps preserve contrast.
Brightness is only part of what makes an image useful. Good contrast can help you distinguish an animal from a complex background. Resolution, edge sharpness, colour rendition and contrast describe different aspects of performance; no single percentage captures the complete viewing experience.
Elevation, windage and parallax adjustment
A riflescope's elevation and windage turrets adjust the relationship between the reticle and the rifle's point of impact. Depending on the model, click values may be expressed in MOA or milliradians. The most suitable turret arrangement depends on how the scope will be used.
If fitted, a parallax adjustment brings the target image into focus for a particular distance and helps reduce aiming error caused by changes in eye position. It performs a different job from the eyepiece dioptre adjustment. Read our practical guide to adjusting riflescope parallax for the steps involved.
Why are scope mounts part of the optical system?
A riflescope must be mounted in the correct position to provide a comfortable sight picture and reliable use. The rifle model, base or rail, main tube diameter, ring height, objective clearance and eye relief all need to work together.
Our guide to riflescope mounts and rings explains how to check these details before choosing a mounting system.
Riflescopes, binoculars, monoculars, spotting scopes and rangefinders
| Instrument | Main purpose | Key considerations |
|---|---|---|
| Riflescope | Combines the target image and reticle for aiming. | Magnification, field of view, reticle, eye relief and mounts. |
| Binoculars | Searches and observes a scene using both eyes. | Field of view, image steadiness, weight and low-light performance. |
| Monocular | Provides compact observation using one eye. | Portability, comfortable viewing and required magnification. |
| Spotting scope | Examines distant detail at higher magnification. | Tripod stability, objective size, image quality and field of view. |
| Laser rangefinder | Measures distance to an observed object. | Measuring capability, ease of use and viewing optics. |
Binoculars for searching and observation
Viewing with both eyes makes binoculars particularly useful for scanning an area and observing wildlife. Beyond magnification and objective diameter, consider the field of view, weight, handling and how comfortably you can use them for an extended period.
Spotting scopes for examining distant detail
A spotting scope commonly offers higher magnification than handheld binoculars. It can help you examine a distant subject more closely, but a stable tripod and careful focusing become increasingly important as magnification rises.
Monoculars for compact observation
A monocular is viewed through one eye and can be easier to carry than binoculars. It may suit occasional observation where compact size matters. Comfort during longer viewing sessions depends on the individual design and the user.
Rangefinders for measuring distance
A laser rangefinder emits a laser pulse and calculates distance from the returned signal. Some models incorporate binoculars. Actual measuring performance varies with the size and reflectivity of the subject, weather conditions and the device itself.
Frequently asked questions about optical systems
Does a larger objective always give a better image?
No. A larger objective can collect more light but can also increase size and weight. Magnification, exit pupil, light transmission, contrast and optical quality all affect the image you see.
Do all optical instruments contain prisms?
No. Prisms are characteristic of many binocular designs. A variable-power riflescope uses a different internal assembly to erect the image and adjust magnification.
Are exit pupil and light transmission the same thing?
No. Exit pupil describes the diameter of the light beam leaving the eyepiece. Light transmission describes the proportion of incoming light that passes through the instrument.
What is the difference between dioptre and parallax adjustment?
Dioptre adjustment adapts the eyepiece to your eye. Parallax adjustment relates to the distance of the target image and its position relative to the reticle. One setting cannot replace the other.
Is more magnification always better?
No. Higher magnification can help reveal detail but usually reduces field of view and makes movement more apparent. Match the magnification to the viewing distance and intended use.
Assess the complete optical system
The numbers printed on an instrument are a useful starting point, but they are not a complete measure of quality. Check whether its magnification and field of view suit your purpose, whether the viewing position feels natural and whether the image remains clear and contrast-rich where you need it. For a riflescope, also consider the reticle, adjustments and correct mounting.
Find optics suited to your purpose
Start with our hunting and shooting optics, then explore binoculars or rangefinders according to what you need to observe or measure.
If you are choosing a riflescope, continue with our guides to FFP and SFP reticles, parallax adjustment and scope mounts and rings.
