Choosing the right optics can make or break a day on the hill. Whether you're glassing a distant hillside at first light, checking boundaries at dusk, or keeping watch over open ground for hours at a time, the wrong binoculars or riflescope will cost you sightings, confidence and time. This guide breaks down what actually matters when choosing optics, so you can make the right call before you head out.
Magnification and Objective Lens Explained
Every set of optics is defined by two numbers, such as 8x42 or 10x42. The first is magnification. The second is the objective lens diameter in millimetres. Lower magnification gives you a wider field of view and better depth of field, useful when tracking movement at close range or scanning cover during vermin control. Higher magnification brings detail at distance but narrows your view. Riflescopes with variable magnification let you adjust for terrain, light and shot length as conditions change through the day.
Why Objective Lens Size Matters
The objective lens controls how much light reaches your eye. A larger lens means better light transmission and stronger contrast in low light, particularly valuable during the first and last hours of the day when deer are most active and when gamekeepers are often out checking ground before dawn or after dusk.
Matching Optics to the Terrain
- For dawn or dusk work, look for an objective lens of at least 35mm to maintain strong light transmission as the light fades.
- For hill stalking and longer-range spotting, a 7 to 10x magnification paired with a 35mm to 45mm objective lens gives better contrast across open ground.
- For woodland roe stalking or close estate cover, lower magnification with strong light transmission suits shaded conditions better than high power.
- For daily estate rounds, boundary checks and vermin watch, an 8x42 or 10x42 pair offers the portability and reliability needed for regular use in all conditions.
Getting this balance right means fewer missed sightings, quicker decisions in the field and a lighter kit bag, whatever the terrain or task.
Understanding the Terminology
Optics specifications can look like a foreign language if you're not familiar with the terms. Here's what the key figures actually mean and why they matter out on the ground.
Coating: Coating refers to the microscopic layer of refractive material, such as magnesium fluoride, applied to the lenses. It improves light transmission and cuts down on reflections, which makes a real difference when you're glassing into low sun or trying to stay unseen.
Exit Pupil: The exit pupil is the small circle of light you see when holding your binoculars or scope at arm's length. It's worked out by dividing the objective lens diameter by the magnification, so a 10x42 gives an exit pupil of 4.2mm. This figure matters because the average adult eye can dilate to around 7mm in very low light. For your optics to deliver their full light gathering potential, the exit pupil needs to match or exceed your pupil size at that moment. In normal daylight, 2 to 3mm is plenty, but for dawn, dusk or deep cover, you'll want at least 5mm.
Field of View: Field of view is the actual width of what you can see through your binoculars or riflescope at a distance of 1000m. It's sometimes given in degrees instead. One degree equals roughly 17.5m, so a field of view of 8 degrees means your visible width at 1000m would be around 140m. A wider field of view helps enormously when you're tracking movement across open ground or scanning for vermin at speed.
Magnification: Magnification tells you how much closer an object appears compared with the naked eye. It's always the first number in the spec, so with 10x magnification, something 1000m away looks as though it's 100m away. With 8x, that same object appears as though it's 125m away. Higher isn't always better. It depends entirely on the ground you're covering and how steady you can hold the optics.
Objective Lens Diameter: This is the diameter of the front lens, measured in millimetres. It determines how much light can enter the binocular or scope, and works alongside magnification, exit pupil, coating and prism type to determine overall light transmission and image brightness.
Twilight Performance: Twilight performance gives a rough measure of how well an optic performs in low light. It's calculated by multiplying the magnification by the objective lens diameter, then taking the square root of the result. It's a useful starting point, though it doesn't account for glass quality, coatings or the number of lenses involved, so treat it as a guide rather than the full picture.
Understanding these terms takes the guesswork out of choosing between similar-looking specifications, and means you're buying based on how and where you actually work, not just the numbers on the box.
Once you know what you're looking for, choosing between binoculars and riflescopes becomes far more straightforward. Explore our Binocular, Telescope & Monoculars collection to find equipment suited to your ground, your quarry and the light you'll be working in.