When choosing binoculars for bird watching, you’ll come across a set of numbers in the specifications. These numbers indicate important factors such as how well you can see birds, how bright the image appears, and how comfortable the binoculars are to use. Understanding magnification, objective lens size, field of view, eye relief, and other key features will help you pick the perfect pair of binoculars for your bird watching adventures.
The first number in a binocular’s specification (e.g., 8×42, 10×50) represents the magnification power, or how much closer an object appears compared to the naked eye.
8x magnification – the image is 8 times closer than with the naked eye.
10x magnification – the image is 10 times closer, bringing more detail but also reducing stability and field of view.
There are trade-offs between higher magnification vs stability and field of view.
A higher magnification (10x, 12x, etc.) brings birds closer but reduces the field of view and increases hand-shake. A lower magnification (7x, 8x) provides a wider field of view and is easier to hold steady.
8x – the most popular choice, offering a good balance of magnification, stability, and field of view.
10x – better for distance viewing (e.g., open fields, coastal birding) but harder to keep steady.
The second number refers to the diameter of the objective lenses (in mm). the larger front lenses that gather light.
The size of the lens affects light gathering and performance. Larger lenses collect more light, improving brightness and low-light performance. However, bigger lenses also mean heavier binoculars.
Compact binoculars (e.g., 8×32, 10×32) are lightweight, but less effective in low light. Standard binoculars (e.g., 8×42, 10×42) are best for all-around for bird watching. Large binoculars (e.g., 10×50, 12×50) produce brighter images but are bulkier and heavier.
8×42 – an ideal balance of brightness, performance, and portability.
10×42 – good for distance viewing but slightly dimmer and harder to hold steady.
The exit pupil determines how bright an image appears, especially in dim conditions. It’s calculated as:
Exit Pupil = Objective Lens Diameter ÷ Magnification
For example:
8×42 binoculars → 42 ÷ 8 = 5.25mm
10×42 binoculars → 42 ÷ 10 = 4.2mm
A larger exit pupil (4mm or more) provides brighter images at dawn, dusk, and in shaded areas, whereas smaller exit pupils (under 3mm) make images appear darker in low light.
5mm+ (e.g., 8×42) – excellent for dawn/dusk bird watching.
4mm (e.g., 10×42) – still good, but slightly darker in low light.
The field of view (FOV) is the width of the area you can see when looking through binoculars. It is typically measured in metres at a distance of 1,000 metres (e.g., 120m/1000m), known as he linear field of view.
The field of view is affected by magnification. Higher magnification binoculars (10x, 12x, etc.) offer a narrower field of view, while lower magnification binoculars (7x, 8x, etc.) have a wider field of view and are better for tracking birds in motion.
The field of view can also be given as an angular measurement in degrees. To convert between the two:
1° ≈ 17.5m at 1,000m
For example, if binoculars have a 7° field of view, they would show about 122.5m at 1,000m (7 × 17.5m).
8×42 – 120–140m at 1,000m (~6.5°–8°) are ideal for tracking birds in flight.
10×42 – 100–120m at 1,000m (~5.5°–6.5°) are better for distant details but harder to follow moving birds.
Eye relief is the distance in mm between your eye and the binocular’s eyepiece at which you can see the full image.
A longer eye relief (15mm or more) is essential for glasses wearers, as it allows them to see the full field of view. A shorter eye relief (<14mm) may cause the edges of the image to be cut off when wearing glasses.
15mm+ eye relief – best for glasses wearers.
Adjustable eyecups – allow non-glasses users to adjust the viewing distance.
The minimum focus distance is the closest distance at which binoculars can focus on an object.
A shorter minimum focus (under 2m) is ideal for bird watching in gardens. A longer minimum focus (over 4m) is better for observing birds at a distance.
1.5–2.5m minimum focus distance – allows close-up viewing of birds at feeders.
3–4m – suitable for general bird watching in open areas where birds are typically farther away.
4m+ – Less useful for close-range wildlife observation but acceptable for distant birding, such as scanning lakes or cliffs.
Binoculars often have waterproof ratings, measured as IPX numbers:
IPX0 (Not waterproof) – not protected at all against water.
IPX4 (Splashproof) – can handle light rain but not submersion.
IPX7 (Waterproof) – can be submerged in 1m of water for 30 minutes.
IPX8 (Fully waterproof) – can be submerged beyond 1m.
IPX7+ waterproof rating – Protects against rain and accidental submersion.
Binoculars often specify the type of glass used in their prisms.
BAK-4 (Barium Crown Glass) – higher quality, provides sharper, brighter images.
BK-7 (Borosilicate Glass) – lower quality, produces slightly dimmer images with a squared-off exit pupil.
Some high-end binoculars specify light transmission (%), which indicates how much light reaches your eye.
Standard binoculars – 80–85% light transmission.
High-quality models with premium coatings – 90–95%, providing brighter and clearer images.
Binoculars may list their minimum interpupillary distance (IPD) in millimetres (mm).
Smaller IPD (50–55mm) – important for users with narrow-set eyes (e.g., some women and children).
Standard IPD range (56–74mm) – works for most adults.
Binocular weight is often listed in grams (g) or sometimes ounces (oz).
Compact (e.g., 8×32) – 500g–700g (good for travel).
Standard (e.g., 8×42, 10×42) – 700g–1kg (best balance for bird watching).
Large (e.g., 10×50, 12×50) – 1kg+, can cause arm fatigue without a tripod.
This page contains affiliate links. For more information please see our disclosure here.
