If you’re new to buying binoculars, you might wonder how are binoculars measured. The numbers on them can look confusing at first. But understanding these figures is the key to picking the perfect pair for your needs. It tells you everything about their power, brightness, and field of view.
This guide will explain the simple system behind those numbers. You’ll learn what each part means and how it affects what you see. We’ll make it easy so you can shop with confidence.
How Are Binoculars Measured
The most basic measurement you’ll see is two numbers, like 8×42 or 10×50. These numbers are the binoculars’ specification, or “spec.” They are always written in the same order. The first number is the magnification. The second number is the diameter of the objective lens in millimeters.
Let’s break down what each of these numbers means for you.
Understanding Magnification (The First Number)
The first number (before the “x”) is the magnification power. It tells you how many times closer an object will appear compared to your naked eye.
- An 8x magnification makes things look eight times closer.
- A 10x magnification makes things look ten times closer.
Higher magnification isn’t always better. While it brings subjects closer, it also amplifies hand shake, making the image shakier. It often means a narrower field of view and can make the binoculars heavier. For general use like birdwatching or sports, 8x or 10x are the most popular and versatile choices.
Understanding Objective Lens Size (The Second Number)
The second number (after the “x”) is the diameter of the objective lens in millimeters. These are the large lenses at the front of the binoculars that gather light.
- A larger number (like 42mm or 50mm) means bigger lenses that collect more light. This results in a brighter, clearer image, especially in low-light conditions like dawn or dusk.
- A smaller number (like 25mm or 32mm) means more compact and lighter binoculars, but with less light-gathering ability.
The combination of these two numbers gives you a complete picture of the binoculars’ core capability.
The Importance of Exit Pupil
Exit pupil is a derived measurement that tells you how bright the image will appear. You can calculate it easily: just divide the objective lens size by the magnification.
For example, 8×42 binoculars have an exit pupil of 5.25mm (42 ÷ 8 = 5.25). A 10×50 model also has a 5mm exit pupil (50 ÷ 10 = 5).
- A larger exit pupil (over 4mm) is better for low-light viewing.
- It also makes it easier to find and keep the image, which is helpfull if you wear glasses.
- In bright daylight, exit pupil is less critical.
Field of View: How Wide You Can See
Field of view (FOV) is another crucial measurement. It’s usually printed on the binoculars body or box. It tells you the width of the area you can see at a specific distance, typically 1,000 yards.
For instance, a FOV of 340ft at 1000yds means you see a 340-foot-wide area from side to side. A wider field of view makes it easier to follow moving subjects, like birds or athletes. Generally, lower magnification and certain optical designs offer a wider field of view.
Eye Relief: Essential for Eyeglass Wearers
If you wear glasses, you must check the eye relief measurement. Eye relief is the distance from the eyepiece lens to the point where you see the full image. It’s measured in millimeters.
Long eye relief (usually 15mm or more) allows you to see the entire picture even with your glasses on. Shorter eye relief means you have to press your eyes directly against the binoculars, which is impossible with glasses. This spec is often listed in the technical details.
Close Focus Distance
This measurement tells you the minimum distance at which the binoculars can focus sharply. It’s vital for viewers interested in butterflies, insects, or detailed nature study. Some binoculars can focus on objects just a few feet away, while others may not focus closer than 15-20 feet.
Prism Type: Roof vs. Porro
Binoculars use prisms to correct the inverted image. The two main types affect the size and shape of the binoculars.
- Roof Prism: Have straight barrels, making them more compact and streamlined. They are often more expensive but are very durable.
- Porro Prism: Have offset barrels (the lenses are wider than the eyepieces). They often provide better depth perception and can offer excellent value for money.
The prism material (BAK-4 glass is superior to BK-7) also influences image quality and brightness, especially around the edges.
Lens Coatings: The Key to Image Clarity
Measurements don’t stop at numbers. Lens coatings are a qualitative measurement of quality. They reduce glare and increase light transmission.
- Coated: One layer on at least one lens surface.
- Fully Coated: One layer on all air-to-glass surfaces. This is a good minimum standard.
- Multi-Coated: Multiple layers on at least one lens surface.
- Fully Multi-Coated: Multiple layers on all air-to-glass surfaces. This provides the brightest, clearest image.
Always aim for fully multi-coated optics for the best veiwing experience.
Putting It All Together: Choosing Your Numbers
Here’s a quick guide to common configurations and their best uses:
- 8×42: The ultimate all-rounder. Great for birdwatching, sports, and general use. Offers a good balance of brightness, stability, and field of view.
- 10×42: Offers more detail for long-distance viewing (like open landscapes or sea watching). Requires a steadier hand.
- 10×50: Excellent for low-light conditions. Popular for astronomy, hunting, or marine use. They are heavier but very bright.
- 8×32 or 10×32: Compact and light. Perfect for hiking and travel where weight matters. Slightly less bright than 42mm models, but fine for daytime.
- 8×25 or 10×25: Ultra-compact pocket models. Great for concerts or keeping in a glove box. Compromise on brightness and ease of view, but extremely portable.
Remember, the best way to choose is to consider where and when you’ll use them most often. Try handling different sizes if you can, as comfort is a personal measurement too.
FAQ: Your Measurement Questions Answered
What does 10×50 mean on binoculars?
It means the binoculars have 10x magnification and 50mm diameter objective lenses. They make objects appear 10 times closer and have large lenses for gathering a lot of light, making them good for low-light situations.
Are 10×42 binoculars better than 8×42?
Not necessarily “better,” but different. 10×42 gives you more magnification for detail at distance, but the image is shakier and the field of view is narrower. 8×42 offers a wider, steadier view, which many prefer for general wildlife and sports.
What is the best binocular size for bird watching?
Most birders prefer 8×42 binoculars. They provide a wide field of view for finding birds, enough magnification for identification, good brightness for forest shadows, and a stable image. The 8×32 size is also popular for its lighter weight.
How important is the exit pupil number?
Very important for low-light performance. In daylight, your eyes pupil is small (2-3mm), so any exit pupil larger than that is fine. In dim light, your eye opens to 5-7mm, so a larger exit pupil (like 5mm from an 8×40 or 10×50) will deliver a brighter image that feels more comfortable.
Can I use binoculars with my glasses on?
Yes, but you need to look for binoculars with long eye relief (typically 15mm or more). Also, make sure the eyecups twist or fold down. This allows you to get your eyes close enough to see the full picture without vignetting (dark rings around the edges).
What does “field of view at 1000 yards” mean?
It means if you look at something 1000 yards away, the width of the scene you see through the binoculars from left to right will be the number given (e.g., 300 feet). A wider number means you can see more area without moving the binoculars, which is great for scanning.
Understanding these measurements takes the guesswork out of shopping. Now you know that those numbers are a simple code telling you exactly what the binoculars can do. You can match the specs to your favorite activities and choose the perfect tool for bringing the world closer. Just remember to check the eye relief if you wear spectacles, as its often overlooked.