The Binocular Site
Frequently Asked Questions about BinocularsHow do I choose the right magnification?
How do I choose the right lens size?
Should I choose binoculars with Porro prisms or roof prisms?
What is field of view (FOV)?
What is eye relief?
What is an exit pupil?
What is phase shift?
What do BAK4 or BK-7 mean?
Why do some binoculars have coatings?
How do I find an optical device that is comfortable to use?
Should I buy wide angle binoculars?
How do I choose between binoculars, scopes and telescopes?
What binocular accessories are available?
Magnification is shown on binoculars as the first number in labels such as 7x50, 8x25, and 10x42. If you see two numbers separated with a dash, that means the binoculars zoom (ex. 7-25x42). If you see two numbers separated by a slash, that means there are different eyepieces to allow different magnifications (ex. 7/25x42).
Higher magnifications let you see further, but reduce how much area you can see through the binoculars at one time. Therefore lower magnifications are better for some uses.
- Magnifications of 2x to 5x are great for monoculars.
- Magnifications of 7x to 10x are perfect for most uses, including hiking, hunting, and birdwatching.
- Spotting scopes will have higher magnifications around 20x.
- For astronomy binoculars, magnifications of around 10x are fine. You may wish to look for telescopes or binoculars with magnification from 20x to 30x and up though. Some telescopes have magnifications as high as 160x and interchangeable eyepieces so you can use different magnifications, but often the larger options also need to be mounted on a tripod.
Objective lenses collect more light as they get larger, making binoculars and other optical devices effective even in lower light. That makes them more effective when skies are gray, the air is foggy, or at dawn and dusk. However, larger lenses are both more expensive and heavier, so the largest ones can require mounts or tripods to be used. 20 mm lenses are often fine for birdwatching and hunting during the day, while 60 mm, 80 mm, and even 100 mm and larger lenses are best for stargazing.
Porro prism binoculars are designed using prisms in a Z-shape. The prisms themselves encompass a wider area, so the objective lenses are spaced further apart. This type is usually more bulky but often perform better at a lower price. They yield a better three dimensional and wider field of view, but may be more likely to be knocked out of collimation when bumped or dropped. Roof prism binoculars are more streamlined, compact, and lightweight, but must have anti-reflective coatings to work effectively. They are more durable, less likely to fog up or allow dust into inner mechanisms, and their internal focus system gives them better integrity as well.
You may wish to choose Porro prisms when you want to start with a lower priced model that still performs well. If you are hunting or using binoculars on a boat or in the military, the extra investment in tougher, lighter roof prism binoculars is worth it. Ultimately either type can give clear images and offer good magnification and light collection, as these are more a result of objective lens size and all the mechanisms working together rather than one prism design or another.
The field of view is the area you can see while using an optical device. It is typically given in feet, and tells you how many feet of an area you can see 1000 yards away from you. Wide angle lenses can increase the FOV.
Eye relief refers to the distance you must hold binoculars or scopes away from your eyes to use them. Larger eye reliefs mean you can use your binoculars while wearing glasses.
The exit pupil refers to a virtual opening in each lens that light moves through. It should closely match your own pupils, otherwise the light that comes through is simply lost. Night vision binoculars and those made for astronomy have a bigger eye pupil to compensate for your eyes dilating in the dark.
Phase shift is a problem that is unique to roof prism devices. The prisms within these devices can cause a shift in light rays, leading to low contrast and poor resolution. In short, poor light focus within the binoculars results in a dim, blurry image. That said, high quality roof prism devices will use special phase coatings throughout. The coatings force light beams into phase again, making a sharp image where fine details are visible.
BAK4 and BK-7 are common types of glass used in prisms. Both are types of crown glass, but BAK4 has a slightly higher level of refraction than BK-7. BAK4 is more expensive but is considered the better option because it creates a smooth, rounded exit pupil and a sharper image.
Every time light moves through glass, naturally a small percentage is reflected. In a set of binoculars the number of glass surfaces – lenses, prisms, and so forth – is high, and without coatings, that means a lot of light could be lost. This is particularly true for roof prism binoculars, which simply cannot work effectively without coatings.
When the first binoculars were designed without coatings, light reflected throughout the barrels, making images hazy and dim. Although bigger lenses could compensate to some degree, they are heavier and more expensive, whereas thin lens coatings are almost weightless. These days up to 95% of light is conserved throughout the binoculars when coatings are applied carefully yet thoroughly. For example, roof prisms offer much more contrast to the eye when they have anti-phase shifting coatings.
Coatings are described as follows:
- Coated - One or more surfaces are coated.
- Fully Coated - All air-to-glass surfaces are coated but plastic lenses may not be.
- Multi-Coated - One or more surfaces have been treated with multiple coats.
- Fully Multi-Coated - All air-to-glass surfaces are treated with multiple coats.
Try to handle binoculars and scopes in store to get an idea of their weight, shells, eye cups, and so forth. Soft eye cups and focusing mechanisms are important when you use your device a lot. For carrying binoculars in hand, you will want a set that weighs up to about 2lb to 3lb. Otherwise you may need to choose a mount as well. Binoculars are often more comfortable to use since they have barrels rather than the one found on a scope or monocular.
When you look through an optical device such as binoculars, you are limited to viewing a certain area at one time. This area is known as the field of view (FOV). Wide angle lenses allow for a larger FOV than seen in standard binoculars. They achieve this using lenses and prisms to take advantage of light across more degrees. While you may see a FOV of 350 feet at 1000 yards in regular sets, wide angle models could offer a FOV of 500 feet. This is particularly useful when you are surveying the sky or watching a race, and need to see over more area at a time.
Read our guide on the types of binoculars and scopes for explanations and advice. Also look at our detailed guides for astronomy binoculars, birdwatching binoculars, hunting binoculars, marine binoculars, and military binoculars.
There are many binocular accessories out there, from cases to straps, specialized mounts, comfortable eye cups, interchangeable eye pieces, cleaning kits, and so much more. When using your binoculars on the move it can be worthwhile to look for heavy duty harnesses and straps to help keep them comfortable to carry. If you're planning to stargaze though, choosing a heavy duty mount with adjustable angles is a great idea.
Interchangeable eyepieces are often available for bino-telescopes and other scopes, and can allow you to view objects at different magnifications as needed. Since zooming is still considered imperfect and problematic, interchangeable eye pieces are a cost-effective and useful accessory to consider buying. Just make sure your binoculars and eye pieces will work together.
Lens cleaning kits are almost inevitably a great idea if you are using high-end optical devices such as telescopes or high powered binoculars. You don't want to scratch up your lenses when you've invested in a good set of binoculars. You might also get one for stashing in your glove compartment or tent if you use binoculars while birdwatching, hiking, hunting, or on a boat.
We have more details about these and other options on our binocular accessories page.