Binoculars Buying Guide
The Basics of Binoculars
When you look through a telescope, you see objects upside-down and backwards, which is fine for objects in space, where there is no up or down. Binoculars contain prisms that flip objects over to the way you would expect to see them down here on Earth, “right-side up.” There are two types of prisms, roof and porro, and each type affects the size and shape of the binoculars.
Porro prisms create a brighter, more realistic, three-dimensional image, which is better at night, making them preferred for astronomy. These have an offset design which makes them larger and heavier than roof prism binoculars.
Roof prisms are smaller and straighter than porro prisms, so the binoculars are more compact and portable. Some of these compact binoculars weigh as little as 9 ounces. Roof prism binoculars can be waterproof or water-resistant. These tend to be more expensive because smaller prisms are more difficult to grind.
This is the outermost lens, the one that is closest to the objects you are looking at. It is measured in millimeters, and the larger the objective lens, the more light the binoculars let in and the brighter the image will be.
It is the measurement of how much larger an object appears when viewed through a lens; on binoculars, the magnification is given with a number followed by an “x,” such as 7x or 12x. Larger magnifications let you see less in your field of view, but the objects you see will appear closer. Larger magnifications can also make it harder to hold the binoculars steady enough to focus; use a tripod for anything over 10x.
The aperture tells you the magnification and the objective lens diameter; for example, 7×35 magnifies seven times and the objective lens has a 35 mm diameter. Any pair of binoculars should list the aperture in the product description. In general, these apertures will be appropriate:
|Indoor concerts, theaters, and small sporting events||about 6×30|
|Hiking and nature walking||about 7×25|
|Outdoor concerts, large sporting events, and general purpose||about 7×35|
|Astronomy, hunting, and birding||about 7×50 or 8×40|
The eyepiece is the part you look through.
The eye relief is the distance between the eyepiece and the user’s eyes. Eyeglass wearers should choose a long eye relief, at least 13 mm. Some binoculars have an adjustable eye relief for multiple users, using methods such as rubber eye cups which fold back.
This is the distance, in millimeters, between the two eyepieces. The distance between the average person’s pupils is between 60 mm and 72 mm. Make sure you choose binoculars that will adjust to your size. You can have someone measure the distance by looking straight forward and using a metric ruler.
The beams of light that exit the eyepieces and hit your pupils are called exit pupils; these help determine brightness: Larger exit pupils let in more light. Divide the diameter of the objective lens by the magnification to find the size. For nighttime use, you will want a larger exit pupil, around 7 mm.
Many binoculars focus with a central knob to adjust both sides at one time; however, some binoculars have a fixed focus, also known as “focus free,” so they will not have a focusing knob. Others will have a diopter on one barrel of the binoculars, so you can customize the view through each eye.