How to choose binoculars
Are you going on a vacation tour to the Grand Canyon and you want to take binoculars with you, but you are not sure want kind to buy? Although, recommending a particular pair of binoculars for someone is an impossible task because what works for one person may not work at all for another, we have prepared certain criteria anyone purchasing binoculars should use in order to find the right pair for them.
To make choosing a pair easier let’s first consider how they work. Binoculars are a relatively simple device that have been greatly improved since their inception one hundred years ago. They are made up of three simple components that combine to create a magnified view of whatever they are focused on. The components are two magnifying glasses and a prism (some have two prisms). Light travels through first magnifying glass called the Objective lens, but just like our eyes the image is upside down and backwards. This light is then reflected into a prism or series of prisms that correct the image and reflect it to another magnifying glass called the eyepiece. All binoculars function in effectively the same manner. This makes it hard to distinguish between the various products available on the market today. Soon there will be digital binoculars with all sorts of different options. But this is a different topic altogether. So for now let’s focus on the criteria to use to differentiate between products.
The following is a list of criteria to consider when purchasing a pair of binoculars. It is important to use these criteria for two reasons. One, binoculars are a relatively expensive piece of equipment, and, two, because they do not break very often, any pair you purchase will probably be with you for a very long time. But just like any piece of gear, it is a lot of fun to shop for binoculars once you know what to look for.
Magnification – The first numbers generally seen on binoculars are something like this “7X42”. The first of these numbers is the magnification of objects. The standard range is anywhere from 7 to 10. The larger this number the larger objects will appear when looking through the binoculars. However, with more magnification it becomes more difficult to keep the image steady. Anything over ten would require a tripod.
Objective lens – The second number is the size of the objective lens in millimeters. In general, the larger the objective the more light that is allowed to pass through the binoculars giving better image resolution.
Eye relief – This is the distance the binoculars can be held away from the eye and still provide the entire field of view. This number is most important to people who wear eyeglasses in that eyeglasses will automatically increase the distance between your eye and the binoculars. Look for devices with an eye relief around 15mm. This really only becomes an issue with the smaller more compact products. Also, look for folding, rubber eyecups that help to reduce the distance from the eye while wearing glasses.
Exit pupil – Another number that is important in selecting binoculars is the exit pupil size. This is calculated by dividing the size of the objective lens with the magnification power. This number is generally between 2 and 6. The larger this number the better low-light resolution the binoculars will provide.
Field of view or view angle – This is the size of the field of view you will see when looking at an object 1000 yards away. Generally, higher magnification binoculars will have a smaller field of view. The number is represented either in feet or a view angle. If you want to calculate the field of view in feet from and angle, the math is rather simple. The circumference of the circle 1000 yards away from you at all points is 18840 feet. Divide this number by 360 degrees and you get approximately 52.3. Multiply this number by the view angle on the binoculars and you get the field of view in feet.
18840ft / 360 = 52.3ft/degree * view angle = Field of view in feet
The main idea is that the larger the field of view the less detail you will be able to see.
Exterior grip – Almost all binoculars have a rubber exterior to make the grip comfortable. Choose the one that feels comfortable to you.
Prism – There are two types of prisms in binoculars today. The standard being the Porro prism and the other the more advanced Roof prism. While Roof prisms tend to be more on the high-end devices, either type should work fine. However, if you choose a Porro prism device be sure that it uses BAK4 prisms as opposed to BK7.
Coatings – In addition to knowing what type of prism a device has, it is also important to know which of the glass pieces have been coated. The coating of glass surfaces in the binoculars reduces the amount of light lost inside the binoculars increasing their image resolution. There are many different levels of coating though. Some devices only have one lens coated while others have of all lenses multi-coated which is even better.
Other criteria to consider
Diopter – Almost all binoculars made today have a diopter. This is a focus setting for one eye to adjust for a variance in vision between eyes.
Weight – Binoculars can weigh anywhere from 10 to 40oz. Depending on your purpose and length of use it is important to think about when purchasing. A heavy pair may not seem bad in the store, but feel like a sand bag after 10 hours in the field.
Waterproof – Decide if it is necessary for your purposes.
Return policy – Many times a pair of binoculars will seem just right in the store but may expose some of its weaknesses in the field. During prolonged use you could feel some eyestrain. Maybe the pair you have chosen is too heavy. Perhaps the magnification is more than you need. For all of these cases be sure to find out about the return policy for any significant purchase, especially something that you will probably have for the rest of your life, like binoculars.
The best place to shop for binoculars is a camera shop, sporting goods store, boat and marine supply store, or a store dedicated to birdwatching.