Buying Binoculars, Part II — By the Numbers

A lot of numbers are involved in buying binoculars. Most important are the numbers after the dollar sign. But there are some others that you should know just a little about before going off to invest in a pair of binoculars. You won’t have to be a physicist to understand them, although they all come from the laws of physics.ab_equinoxhp_8x42_200.jpg

First is the “magnification” number. That is the first number in the “8 x 42″ or “7 x 50″ or “10 x 42″ or any of the others you’ll see when looking at binoculars. For birding, the ones you buy should be in the 7 times to 10 times magnification range. Binoculars that magnify more than ten times cannot be held steady by a normal human. Anything above 10x needs a tripod to hold them steady and often there is not time to get a tripod set up before the bird is gone.We recommend you choose either a 7x or an 8x magnification. And there are two more numbers/reasons not to buy anything over 10x magnification: depth of field and field of view. We’ll get to those numbers in a minute.

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The second number is the size of the lens in millimeters at the front of the binoculars. The size of the lens determines how much light is admitted to the binoculars. Up to a point, the more light the better. However, there are trade-offs. Any lens larger than 50 millimeters is likely to make the binoculars weigh more than they should to be comfortable to hold. Moreover, there is no appreciable gain in color or resolution above that size.

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A third number is derived from the first two. You get it by dividing the second by the first. For instance, if you have a pair of “8 x 42″ binoculars in your hand you divide 42 by 8 to learn what something called “the exit pupil” is. For a pair of 8 x 42s the exit pupil will always, must always be precisely 5.25 millimeters. For a pair of 10 x 42 binoculars, that number is 4.2 millimeters.

Why do we care? Because that is the size of the hole through which the light that gets to your eyes must come. In other words, it determines how bright the image will be. The human eye cannot open much wider than 5 millimeters so an exit pupil somewhere in that range is as good as it gets. On the other hand, an exit of 2 millimeters is much smaller than the pupil of your eye so will not admit as much light and will be experienced by you as dimmer than it should be.

For birding, what that means is that you should look for binoculars with magnifications of 7x or 8x and objective lenses between 40 and 50 millimeters. (With  one exception which is coming below.)

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The fourth number you may see is one for the “field of view.” For example, you may see something like 446 feet at a 1000 yards. Or, if the manufacturer wants to confuse the issue you may see something like “Field of View = 8.5 degrees” which is the same thing at 446 feet at a thousand yards. Your naked eye sees about 165 degrees, so binoculars reduce it greatly.

Field of view simply means how much the binoculars will allow you to see when you hold them up to your eyes. The numbers are determined by the magnification and by the design of the eyepiece. Anything less than about 5 degrees will make it hard to find in the binoculars what you are looking at with the naked eye; anything more than 8.5 degrees is carrying coals to Newcastle. Get something with a field of view between 242 feet and 446 feet at a 1,000 yards and you’ll be fine.

What’s that I hear you say? You won’t be looking at birds much from that far away? I know. So do the manufacturers of the binoculars but the math is the math and that’s how they do it.

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But there is still at least one more number you should know about: “eye relief.” When you put the binoculars up to your eyeballs, your eyeball has to be a precise distance away from the exit pupil to see all the light coming out of the binoculars. This distance varies from person to person, especially if that person wears glasses. If you don’t wear glasses that number is probably about 10 millimeters. If you do, the number is about 20 millimeters, although something in the range of 15 millimeters will work pretty well. Almost all binoculars come with some kind of adjustable eye cup which can be folded down or twisted away for people wearing glasses.

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That is about it for the numbers. Except for 4%. That is the percentage of light that would get bounced around inside the binoculars if the glass of the lenses were not coated. Which would mean you would lose that much sharpness reaching your retinas. All but the cheapest binoculars have coated lenses. If the lenses are not coated, don’t buy them. Period. I don’t care how little they cost.

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We’re just about there now. Remember the exception I talked about. You could buy little compact binoculars which are known as “porro prism” binoculars. Because of their small size, the glass inside them has to be arranged far differently than in regular full size binoculars. We recommend that you not buy the compacts for birding EXCEPT if price is an issue. You can get perfectly good porro prism binoculars for less than $100.00 and you can’t get good regular binoculars for that. The drawbacks to compact binoculars are many: they are harder to use because their field of view is quite limited so it is more difficult to find that bird you can see with your naked eye and they are lighter which makes them, ironically, harder to hold steady. However; a few years ago we bought compacts for two women in their eighties and they loved them. They found regular binoculars too heavy to hold.

If your budget is less than $100, buy compacts.

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When we started these posts about how to buy binoculars we told you we would give you our personal recommendations. Here they are:

Compacts: Nikon Travelites. They come with a magnification factors of 8x,9x, and 10x. Any of those is fine, even the 10x since these binoculars are so lightweight, but we prefer the 8x.

Regular size: When we bought our current birding binoculars we chose the Nikon Monarch series and remain quite happy with them. They cost slightly less than $300 in most places, work fine and have endured a lot of birding successfully. At the time we bought them we did not feel that the Nikon Premier line, which then cost about $1000 per pair, were enough better to justify the additional $700 per pair. Since that time; however, Nikon has improved — and raised the price — of their Premier line and some reviews rate them as highly as Zeiss or Leica or Swarovski binoculars, all of which cost more than $1700. Nikon Premiers sell for about $1300.

Remember our advice: buy the best you can afford. If you can afford the best, we recommend either the Swarovski 8x50B SLCNew (or the older 8.5×42 SLC) or the new Zeiss Victory 8×42 T*FL. You should be able to find the Zeiss binoculars for about $1750.00 and the Swarovski’s for about $1700.00.

Finally, remember what we said in our first post about buying binoculars: Try them out. No matter how many numbers or physics are involved, there is something subjective about binoculars. If you have a chance, go on a local bird walk and ask the other participants to allow you to look through theirs. When you go to the store to buy some, insist on taking them outside and looking around for as long as possible. Then buy the ones you like best that are within your price range.

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