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While an 8x42 binocular remains the most versatile all-purpose optical instrument for birding, there are situations when you’ll need a binocular with higher or lower magnification, or perhaps even a spotting scope. In this article I’ll provide guidance selecting optical equipment that best matches your birding style, locale, and habitat. Before we sort out specific options, I’ll explain more about magnification and aperture.
The first number in a binocular specification is magnification, or how much larger objects appear versus the naked eye. Therefore, “8x” or “8 times” means objects are eight times closer or larger and “10x” means objects are ten times closer or larger. The second number is the aperture of the binocular's objective lens measured in millimeters. The larger the aperture the more light the binocular lets in, and subsequently, the brighter things appear to you. For example, an 8x50 lets in more light than an 8x42. Be mindful that the objective lens size is also an indicator of the overall physical size and weight of the binocular, so an 8x50 would be larger and heavier than an 8x42.
Another optical specification of great interest is the exit pupil, which is the width of light in millimeters that exits the eyepiece that enters through your pupil. It’s calculated simply by dividing aperture by magnification. For example, an 8x42 has an exit pupil of 5.25mm, while a 10x42 has 4.2mm. The larger the exit pupil, the better the binocular will perform in low light. Since the exit pupil doesn’t take glass or optical coating quality into consideration, it’s only useful when considering various magnification and aperture sizes with respect to one another.
When I think of my favorite type of birding, I like to imagine myself in a woods during the peak of spring migration in May. Searching for the array of warblers, vireos, flycatchers, and thrushes this time of year can be challenging under a forested canopy. What if there’s a skulking Connecticut or Mourning Warbler in the shadowy tangle of the forest floor? Having a large exit pupil (5mm or greater) offers an advantage over higher magnification binoculars with the same aperture size. I would rather bird with an 8x42 or 7x42 than a 10x42 because of the brighter image I’ll see. Not to be overlooked are high-end 8x32 binoculars which offer comparable brightness to many mid-priced 42mm ones.
Simply stated, avoid compact binoculars! The obvious benefit of a compact is its portability, but it’ll render an extremely dim image in the dense rain forests of central and South America. Even mid-aperture binoculars (30mm to 35mm) will leave you craving a brighter image. As in woodland birding, you’ll desire the large exit pupil that a 7x42 or 8x42 delivers. Lastly, and perhaps most importantly, be sure your rainforest binoculars are waterproof and nitrogen purged for fogproofing.
Perched grassland birds can be seen from hundreds of yards away in the open expanse of the prairie. Bobolinks, Dickcissels, Meadowlarks, Upland Sandpipers, and Grasshopper Sparrows are some of the habitat specialists you’ll encounter. While some bird species might tolerate a close approach (because they’re generally defending nest sites), most will take wing long before you can get a satisfying look through binoculars. A spotting scope is a prairie enthusiast's best friend. Complementing a spotting scope, just about any binocular can be used to scan the fields for perched birds. Once you’ve located your quarry, your best views are going to be observed at higher magnifications that only a spotting scope can deliver. However, if you plan on bringing only a binocular to the prairie, a 10x is your best choice.
These are my personal recommendations for three specific birding habitats I enjoy the most. At Eagle Optics we strive to match optical products by need and application, but our recommendations are by no means a rule. If you personally match up with a 10x50 under a woodland or tropical setting, it shouldn’t be regarded as the wrong choice; it’s what works best for you!
Article and photo Yellow Warbler contributed by Mike McDowell, an avid digiscoper, amateur naturalist, and Eagle Optics employee.