In browsing through FeatherEdgeOptics you’ll see binoculars ranging from 6x to 15x power, and each has its application. But most of us, most of the time, will be best served by either an 8x or 10x binocular. We’re mostly watching birds, butterflies and dragonflies, or scanning broad horizons to find mammals. This post may help you understand why we’re using 8x or 10x, and hopefully to select what will work best for you.  I’m going to make a case here for one of those over the other, but you’ll have to make the final decision.

When deciding between 8x and 10x, be sure to compare apples to apples, by which I mean 8x32 vs. 10x32 and 8x42 vs. 10x 42 [and yes, the 32 mm versus 42 mm objective will be the topic of another blog post!]

So, right now we’re considering an 8x42 and a 10x42.  There are many brands and levels of quality, but there are generalities consistent among them.  If the manufacturer is honest with power and hasn’t undersized the prisms, the 8x42 has a 5.25 mm exit pupil while that of the 10x42 is 4.2 mm.  On a sunny day, either supplies the eye with all the light it needs.  But at dawn or dusk, or cloudy days under forest cover, your pupil will naturally open wider, and the 8x42 will be brighter than the 10x42.   I’m going to call that one point for the 8x; if you’re a fair weather nine-to-five birder, you might demur.

Next, while you might find a Brand A 10x42 with a wider field of view (FOV) than a Brand C 8x42, within any line, the 8x will always have a wider field.  A wide field helps you to get on and follow moving targets.  Quality and eyepiece design enter here, since the utility of a wide field decreases if it’s only sharp in the center.  It pays to pay more here: with a wide field that’s sharp edge to edge, that meteoric White-throated Swift is yours.  A 10x42 with field of view under 300’ @ 1,000 yards which is only sharp in the center is best reserved for watching feeders.

I know whereof I speak, since I’ve just described my first two binoculars. They were 10x50’s (see below, the same model as my first bins in 1971) which, though optically not bad for the time, had non-adjustable hard Bakelite eyecups, scant eye relief (future blog post) and so narrow a field that most flying birds were in the next time zone before I got on them.  During my first five years of birding I despaired that field guides weren’t filled with plates of edge-on views of departing birds, since that’s mostly what I saw. I’ll call that another point for the 8x, but again, if you monitor an eagle nest you might say otherwise.

I’m not a tripod.  While I’m steady on my feet and don’t have an essential tremor, when I use a 10x binocular I’m often aware of image movement. It’s rarely a bother, and I usually notice it only when I’m looking for a field mark on a distant bird.  Um.  Gosh. I guess I do that often, don’t I? I can rest my elbows on the car roof or a deck railing, but they’re not always available.  I’ve carried 10x and 8x into the field together, switching back and forth, and found that for me the detail provided by the slightly larger image of the 10x can be lost in the perceived motion, but seen in the more stable image of the 8x.  Quality matters here, too, since a top shelf 8x can out-perform an entry level 10x. Even though I carried 10x for over 30 years (and my wife still prefers 10x), for me at least, this is a third point tilting towards an 8x, and why I have carried them for the past decade. 

That covers the main points to think about in deciding between 8x and 10x. You can stop here and decide or read on for a bit about all those other binocular powers!

So, what about the outliers, meaning binoculars other than 8x or 10x? The earliest “field glasses” were paired Galilean telescopes. Because of their length, about 4x to 6x were the highest magnifications practical for handheld use. They were popular from the mid-1800's until after World War I*. The development of Porro prism and roof prism binoculars allowed for significantly more compact designs and higher powers. We still offer 6x binoculars because they’re a good first binocular for children, as the wide field of view and “low” magnification make them easy to use. They may also work well for anyone with an essential tremor.

Binoculars of 7x magnification (7x35, 7x50) have been made continuously for over 75 years. Today you’ll mostly find cheap 7x35s at sporting events, as many major league players are larger than warblers and sport jerseys with foot-tall numbers. On board a ship, where markings on tertials don’t matter, but seeing a channel marker in the gloaming or a darker darkness off the bow at three in the morning could be critical, 7x50 and 10x50 rule.

The advent of the new (again) 7x42 format in the 1990’s started by the popularity of the Zeiss Dialyt 7x42 T*B, Swarovski 7x42 SLC, and Leica Ultravid 7x42, gave 7x glass another renaissance. The fantastic wide field was the main reason, though depth of field and great dawn & dusk performance played a part.  And just to carry the arguments above a little further, when using a 7x42 image movement is even less noticeable, depth of field greater, FOV usually wider, and the 6 mm exit pupil nearly equals your eye at deep, deep dusk.

The Zeiss and Swarovski 7x42 are gone again. Why?  With continued improvements in glass manufacture, even better-quality glass and coatings and advanced lens systems, many of the same qualities of yesterday’s 7x are now to be had in 8x formats. The 450’ FOV of the Zeiss Dialyt 7x42 T*B is essentially equaled by the 8x42 Victory SF’s 444’, the SF is waterproof where the Dialyt isn’t, and the SF has cut the close focus from 15’ down to a butterflying-worthy 4.9’.  But if a 7x42 still intrigues you, we still carry the Leica Ultravid HD+ 7x42. Check them out!

I have hand-held 12x50 binoculars and maybe you can too.  But if you just carry on with the arguments above you’ll know these will have a still-narrower field of view, be less bright in low light, show visible image shake, and not focus as close. I wouldn’t carry them up a mountain or use them for warbler watching. They’re great for a sea watch, though, and scanning dunes, mudflats, and bays.

When we go up to most manufacturer’s biggest gun, a 15x56, pretty much everyone will be tripod mounting them and using them as a low-power binocular telescope. If big box stores sold them, they’d be labeled “Team Lift”. They’re fabulous at the Avalon Sea Watch, and they’d be great for catching Cackling Geese in Cranbury farm fields or picking out a Pink-footed Goose at Middle Creek WMA.  But don’t bring these to the Biggest Week in American Birding unless you’re equipped with a back-up beeper.

*For fun, type “history of prismatic binoculars” in your search bar on Friday after dinner, and make sure your calendar is open through Sunday night! You’re welcome!