Binoculars

Optics

Binocular Power – Too Much of a Good Thing
By Ron Spomer

A version of this article first appeared in Gun Hunter magazine

An axiom in politics is that power corrupts and absolute power corrupts absolutely. It’s pretty much like that with binoculars, too.

We hunters, crazy to see every hidden tiger, crouching whitetail, figure if 7X is good, 10X has to be better and 15X has to be twice as good.

Wrong.

To a large degree, the higher the power, the less we see.

To understand how this could possibly be, we must consider how the human eye works, which is like this:

1. Waves of the electromagnetic spectrum enter through pupils that shrink to about 2.5mm in bright light, dilate to about 7mm in low light. This dilation declines about 1mm per decade after age 40 or 45, reducing low-light vision.
2. The inside wall of our eyes, the retina, are covered with rod and cone cells. Rod cells are sensitive to brightness, enabling us to see in really low light, but only in black and white. Cone cells are less sensitive to dim light, but enable us to see the colored part of the electromagnetic spectrum, roughly 400 to 700 nanometers or all the colors of the rainbow but not infrared light, radio waves, gamma rays and all the rest of the amazing electromagnetic spectrum. Cone cells are most abundant near the center of the retina, rod cells proliferate everywhere else, which is why it’s easier to see really dim stars at night if you don’t look directly at them.
3. The lenses behind our pupils bend the incoming light waves to cover or “see” about a 150-degree angle. The farther toward the outer edges of this wide angle of view (peripheral vision), the less sharp and richly colored things appear and the more difficult they are to notice unless they move. When you catch motion on the rod cells in your peripheral vision, you turn to look at it because.
4. Covering the central 10-degrees of the retina is an area called the macula, which has more cone cells and “sees” a shaper image. Within a 2-degree angular field in the center of the macula is the fovea with our sharpest vision. This is the area we use to stare with and detect the finest details. You’re using your fovea to read this. Stare at this page and notice how you can see sharply only two, at most three words at once. Those are the words that fall within the 2-degree angle of view of your sharp fovea.

    

   With this information we can begin to understand how little of the world we see sharply at any moment. Yes, our eyes are taking in light waves over a 150-degree area (180-degrees when stereoscopic vision of both eyes are taken into account) but our brains are really only “seeing” at best what falls within the 10-degree arc of the macular field and we’re truly concentrating on just the 2-degree area of our foveal field of view (FOV). At 100 yards, this foveal FOV is just under ten-and-a-half feet wide/tall. The macular FOV is just over 52 feet wide.

This should suggest that an extreme wide-angle-view binocular image may not be all that useful, but that’s not quite the case because, even while we’re scouring the countryside with our tiny, 2-degree fovea, our 10-degree macula remains able to pick out fairly obvious objects such as a dark-bodied deer standing against yellow grass. And our 150-degree retina should catch the motion of a bobbing head or flicking tail. Why then, do we need binocular magnification at all? Because of limitations in resolution.

The human eye can resolve (clearly, sharply see) at best one minute of angle (MOA.) That’s an object just over an inch tall (1.047″) at 100 yards. Because the chest of a big buck is 18 inches deep, we have no problem seeing it at 100 yards, but at 500 yards our MOA resolution subtends five-inches, so that buck starts looking awfully tiny. At 1,000 yards an object needs to be 10.47 inches high before our eyes can resolve it. A buck at that distance would look like a 2-inch high toy at 100 yards. A prairie dog would disappear. Bring on the binoculars!

Magnification helps us see distant objects better by increasing their apparent size so that they fill more of our view. In basic terms, they’re bigger so they’re easier to see. Of course, this only works if the binocular itself has resolving power equal to or greater than our own. For the rest of this article, we’ll assume it does. At 10X magnification an 18-inch buck at 1,000 yards would appear to be 10 times closer than in reality, or 100 yards away. At 8X it would seem to be 125 yards out, and at 6X it looks 167 yards distant.

Those numbers suggest 10X beats 8X a bit and 6X quite handily. Why not make everything we look at 10 times closer so it’s easier to see? Because sometimes 10 times closer is too close, thanks to our narrow foveal angle of view. Let’s say we’re scanning the countryside for game with a 10X glass. At 100 yards our naked-eye foveal FOV covers 10.47 feet, our macular FOV 52.36 feet. Multiply those by the 10X REDUCTION in FOV (due to the 10X enlargement) and at 100 yards we’re seeing just 1.04 feet wide with our fovea, 5.2 feet with our macula. This is great if we want to count tines and see sticker points, not so great when we want to pick out the gross outline of a buck’s body in a varied landscape.

While it’s true that seeing a 1-foot patch of deer hair makes it easier to identify as deer hair instead of branches and dirt, while we’re staring at it we could miss Noah’s parade of animals marching two-by-two just 40-feet to either side. And this is the key. If we’re selecting a binocular for finding game, less power means we see more because a greater FOV at all distances falls within our critical foveal and macular FOV.

In simple terms higher magnifications give us a smaller slice of the visual pie. While this can be good when studying detail or seeking game on a mountainside three miles away, it’s overkill for the bulk of hunting where we glass from 25 yards to 1,000 yards. Look at it this way: you are scanning a mountainside 1,000 yards away with your 10X bino, which takes in a 314-foot wide span of countryside. As you stare, a mule deer buck climbs out of a defile at the 315-foot mark, walks across an opening to the 330-foot mark and beds down behind a bush. If you’d been looking through an 8X bino with a 330-foot FOV, you’d have caught the motion, focused on it and spotted the buck. At that point you don’t care if it has three two-inch stickers on the left side. You have to find it before you can size it up. Binoculars are for finding first, studying fine details later, if at all. Most experienced trophy hunters hire a superbly sharp 7X or 8X binocular to find game, then switch to a 20-60X spotting scope to really study it. But there are exceptions we’ll cover later.

Another disadvantage in high magnification binoculars is limited depth-of-focus. In any optical instrument, the higher the magnification, the shorter the depth-of-focus (DOF), that distance in front of and behind the subject that appears sharp. These distances shrink at close range, increase at long range until everything looks sharp at infinity. In hunting, a limited DOF means that when you focus on a tree or rock at 100 yards, objects at 30 yards or 200 yards might be too fuzzy for you to recognize. Thus, if you sweep past a buck in the foreground or background, you might miss it. The higher the magnification, the greater this problem is. In practice, I’ve found limited DOF a problem only when woods hunting under about 200 yards. It’s most critical out to 100 yards, which is a major reason I prefer a 6X or 7X binocular for woods hunting whitetails, elk or anything else. Also, higher magnification makes trees and brush so big that I lose perspective. Tree trunks loom too huge and take up too much space. It becomes difficult to scan and understand which part of the woods you’re seeing. Was it that tree 25 yards in front or the one 50 yards beyond it?

In short, most of us would find more game with a 7X or 8X binocular than anything more powerful. The exception involves scouring distant cover for bedded game, commonly done by Coues deer and sheep hunters. These hunters often use 15X and even 20X binoculars on tripods while studying shadows for hours and hours to identify every boulder, rock and twig. They often find animals by seeing an ear, tail, rump or patch of hair. Big binoculars work better than spotting scopes because they provide binocular vision and are easier to use with both eyes open.

If you must limit yourself to one binocular for general purpose hunting everywhere, get a 7X or 8X. If you’ll specialize in open-country hunting for mule deer, pronghorns, sheep and the like, you might justify a 10X. For woods hunting, go with 6X or 7X. For more versatility, look into dual or switch power binoculars that flip from 7 or 8X to 12 or 15X.

Sidebar — Resolving Power and Contrast Help

Magnification means little if resolving power isn’t high. A deer that appears ten times closer through a fuzzy 10X will not be as easy to detect or study as one that’s only 7 times closer through a superbly sharp 7X binocular. Resolution is determined by the quality of the objective lens, and bigger ones are easier to build to high quality than small ones. To test resolution, try to read letters one-inch high at 100 yards or 1/4-inch high at 25 yards with test binoculars settled solidly on a table, bean bag or tripod.

Contrast also makes it easier to see game because it brings out the subtle color and brightness differences between fur and dirt, antlers and limbs, etc. High contrast results from excellent flare control which is mastered by the best multiple layers of anti-reflection coatings on all lenses. By minimizing reflected light bouncing around within the binocular, coatings minimize contrast-reducing flare and glare.

Fields of View in Degrees & Linear Feet at 100 Yards