Binoculars Part III: One Eye Versus Two

One of the reasons that I postponed purchasing 15x70 binoculars so long is that I already owned a 70-mm refractor that I routinely use at 16X for wide-field browsing. Would the binoculars duplicate a capability that I already have? Or would the benefits of using two eyes make binocular viewing an entirely different experience?

The answers, not surprisingly, are yes and yes. There's a huge amount in common between 15x70 binoculars and a 70-mm telescope operating at 16X. And the telescope's ability to achieve different magnifications by interchanging eyepieces makes it a far more versatile instrument. Even so, there are some aspects of binocular astronomy that cannot be duplicated by any instrument with a single main lens or mirror.

My first impression on comparing views between the two instruments was how much brighter everything appeared in the binoculars — both the targets I was viewing and the sky background. To verify this, I tried covering each of the binoculars' main lenses in turn. And sure enough, as soon as I restricted my binoculars to single-eyed use, the sensation of brightness was greatly reduced.

At some level, this is an illusion. The image formed on one retina doesn't change when the other retina is taken out of service. But somehow things seem brighter when the brain combines the images from the two eyes. You can see if this is true for you with any set of binoculars; covering one lens of 7x35 binoculars should have exactly the same effect as covering one lens of 15x70 binoculars. I find the change in apparent brightness is smallest in an urban environment, where everything seems bright even with one eye, and greatest under dark skies.

What does this mean in practical terms? Browsing through the Virgo Cluster, several galaxies that were quite evident through the binoculars were difficult or impossible to see through the telescope at 16X. But 16X is far below the optimal magnification for seeing faint fuzzy objects through a 70-mm instrument. Boosting the telescope's magnification to 40X made it easy to see all the galaxies that were visible through the 15x70 binoculars — and many more besides.

That was the general pattern. I viewed dozens of objects with both instruments, and for the vast majority, the improvement yielded by using two eyes paled in comparison with the improvement from increasing the magnification. But there were two exceptions, which I found quite instructive.

The Beehive Cluster, M44, contains several stars that are strikingly orange or reddish when viewed through large instruments. (In most other respects, a large telescope fails to do this cluster justice, because its field of view is to big to fit and frame the cluster well.) I could make out just a hint of these hues through my 70-mm refractor, and boosting the magnification made little difference. But through the binoculars, the colors stood out well on first glance. Apparently using two eyes is a big help for seeing color in faint objects.

The other case was the globular cluster NGC 5053 in Coma Berenices. It's quite large — nearly as big as neighboring M53 — but it's just one-fifth as bright, giving it very low surface brightness. And its borders are vague. That's exactly the kind of object where extra magnification is least useful. Indeed, I could not see NGC 5053 through my 70-mm refractor at any magnification. But there it was in my 15x70 — extremely subtle and hard to see, but visible nonetheless.

That replicates the common experience with other large, diffuse, low-surface-brightness objects such as the Triangulum Galaxy, M33. This galaxy is readily visible under dark skies, but novices usually have great difficulty spotting it under suburban conditions. And it's famously easier to see through binoculars, even quite small ones, than through telescopes with much larger apertures. The single situation where two-eyed vision is most helpful is for viewing large features with very subtle contrast. That's my experience, anyway.

Still, working better on two objects out of several dozen isn't a lot. And in many ways, my 70-mm refractor is much easier to use than the 70-mm binoculars. There's no practical way to hand-hold it, but it's designed to fit easily on a photo tripod, whereas photo tripods and binoculars are an awkward match at best.

In binoculars, you look directly toward your target, but the telescope is equipped with a "star diagonal" to let you look at right angles to the way the scope is pointed. This cuts both ways. As I said in an earlier article, the connection with the unaided-eye view that's so strong in 7X or 10X binoculars is beginning to stretch thin at 15X — but it's still there. The right-angle view pretty much destroys that connection as far as I'm concerned. On the other hand, viewing objects high in the sky is vastly more comfortable with the star diagonal.

If I really wanted, I could buy a straight-through erecting prism that would let me look through my telescope exactly as I look through binoculars. The fact that I've never so much as considered doing so indicates that the benefits of the star diagonal far outweigh the disadvantages for me.

So what are big binoculars good for? The answer is really pretty simple. Yes, I can see almost every object better through my 70-mm scope than through the 15x70 binoculars. And yes, it gives me an equally large field of view. But I have to switch back and forth between low magnification for the wide-field view and high power for the detailed view. To see both a wide field and detail at the same time, binoculars do better than any monocular telescope possibly can.

That's the bottom line. It's no accident that almost all conventional binoculars deliver extremely low magnifications relative to their apertures — it's because that's where two-eyed viewing helps the most. Binoculars are specialized for bright, ultrawide, low-power views. They're one-trick ponies. But it happens to be a very good trick.

Read the earlier posts in the binocular stargazing series:

A Tale of Three Binoculars: Part I
Three Binoculars: Part II