Why the Human Eye Sees a Total Solar Eclipse Better Than a Camera

As we get closer to the day of the "Great American Solar Eclipse," we'll be hearing more and more about the subject of eye protection and just how important it is to properly observe the eclipse, lest you do damage to your eyes.  

This column, however, is not going to speak directly about the do's and don'ts of eclipse gazing. Instead, I thought that it would be worthwhile to discuss how amazing these optical instruments that nature has provided are.  

Our eyes, after all, help most of us carry on almost all the activities of our daily lives. If you think about it, the eye is also our only sense organ capable of dealing with objects at great distances. Our touch and taste senses require actual contact with a specific object. To smell something, we must be relatively close to an object that emanates odor-causing particles. We can hear things happening off in the distance, like an explosion or a crack of thunder, but to hear things that are happening at greater distances (or lower volumes), we would need a phone or radio. [A Solar Eclipse Can Blind You (Read This Before Looking at the Sun!)] 

But with our eyes, the sky is the limit, both literally and figuratively. We can step outside on any clear night and, with our eyes alone, see stars located hundreds or even thousands of light-years away. And if you know exactly where to look, without any optical aid, you can perceive the light of the great Andromeda galaxy, estimated to be more than 2 million light-years from us!

[Editor's note: Visit Space.com on Aug. 21 for a live solar eclipse webcast from NASA, beginning at 12 p.m. EDT (1600 GMT).] 

A gelatinous blob

Yet, if you examine the eye carefully, the first thing you would likely think to yourself is, "How in the world can we see anything with it at all?"  

Unlike the clean, clear and transparent lenses that are used in eyeglasses or other viewing instruments, the human eye seems to amount to nothing more than a gelatinous blob. And yet embedded in this blob are numerous circular rings — somewhat resembling those in an onion — that make it flexible.

And that is what sets the eye apart from a standard lens in a camera or telescope; that flexibility allows the lens of the eye to change focal length by altering its shape. When we look at close objects, those muscles surrounding the eye lens squeeze and make it more curved. Alas, as we become older, the pliability of the lens diminishes, so we need glasses or contact lenses to help them out. 

To focus an image onto the retina, our eyes make use of a three-element lens system: the main lens, the cornea and the fluid that lies between them (called the aqueous humor). The cornea does the focusing, while the lens accommodates for distance. 

Much better than a camera!

People sometimes compare the eye to a camera, but this comparison is not really apt. Wherever we point a camera lens, the detail that is recorded is virtually uniform throughout the entire image. 

But because of the eye's construction, we're viewing objects within just a very small area in the central part of the retina called the "fovea centralis." It is here that the so-called "cones" — a bundle of resolution elements — are clustered more thickly than anywhere else. The rest of the retina simply "fills out" the picture. 

You can grasp the importance of the fovea centralis when you're reading a book or the text on a computer screen (as you're probably doing right now). A camera is completely stationary when it's used to photograph such media, taking in the entire image of the page or screen. Not so with your eyes! They must move from word to word to place each one individually on the fovea. [How to Film or Photograph the 2017 Solar Eclipse Like a Pro]

Be careful!

Now, here is why you should be very careful when viewing the upcoming solar eclipse: The fovea centralis covers only about half of one degree of angular extent in our visual field. And the angular diameter of the sun happens to also measure half of one degree across. 

So, those who gaze at the sun with inadequate eye protection during the partial phases are courting possible blindness. That is, they're risking the destruction of the fovea by subjecting it to a dazzling source of light that's just the right size to damage it!

You can see it all at once

Another advantage of our eyes over cameras is the far greater versatility of seeing "the whole picture" at once.

Those who attempt to photograph the beautiful corona of the sun during totality know that a single exposure will bring out only certain features. A short exposure will show the inner part of the corona (the sun's outer atmosphere), and if the exposure is very short, the picture might also bring out some of the ruddy prominences (loops of hot material protruding off the surface of the sun) that might be arrayed along the dark edge of the moon. A short exposure might even reveal individual features within the corona, such as brushes, rays, streaks and streamers of light.

On the other hand, a long exposure will show all but the subtle streamers of the outermost corona, but will wash out the coronal brushes and streaks, as well as the prominences. 

Some photographers in our digital era have learned to cheat, by stacking images and creating a composite that shows all the features mentioned above on a single image.

But your eye does not need to stack images to create a composite. You can see everything — the prominences, the delicate streaks and the streamers — all at once, and all with far greater clarity and with much finer detail than even the best photograph can show.  

Some people say, "I don't need to see a total eclipse — I've seen one on television," or, "I have a book with some beautiful pictures of the sun’s corona," but those people don't know what they're missing.

Dark adapt

And lastly, the pupils of our eyes automatically compensate for the amount of extraneous light by getting smaller under bright conditions and growing larger in dark environments. It's the iris of the eye that regulates the amount of light that enters the eye by changing the size of the pupil.  

So, some veteran eclipse watchers cover one eye with a dark patch about 15 to 20 minutes prior to totality and immediately remove that patch once totality sets in. Of course, while the eye was covered, the pupil grew wider.  

When the patch is removed during the darkest phase of the eclipse, the difference in what the uncovered eye sees versus what the covered eye sees becomes fairly dramatic. The darkness of the sky is more accentuated to the covered eye, perhaps allowing you to see a few more stars and possibly see the corona extend out to far greater lengths as opposed to what you would see pictured in a magazine or a book.  

Editor's note: Space.com has teamed up with Simulation Curriculum to offer this awesome Eclipse Safari app to help you enjoy your eclipse experience. The free app is available for Apple and Android, and you can view it on the web. If you take an amazing photo of the Aug. 21 solar eclipse, let us know! Send photos and comments to spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for Verizon Fios1 News, based in Rye Brook, New York.

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