Seeing is not Perceiving: Reflections on Foveal Vision, Photography and Telescopic Observing

In my last post, I embedded a nicely-made video from Nasa about the conjunction of Jupiter and Venus. Something struck me in the middle of that video, when the narrator began to explain why it might be that we find planetary conjunctions so beautiful. The explanation made a lot of sense to me, and cleared up another mystery that I have been wondering about for years. Or at least, it offered a plausible explanation. But first, here’s Dr. Tony Phillips, who wrote (but I don’t think narrated!) that Science at Nasa video:

There’s something mesmerizing about stars and planets bunched together in this way—and, no, you’re not imagining things when it happens to you.  The phenomenon is based on the anatomy of the human eye.

Cold and Spellbinding (fovea)

The fovea is responsible for our central, sharpest vision. “Your eye is a bit like a digital camera,” explains optometrist Dr. Stuart Hiroyasu of Bishop, California. “There’s a lens in front to focus the light, and a photo-array behind the lens to capture the image. The photo-array in your eye is called the retina. It’s made of rods and cones, the organic equivalent of electronic pixels.”

There’s a tiny patch of tissue near the center of the retina where cones are extra-densely packed. This is called “the fovea.”

“Whatever you see with the fovea, you see in high-definition,” Hiroyasu says. The fovea is critical to reading, driving, watching television. The fovea has the brain’s attention.

The field of view of the fovea is only about five degrees wide. Most nights in March, Venus and Jupiter will fit within that narrow cone.  And when they do—presto!  It’s spellbinding astronomy.

I never really knew about foveal vision, but I tried it out. If you concentrate, you can actually see that your eye only focuses on a small area at any given time. I was standing in my kitchen, facing the kitchen sink, and I realized that I could see the Bon Ami canister and an empty can of Yuengling beer sharply, but everything around that was blurry. To see this effect for yourself, you need to do a neat trick I learned as an astronomer, that is, focus your eyes on one thing but concentrate on what’s around that sharp area. It’s called averted vision and it’s how we can spot things that are dimmer than we can detect with our foveal vision, though that’s a topic for a later post. This is how I saw the scene:

How I Saw It (In digital photography, this is actually called a foveated image.)

Interestingly, though I perceived both objects as sharp, when I tried to read the labels on each, my eye had to move slightly from one to the other. The fovea represents just 1% of the area of the retina, though 50% of the optic nerve is devoted to the information it collects. That 1% is not uniform however; the central area of the fovea is the most densely packed area of cones in your eye. So the area of highest resolution is just a small portion of the 5% of foveal vision. To me it looked more like 2 degrees. Most amateur astronomers know that a fist, held at arm’s length towards the sky, is about 10 degrees wide. So I went back to the Bon Ami and Yuengling, and gave it a 1968 Mexico City Olympic Black Power salute:

They are indeed about half a fist-width across, or 5 degrees. What I can read, one or the other, is about 2 degrees.

I never really noticed this before, despite being a photographer, artist and astronomer, mostly because of my brain. It ain’t broke, but that’s the point; it’s just doing its job, and quietly. When I look at that side of my kitchen (to see, for example, deer in the back woods, or to ascertain how many dishes my wife has left for me to clean), I feel like I’m seeing the whole tableau sharply. It looks more like this:

How my brain perceives the scene. And how the camera with a normal lens records it.

This is because most of the time, my eye scans around a scene, sweeping back and forth so my fovea can gather high resolution information about the scene, and then my brain handily integrates all these little sharp views over a wider area that the other parts of the retina are collecting, filling in the broad broad retinal strokes with fine detail.  This integrated image is much wider than the foveal field of view, about 55 degrees, and it’s called the cone of visual attention. I photographed the scene above with the equivalent of a 50mm lens on 35mm film (in reality it was a 35mm lens on an APS-C sized sensor chip, but the field of view is about the same), which happens to have a field of view near 55 degrees. The classic 50mm was often called a “normal lens”, for this very reason.

The human visual system (eye and brain) can actually perceive an area much wider than that (humans have a vision span of about 120 degrees, a lot of that peripheral vision so we can be alert to sabre-toothed tigers, rivals for our mates, and cops) but can only do the integration trick across the cone of visual attention. A fundamental part of our visual perception involves exclusion. Faced with a 360 degree world, we absorb it in 55 degree increments, and scrutinize only 5 degrees in a single moment. As a photographer, this makes total sense to me. Photography, as I always tell my students when I’m teaching travel photography workshops, is as much about what you don’t show as what you do show in the frame. Human vision also explains why super-wide-angle images look unnatural to me and yet can also hold an alluring beauty, sharply capturing a much wider field of view and representing it in a space (a photograph) we can appreciate within our cone of visual attention. It’s not something humans could really do before except perhaps in Persian miniature paintings. I’m oversimplifying a huge topic here, but for  a tad more, read Roger Cicala’s blog post, The Camera Vs. The Eye. (Roger’s website is a commercial site where you can rent lenses, though Roger’s blog posts are real treasures to the photographic community.)

These two concepts, the narrow, high-resolution foveal vision and the cone of visual attention integrated by the brain, explain a few things I have noticed visually in my decade as an amateur astronomer, but never really understood.

At my last observing night for the astronomy class, I showed the students a variety of winter double stars, including Gamma Andromedae, also known as Almach. About a third of the stars in the Milky Way are binary (or larger) systems, meaning stars that orbit around a common center of gravity. When we look at most multiple star systems with our naked eyes, we see them as a single star. But some of these pairs and triplets and quadruplets (etc.) are close enough to us and/0r far enough apart that we can resolve or “split” them with a telescope. Here’s a sketch of Almach by the wonderful telescopic artist, Jeremy Perez:

So back to my observing night. We had cycled through a series of double stars, and I commented (truthfully) that I found double stars to be some of the most beautiful objects to observe in the night sky. I then reflected that I really couldn’t explain why it was that when you stuck two bright pinpoints of light together, and maybe gave each one a different hue, they suddenly, well, got pretty. But it seemed to be the case.

Adding what I know now about foveal vision, it makes much more sense to me. It’s the same concept that Tony Phillips and  Stuart Hiroyasu explain above when talking about the conjunction of Jupiter and Venus. The beauty is in the high resolution points of light, glimmering right in the zone of our vision where we can best appreciate them (and since the fovea is also the area of the eye most packed with color-seeing cone cells, it’s also where can most appreciate the color contrasts that many binary pairs exhibit.) I will now feel entirely justified when someone asks me what’s so beautiful about double stars, and I can also revel in a chance to actually explain (well, okay, suggest a plausible explanation) as to why we see this particular beauty.

Take a second look at Jeremy’s illustration, but this time pay attention to what’s around Almach A and B (as we call the individual stars in a binary system, A being the brighter). If A and B are about 5 degrees apart, then the eyepiece field of view, represented by the circle, is about 55′ wide, or about the same span as the cone of visual attention.

This brings me to my last ah-hah! moment. Each eyepiece design (and there are hundreds) will yield a different apparent field of view (AFOV), or the angular size of the image window that you can see through the eyepiece.  Older eyepiece designs like Kellners, Abbe Orthoscopics and Plossls have an AFOV somewhere between 35 and 50 degrees. When you look through these eyepieces, you can “see” the whole circular image at once. More modern eyepieces, in large part innovated by the optical house of TeleVue, have given us AFOVs as large as 68, 82, 100 and now even 120 degrees. That last eyepiece has such a great AFOV that the telescope and eyepiece effectively disappear. It must be a similar view to that of an astronaut on a spacewalk, his visor turned away from the nighttime earth to the wider cosmos. Indeed, the spacewalk analogy has been used by Televue’s founder Al Nagler to describe the effect he is after in his eyepiece designs.

I’ve used all of these eyepieces at some point in the last ten years. I started in the hobby with a mix of modest orthoscopics and plossls, and have since tried many of the wider-field designs. I even tried the first 100-degree eyepiece released by Televue, the 13mm Ethos. It’s a pretty amazing effect. You look into the eyepiece and pretty much everything you see is the telescopic image. You have to move your eyeball–and even your head!–around to take in the whole view. People have reported getting motion sickness from slewing their telescopes while looking through an Ethos eyepiece.

I get what Al Nagler is after. It’s a neat experience, the technology almost disappearing entirely, allowing people to really lose themselves in the cosmos, so much so that they might end up flat on their faces in the dewey grass if they are susceptible to disorientation!

As much as I enjoyed looking through the Ethos, I still gravitate towards the views the older and narrower eyepieces yielded. And now I think I know why; they neatly frame out the cone of visual attention. I can take the whole view in with my retina, the fovea scanning around to supply the details. With the 10o-degree Ethos, I had trouble relaxing my eye, focusing on a single tableau. It was excess. The old 45 degree orthoscopic, or the 68 degree eyepieces I favor now, seem a happier medium, allowing me to look at the universe one digestible bit at a time. Rather like a photograph taken with a 50mm lens. It enhances my appreciation of the beauty of the universe.

And maybe it does more than that. I have to wonder about the evolutionary roots of our visual system. Why did evolution favor our 120 degree eyes, less than half of which we would “see” clearly at any given time, and only 5 (or even just 2!) degrees of which we could really resolve in high definition. How would our civilization and worldview be different if instead, like rabbits, we had nearly a 360 degree visual span? I’ve never seen a rabbit do calculus or paint a painting, so maybe that’s the answer. We learned about the world by concentrating on small bits of it at a time, and integrating those little bits into a larger picture. Science itself works like this, and maybe it’s a way of transforming information into knowledge that’s wired into our brains–and our eyes.


5 thoughts on “Seeing is not Perceiving: Reflections on Foveal Vision, Photography and Telescopic Observing

  1. Doug,
    When I was in the Army, we were told that we would see better in low light if we would focus our eyes slightly above whatever it was were trying to see. It seems that this would place the light or image on a part of the retina that was more sensitive… It seemed to work.

    Hope you and family are doing well. Really enjoy your blog. You must come out here and do a workshop.

  2. Doug,
    These articles are greatly superb reading and
    they’re very educational and benefit for our references.

    Ken Kressler

  3. Ray,
    They were right about that! That’s averted vision. The fovea has no rods in it, the cells that are really sensitive to light intensity, but the rest of the retina, especially the part the records peripheral vision, is packed with them. There are some astronomical objects (the Blinking Planetary) where, if you look directly at it, it disappears, and when you avert your gaze, it pops back into view. Great to hear from you!

  4. Good article Doug. A new teacher developes averted vision early for survival purposes. Eventually it spreads to “eyes in the back of thier head”.. ,. . Don

  5. Doug, Thanks for making some of the connections here with Foveal and Averted vision. Very insightful. It gives a whole new meaning to ” Beauty is in the Eye of the Beholder”


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