“Physiology of the Eye” — a return to first principles

[Heroique]

All the threads lately about the “subjectivity of art” are, naturally, provoking a lot of frustration and bewilderment, so I thought one about the “physiology of the eye” might provide a settling influence – though I recognize that’s a tall order. ;)

Here are a couple of interesting views from people we know, and I’d enjoy hearing your reactions…

The camera, AA says in The Camera, is “analogous” but not “identical” to the eye, and he then offers a few first principles about how the eye actually works:

“The camera, for example, does not concentrate on the center of its field of view as the eye does, but sees everything within its field with about equal clarity. The eye scans the subject to take it all in, while the camera (usually) records it whole and fixed. Then there is the film, which has a range of sensitivity that is only a fraction of the eye’s. Later steps, development, printing, etc., contribute their own specific characteristics to the final photographic image.”

I’ve also come across the following piece by art critic Robert Hughes who has his own claims about the eye. The following remarks (from The Shock of the New) lead-up to his discussion of Cezanne, but like AA, he wants to establish some first principles about how the biological eye works:

“Look at an object: your eye is never still. It flickers, involuntarily restless, from side to side. Nor is your head still in relation to the object; every movement brings a fractional shift in its position, which results in a miniscule different of aspect. The more you move, the bigger the shifts and differences become. If asked to, the brain can isolate a given view, frozen in time; but its experience of the world outside the eye is more like a mosaic than a perspective set-up, a mosaic of multiple relationships, none of them (as far as vision is concerned) wholly fixed. Any sight is a sum of glimpses.”

-----
It seems to me that Ansel Adams and Robert Hughes want to make the most simple and objective claims about the physiological eye, before going on with their instructive work – but can one reconcile their claims? One of my initial reactions is that while AA says the eye “scans,” Hughes would “roll his eyes” at the claim that it “concentrates on the center of its field of view.” Just not possible, one might hear him reply. Likewise, AA might be skeptical about all of Hughes’ talk about “mosaics.”

Or, perhaps the correspondence between their views is greater than any difference.

How would you clarify the matter?

[Drew Wiley]

I had huge arguments with my aunt about this. Seems that this kind of thing was a
vogue idea in the 1930's. She was blind in one eye so rolled the good one around to
capture the whole view, so was hardly objective in relating to how ordinary folks see
things. I really believe that at times I grasp a vast number of details all at once, and
it can be a bit spooky. Now I try to relax more and take my time looking. But either
way, it is the view through the groundglass which determines actual composition
and not my initial reaction.

[paulr]

Cezanne's ideas are much closer to contemporary understandings of vision. Ansel gets the physiology right, but not the neurology, which is more important. Possibly the bigest difference between human vision and photographic vision is that our eyes scan a scene in pieces, and our minds assemble a composite ... more collage than mosaic, because the pieces aren't from precisely the same time. Or even the same vantage point.

It's one reason you can drive past a spot and think "I saw a great picture there," but when you return with a camera, nothing you see corresponds with memory.

[mdm]

The eye is exactly like a camera, only difference is that what you actually see has first been run through the old super computer between the ears and as a result reflects years of conditioning, learning and many assumptions. Lots of times we see something but we only assume we have seen it, or we see something because we expect to see it, or we heard it or felt it or sensed it, so we see it. And in that respect a blind person sees just as well as you or I. Perhaps a photograph is the only thing a blind person cant see. So in my opinion none of the senses should be considered in isolation.

David

[Vaughn]

Memory, for some reason, has great depth-of-field, and can handle very high and very low light and contrast situations. Now if only I could have the film and equipment to match what memory can easily master...

The eye is exactly like a camera, but of course totally different.;) Since the eye was here first, I would say that the camera comes close to being like an eye. A movie camera a bit more like an eye. I don't know where the 360 degree movie at Disneyland fits into it all (may not even be there anymore). Then there is 3D, but as the Romans said, "That's entertainment!"

[rguinter]

As a human factors engineer, interested in understanding the role of human beings in the workplace and how to adapt the workplace to better fit the human design, I have studied the physiology of vision very thoroughly.

It is like a camera in some very simple ways, but mostly it is vastly different. And a complete treatise on its physiology would fill hundreds of pages.

But there are some simple concepts that can be related here that explain both Ansel Adams’ and Robert Hughes’ statements.

The eye itself is really (physiologically) two eyes in one. Our retinas are both scotopic and photopic which means in general terms, night vision instruments and day vision instruments respectively.

The neural connections from the sets of rods and cones (that produce scotopic and photopic vision) are completely separate circuits and are analyzed separately as day and night vision. Actually during the bright light of day, most of the rods of scotopic vision, which are extremely sensitive, are saturated and inactive. And at night they become active when the cones, which are many times less sensitive, become inactive due to insufficient stimulation.

So the bottom line is we see reasonably well in both daytime and night-time using one or the other set of circuits in our dual retinas; but we don't see extraordinarily well either day or night because of this compromise.

Regarding photopic vision it is trichromatic with 3 different cones of different sensitivity curves to wavelengths of light: one at 420-, one at 534-, and a third at 564-nanometers. These are the primary visual colors blue, green, and red.

There is significant overlap among the three curves and the relative stimulation of all three types of cones at once allows the brain to differentiate the palette of colors that we know generally as red, orange, yellow, green, blue, and violet.

Another major compromise is that the neural circuit wiring of the retina is not a "one-for-one" mapping of cones and rods into the visual cortex of the brain.

Our fovial vision (the central part where only cones reside) are in general mapped one-for-one. This gives high resolution and acuity for fovial vision in an analogy equivalent to individual pixels in a digital camera. But fovial vision is a very small area compared to the total retina and allows for only a very narrow field of clear view.

As an experiment to see what I mean, try closing one eye and focusing on a word in the middle of the line above. Don't let your focus drift to other words alongside. Then, while focused on this word, try reading words to the left or right without moving your eye.

If you are able to keep your eye still long enough you will see how very narrow fovial vision really is. The words to the left and right are in peripheral vision and essentially unreadable.

Neural circuit connections in peripheral vision include many receptors connected in parallel that are not mapped one-for-one into the visual cortex. This is why both Ansel Adams and Robert Hughes quoted above suggested that the eyes have to move around a scene so that the brain can assimilate a clear picture of it over a period of time. The phenomenon is known as physiologic nystagmus.

Bob G.

[mdm]

Thank you for an excelent post.

[ic-racer]

The fovea has a pretty limited angle of view, so your eye has to scan around quite a bit to make any sense of the world. The lens totally sucks when used wide open. The chromatic aberrations are pretty severe.

[Heroique]

...our eyes scan a scene in pieces, and our minds assemble a composite...more collage than mosaic, because the pieces aren't from precisely the same time. Or even the same vantage point...

This might explain why that collage-mosaic feel of Cezanne (below) seems so “real” to me.

Now I see where Hughes was going with his eye-physiology.


...the rods...at night become active when the cones, which are many times less sensitive, become inactive due to insufficient stimulation...

And this helps explain why at night, faint stars in my peripheral vision seem to “fade-out” when I try to look straight at them.

Maybe my night-vision and art-vision see more reality than I thought.

[Bill Burk]

Here is a book worth reading on the topic. Bob, if you haven't read it - I think you would really enjoy it.

The Eye: A Natural History: Simon Ings

It is highly readable, covers a broad variety of visual topics, evolution of the eye (several independent times), historical beliefs, early drawings from scientists and recent work.

[Drew Wiley]

Perception is only partially optical. The eye and mind interact in very complex ways
which are conditioned by training and experience, among a lot of things that are
relatively mysterious. Symphony conductors sometimes develop the ability to hear
many different individual instruments at the same time they recognize the tenor of
the entire orchestra. Visual artists sometimes develop analogous abilites, but connected to visual perception. It's very intriguing. The need for rhythm, harmony,
order, connection, seem to be universal characteristics of mankind. But there are
different ways we solve this.

[Bruce Watson]

This topic gets an amazingly through review in the Margaret Livingstone book Vision and Art: The Biology of Seeing (http://www.amazon.com/Vision-Art-Biology-Margaret-Livingstone/dp/0810904063). While not aimed at photography, it nonetheless applies to photography. It explains the human visual system in depth, and explains how some iconic paintings work so well with the human visual system. Dr. Livingstone knows her stuff.

Between the Livingstone book and Richard Zakia's Perception and Imaging (http://www.amazon.com/Perception-Imaging-Third-Photography-Seeing/dp/0240809300/ref=sr_1_2?ie=UTF8&s=books&qid=1288441544&sr=1-2), you'll have plenty to think about.

[BetterSense]

Perception is only partially optical. The eye and mind interact in very complex ways
which are conditioned by training and experience, among a lot of things that are
relatively mysterious

For a cool experiment, go into your darkroom or a darkened bathroom at night, and stare straight ahead, into a corner or something. While you are doing so, fire off a camera flash, or flick the light on-and-off quickly. Do not move your gaze. If you can succeed in not moving your eyes, you will be abe to see a complete image of the room for many seconds after the light is gone. The instant you move your eyes, though, it disappears to blackness again. A very interesting feature indicating that your brain keeps visual information in memory for as long as it 'could be' valid, but then erases it as soon as the next update is called for.

[rguinter]

For a cool experiment, go into your darkroom or a darkened bathroom at night, and stare straight ahead, into a corner or something. While you are doing so, fire off a camera flash, or flick the light on-and-off quickly. Do not move your gaze. If you can succeed in not moving your eyes, you will be abe to see a complete image of the room for many seconds after the light is gone. The instant you move your eyes, though, it disappears to blackness again. A very interesting feature indicating that your brain keeps visual information in memory for as long as it 'could be' valid, but then erases it as soon as the next update is called for.

Yes that is an interesting phenomenon. And works best if the eyes have been (partially) dark adapted.

It is also partly physiological. The change of chemical state that occurs in rhodopsin when illuminated is rather slow to reverse itself when illumination is removed... The reason for afterimages and the controlling factor in the process of "dark adaptation."

Cheers. Bob G.

[Greg Miller]

I had huge arguments with my aunt about this. Seems that this kind of thing was a
vogue idea in the 1930's. She was blind in one eye so rolled the good one around to
capture the whole view, so was hardly objective in relating to how ordinary folks see
things. I really believe that at times I grasp a vast number of details all at once, and
it can be a bit spooky. Now I try to relax more and take my time looking. But either
way, it is the view through the groundglass which determines actual composition
and not my initial reaction.

Your Aunt's vision was actually closer to a camera than the rest of us. Most of us have stereo vision, but your Aunt and most of our cameras had/have mono vision.

Beyond that we have to consider the properties of our film and paper. Most color film has dramatically less dynamic range than the human eye, so scenes are rendered with more contrast than we see with the eye.

And then the brain's ability to self color correct (which film does not). And our memory of color is quite poor and also degrades as time since viewing the scene increases. The list goes on and on...

[Heroique]

Yes, stereo visualization often interferes with my pre-visualization.

I close one eye. I am Cyclops.

It’s interesting to note that both AA and Robert Hughes, in their “first principles” of physiology, use only the singular word “eye,” never its plural form. In quite the opposite direction – yet to confirm AA and Hughes – Bob takes one eye, and divides it into two: “The eye itself,” he says, “is really (physiologically) two eyes in one” (post #6).

There’s a frightening consistency going on.

[Mike Anderson]

Yes, stereo visualization often interferes with my pre-visualization.
....

I'm pretty new to photography and I think one of the hardest things to learn is how to imagine (or predict, or visualize) a scene in 2 dimensions.

...Mike

[Leonard Evens]

It is in fact impossible to see anything in the normal sense without your eye scanning the scene. An interesting experiment involves using some sort of apparatus to fix an object so that it moves with your eye. After a short while the object disappears from view.

[Heroique]

An interesting experiment involves using some sort of apparatus to fix an object so that it moves with your eye.

After a short while the object disappears from view.

I wish some of my landscapes had moved w/ my camera.

Then they’d come into view.

:rolleyes:

[paulr]

I'm pretty new to photography and I think one of the hardest things to learn is how to imagine (or predict, or visualize) a scene in 2 dimensions.

This is a way in which view cameras make photography easier. When you look at the ground glass, you perceive it as a picture that's already been flattened to a 2-dimensional projection.

For some reason when you look through a viewfinder (even though the viewfinder may actually be based on a ground glass, like in an slr) the sense of looking through a window at a 3-dimensional world persists.

[cowanw]

The fovea has a pretty limited angle of view, so your eye has to scan around quite a bit to make any sense of the world. The lens totally sucks when used wide open. The chromatic aberrations are pretty severe.

Probably makes a great soft focus lens and explains why we always think our Number one squeeze actually is attractive.
Regards
bill

[rguinter]

The fovea has a pretty limited angle of view, so your eye has to scan around quite a bit to make any sense of the world. The lens totally sucks when used wide open. The chromatic aberrations are pretty severe.

Yes they are (the chromatic aberrations) that is. And the refractive index of the vitreous humour varies with the wavelength of light rays passing through. This can be quite a problem when one is viewing a field with significant amounts of both red and blue which are at opposite ends of the visual spectrum.

I've seen some professional (powerpoint) presentations done with the authors using red letters on a blue background and vice versa. Very hard to focus on their slides. Bob G.

[rguinter]

It is in fact impossible to see anything in the normal sense without your eye scanning the scene. An interesting experiment involves using some sort of apparatus to fix an object so that it moves with your eye. After a short while the object disappears from view.

Yes the disappearance is quite interesting indeed. I've heard about the experiment but not participated in it.

Nystagmus and saccadic eye movements are necessary because of the long half life of cis-retinal. The conversion of trans-retinal to the cis- isomer occurs almost instantaneously (in femtoseconds) when it is struck by photons. And this conversion is what initiates a signal in the attached nerve fiber to the brain. But the process of getting back to the trans- state takes much longer when the photons stop coming.

So if one could keep the eye perfectly still with the same image constantly on the same part of the retina... all the receptors constantly receiving photon stimuli would become saturated (i.e., all their trans-retinal would become converted to cis-retinal) and those receptors would cease sending signals to the brain.

Bob G.

[Heroique]

...fix an object so that it moves with your eye. After a short while the object disappears from view.


Yes the disappearance is quite interesting indeed. ...The receptors...would cease sending signals to the brain.

When this happens, what exactly fills-in the place of the “vanished” object?

Would it look like the “blind spot” we all have on the retina?

This is beginning to sound like magicians and rabbits.

[Mike Anderson]

This is a way in which view cameras make photography easier. When you look at the ground glass, you perceive it as a picture that's already been flattened to a 2-dimensional projection.

For some reason when you look through a viewfinder (even though the viewfinder may actually be based on a ground glass, like in an slr) the sense of looking through a window at a 3-dimensional world persists.

I'm starting to understand that. I think the important difference is looking at the flattened image with 2 eyes is more informative than looking at the flattened image through one eye.

...Mike

[rguinter]

When this happens, what exactly fills-in the place of the “vanished” object?

Would it look like the “blind spot” we all have on the retina?

This is beginning to sound like magicians and rabbits.

My guess is yes it probably would.

The blind spot is a true "blind" spot for vision. It is the off-center position on the back of the retina where the optic nerve is attached.

There are plenty of text books with diagrams that can be used for demonstrating the blind spot. A google search will pick some up. Me I used a picture of an orange and an apple when I ran seminars in human-factors. While focusing on the orange with one eye open and moving the picture around a bit one sees the apple disappear.

While doing this one can actually become aware of what the brain does constantly... it overlays adjacent visual information to fill in the spot. Much the way a "clone stamp" tool works in photoshop.

With this process we are never aware of the blind spot except for those few moments when doing the demonstration I described above.

Cheers. Bob G.