Vision for Photography

The eye is not a camera but there are some similarities.

The focal length of the eye is about 17 mm (2/3"). We see almost 180 degrees wide, 120 degrees high, in a goggle format.

PUPIL/ Iris/ diaphragm: restricts light. From f-2.0 to f-10. A ratio of 25 to 1 or 5 f-stops. Open is dilated, closed is constricted.

RODS: The more sensitive RECEPTORS of light in the RETINA. There are 120 million rods and they are color blind. The RODS drastically change SENSITIVITY with different light conditions. They need 50 minutes to completely DARK ADAPT and about 30 seconds to LIGHT ADAPT. The eye has a range of SENSITIVITY of 10 million to one. A candle 17 miles away can be seen on a clear night by a dark adapted eye. On the bright side we can VERY CAREFULLY look at the sun partly obscured a cloud. DO NOT LOOK AT A BRIGHT SUN DIRECTLY.

CONES: The less sensitive COLOR RECEPTORS in the RETINA. 6 million total. Three kinds of cones, red blue and green, together they can discriminate 200 different hues (colors) of light.

FOVEA: A 1/50 inch (mechanical pencil lead size) area of all CONES in the back center of the eye's retina. This is the only area of the retina with high resolution. Most of our "seeing" is dependent upon the fovea "scanning" our environment. Each of the 35,000 CONES in the FOVEA has it's own nerve to the brain and can send whole images to the brain at one time. A camcorder CHIP has about 400,000 receptors (pixels) but sends information in a stream. The FOVEA "sees" an angle of only 1.7 degrees, about the size of a pen held at arms length. Outside of the fovea the eye's resolution is very poor.

DEMO: Hold your hand outside your central vision with the fingers extended. Try to count the number of fingers on your hand without shifting concentration to them. You hand is just a blur.

RESOLUTION: The eye can see a wire the size of a pencil lead at the end of a football field. That is an angle of .004 degrees. (The fovea "sees" 1.7 degrees). If a TV set is 8 feet away and is 16 inches high your eye should be able to resolve the 400 lines the set is capable of. If the set is further away the TV set's resolution is wasted. The eye has the best resolution in bright light.

The eye has no FRAME LINES. Although the eye can make very fine discriminations in color and brightness, it cannot make absolute brightness measurements. It won't do as a light meter unless it has an object of known brightness in the scene to compare with.

Our eyes scan the world at an amazing rate. They move as fast as 1/50 of a second to look at some detail then stops for as short as 1/20 of a second, that information is sent to the brain and then the eyes scans again for new details.

The eye's scan is controlled by our brain, often with no conscious effort. Scanning is directed by our brain to things that interest us. Scanning is accomplished by the muscles in our eye socket that move the eye so rapidly. Things relevant to our physical survival are scanned first. Next we scan for our species survival. (The opposite sex.)

If we can see 180 degrees wide with the whole retina and only 1.7 degrees in high definition (the fovea telescope - spot light), you might say that the eye has a 1.7 to 180 degree zoom ability. That's equivalent to a 8 mm to 800 mm lens on a still camera or a 100 to 1 zoom ratio. This all happens automatically without our thinking about it. What's missing are the frame lines that cameras, TVs, movie screens and pictures have.

What's the point? Our eye does all this automatically and we aren't even aware of it. So it's easy for our eyes to fool us. The eye is not a camera, cameras only includes what we tell them to include.

If the fovea is for high detail seeing, what is the rest of the eye for? The rest of the eye helps us not trip into things, helps us drive down narrow alleys without hitting the walls or trash cans, or duck a fast ball. It helps us orient our bodies in space.

If the eye gave us only a foveal vision, like a telescope, life would be tough. Cup your hands to see a small opening and imagine walking around the world. We would trip, get hit and generally have a tough time. Having the fovea scan different images (accomplished by moving the eyeball in its socket) is a lot easier than moving our head around to see different things as most birds have to do.

Outside the fovea the rest of the retina has 120 million rods that only see in black and white. They help tell our brain how light or dark an area is relative to the rest of the image, remember, only the cones see colors. The rods are the sensors of the retina that control the pupil / iris so that the right amount of light enters the eye and doesn't over saturate the cones. Just like auto exposure on a many cameras the rods tell the brain to tell the iris to open or close the pupil. Along with the rods in the rest of the retina there are about 6 million more cones. These rods and cones combine to send a much less detailed images to the brain, but from a very wide angle of view. The information sent by the non-fovea area is shape, pattern and recognition information, but not fine detail. Much of this information is used unconsciously and is dependent upon previous learning to make sense of the images we have seen.

It is probably to our advantage that the rest of the eye sends only vague information, because an avalanche of information would be produced if the whole eye sent high quality information. Our brain would be overloaded.

How does this fovea, scanning so fast, form a still sharp image in the brain? Scientists have spent years studying it. Let's back track a little. The early thinkers thought that the eye sent out rays like light from the eye to perceive things. Sort of like touching things with our hands. Then in the middle ages someone thought that light rays formed an image in the eye and proved it by scraping the back layers off of the back of an ox's eyeball to expose the retina. An inverted image of the world was seen on the retina of the ox's eye. This was a break through and the theory that images were "seen" in the eye ball prevailed for some time. Now we know that images are formed in the eye and "seen" in the brain. Our seeing is dependent upon images acquired from birth on. What we see is dependent upon images seen before. People blind from birth don't see right away if ever at all if their sight is restored.

Here is a demonstration of why we DON'T see in the eyes. Look straight ahead and pan your head side to side. The image that you see pans just as it would in your camera. Now face straight ahead, but don't move your head and scan the area in front of you with only your eye balls. Don't move your head. Notice that the image that you see stays stationary even though your concentration moves and the image on you retina is moving. This proves that the image you see is not in your eye, but somewhere in the brain.

Good books on perception and seeing: In Library/"Eye and Brain" Gregory, Princeton Science Library. And "Sensation and Perception" by Goldstein. (Expensive)

© Copyright 1999-2004 Ron Dexter. All Rights Reserved.