In most people, vision is the most highly developed of all the senses. It provides a large amount of our daily input of information, and occupies much of our memory. More than this, however, we select and discriminate visually. We respond emotionally to images, finding pleasure in some while rejecting others.
Photography provides a technical means of recording the images that we see, using processes very similar to the mechanism of the eye and brain. Formation of an image, inside a camera body and the eye, depends on optics - the physics of light. Recording the image involves a different and more complex process, photochemical in the case of film, electronic in the newer field of digital imaging. (Show Photo) Also, in the eye, photochemistry is responsible for fixing the image briefly. However, in this case the information is then passed on to the brain by means of electrical impulses and chemical transmitters.
Thus the mechanisms of photography parallel those of sight very closely. Ultimately, however, when the photograph has been seen, composed, taken and produced, whether as a print or a slide, or displayed on a screen, it is processed all over again through the eye of the person looking at it. The final part of the process involves our interpretation of the photograph; does it give us satisfaction, pleasure, a visual jolt or a fresh insight?
Eye and camera
The eye and a still camera are both optical systems that produce, in the end, the same result: a recorded image. Naturally, there are many differences, but a knowledge of one helps an understanding of the other. The most significant difference is that the eye's operation does not end at the retina, which receives the image; the optic nerve connects the eyeball to the brain, where the image is processed and interpreted simultaneously (the psychology of vision is every bit as important as the physiology). The photographic image is processed through the camera, and then through the eye and brain. Both manipulate the same sequence of events: a lens system focuses the disorganized light from a scene by means of refraction; the resulting image is focused upside down on a light-sensitive surface; a tightly packed array of individual sensors use chemical reactions to fix a record of the image.
Forming an image
A lens, in both the eye and the camera, is a device for manipulating light, and is able to do this because of two factors: the difference in composition between it and the air, and the shape of its surfaces. Light travels in waves. A ray of light travels, with a waveform, in a straight line until it strikes a surface. Most materials deflect light (a ray bounces off them at an angle) but some transparent materials like glass and clear plastic allow the light to pass through.
The speed at which light travels is not constant, but depends on the density of the medium through which it passes. As light from the sun travels towards us through space, the earth's atmosphere slows it down a little, to just under 300,000 km per second. The extent to which a material slows down light passing through it is measured in relation to the speed of light through air, and is known as its refractive index. For example, glass, which slows light down to about 200,000 km per second, has a refractive index of 1.50.
The power to bend light is known as refraction, and is the basis upon which a lens works. As the ray of light leaves the glass on the opposite side and enters air again, it is bent. By shaping the glass so that the angle of its surface changes gradually- producing a curve, in other words - the direction in which the many different rays are bent can be controlled. In particular, they can all be made to converge on one point.
An object, any object, from a person to an entire landscape, can be thought of as an assembly of many points. Each is focused somewhere behind the lens, in a slightly different position from any other. Added up, the result is an inverted, two-dimensional reconstruction of what is in front of the lens, in other words, an image.
The eye
The eye contains a lens that acts in the same way as any other lens, although most of the refracting is done by the cornea at the front of the eye, because its surface has the greatest slowing effect on light as it enters from the air. The lens is responsible for the fine focusing movements needed to deal with nearer objects, and it does this by changing its shape.
The retina, at the back of the eye, is the image recorder, the equivalent of film or an electronic imaging chip. Both silver halide film and electronic imaging are now part of photography, and it is interesting that the retina manages to combine elements from both: photochemical reaction, digital sensors and electrical transmission. It contains a dense pattern of light-sensitive receptor cells, known as rods and cones, and the density of the photo-receptors helps determine the eye's ability to resolve fine detail. In the central region of the retina, the fovea, they are packed tightly together, and it is this that gives us our sharpest vision. Away from the fovea, the cells become more spread out, and correspondingly our peripheral vision is more blurred. In some ways, the rods and cones are the equivalent of grains in the emulsion of a film. All contain chemicals that undergo a reaction when struck by light, triggered by the electrons being activated. The individual rods and cones react in proportion to the light intensity, and this allows the eye to discriminate between tones. To make color vision possible, the cones are of three different types, each sensitive to one color: blue, green or red. The eye and brain assemble the fine pattern of these three colors into a full color image.
Because of their different sensitivities, the rods and cones are like two interlocking emulsions. The cones, for daylight color vision, have one pattern; the rods, for total vision, have a more widely spaced, coarser pattern. To stretch the analogy, it is like a combination of slow, fine-grained color film and fast, grainy black-and-white emulsion. As the light alters, stalks contract and push either the rods or cones in front of the others, in order to get the best balance of vision.
In fact, a closer comparison can be made with an imaging chip. The 125 million rods and 5.5 million cones in a retina receive the image like a CCD array. Their neural connections, like the circuitry on the chip, send impulses to the brain for processing into an image. Away from the fovea, the neural connections for numbers of rods and cones converge onto fewer optic fibers; this is a kind of image compression, and results in peripheral vision. In the fovea, where there are only cones, there is little convergence, and the signals from individual cones give much sharper vision. In all, there are only about l million optic nerve fibers leading to the brain. In addition, there are lateral connections that help the eye distinguish the contrast between edges.
Color sensitivity
In reasonably good light, the eye can distinguish millions of colors. It manages this because the cones are divided into three types: those sensitive to blue, to green and to red. The same thing happens in film, although there it works in the form of three layers of emulsion, each sensitive to one of the three separate colors. Blue, green and red together embrace the full spectrum, so that by varying the input from each, any other color can be made. In the case of human vision, this occurs in the brain.
The three kinds of receptors are not equally efficient. We are least sensitive to blue and the overall result is that the eye is most sensitive to yellow-green. We cannot see ultraviolet, to which film is sensitive, because the eye has two yellow filters, one over the lens and one over the fovea. This probably evolved to reduce chromatic aberration, at its greatest in violet and ultraviolet light.
Perception
The greatest difference between human perception and a camera lies in the way the brain builds up and recognizes an image. In each eye, the resolution falls away sharply from the fovea, which is smaller than a pinhead and covers an angle of only 1.7 deg. The views from both eyes overlap and combine. Although such a view is fine for concentrating the attention on one part of the scene, it would be useless for taking in the whole sweep of, say, a landscape, were it not for the way in which the eye works. While we are rarely conscious of it, the eye actually scans a scene constantly with small, jerking movements that each last only a few milliseconds. By means of these saccadic movements, as they are called, the eye and brain build up a sharply perceived pattern of a large scene, in a way quite unlike that of a camera.
