How human vision works
How human vision works | ||
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In all the photographer talk about resolution, focus, and depth of field—which we'll get to after we lay a foundation on these pages—it's easy to lose sight of the ultimate restriction to how much detail you can see in a photo—the human eye. The biggest asset in seeing detail is imagination, the ability to make logical connections to fill in the details that are smaller than digital film can capture or photo printers can reproduce or the eye can discern. The tricks our perceptions pull on us are particularly important in digital photography because the technology squeezes the size of digital 'film' to an area smaller, usually, than even the 35mm frame. That pushes the concepts of resolution and detail into even finer distinctions and even smaller realms. That's where human vision encounters the, no kidding, Airy discs and circles of confusion…strap yourselves in and keep your hands inside the cart at all times for this one. A point of light, like other points having no width or breadth, is an abstraction. The closest thing to a physical point of light is a spot of light called an Airy disc (after astronomer George Airy, who first noticed the phenomenon). An Airy disc is an artifact. It doesn't exist in nature, but instead is created by the interaction of light diffraction and human vision. What began as a point of light changes when it passes through air, a lens, and an aperture. (That includes the eye's lens and pupil. An Airy disc doesn't exist at all unless someone or something looks at it.) Diffraction is caused when impurities in the air and lenses scatter light coming from a point. The pattern that the diffraction creates is predictable. It is concentrated in the center of the disc, fading out in soft, con- centric circles until it reaches the disc's rim, where the light is again concentrated. If you back away from any two objects, perspective makes them appear to move closer together. As two Airy discs approach each other, they reach a point where the distance between them is equal to the radius of one of the discs, about 0.27 microns (Rm). This is called the Rayleigh limit When objects cross the Rayleigh limit, they can no longer be distinguished as separate objects. They lose visual information in the form of detail, but they might produce new, gestalt information. If you look closely at a brick wall, you'll see that it is made up of individual bricks with rough textures and a mixture of reds, all surrounded by mortar of a different color and texture. Back off from the wall and the points of light from the bricks cross the Rayleigh limit, blending their visual information. First the wall's color becomes more uniform. Back off farther, and the individuality of the bricks blurs into one solid wall. Although you might lose visual information, such as the type of bricks used in the wall or even that it is made of bricks at all, you now know that the wall is of a two-story building, something that even a close knowledge of the parts of the wall couldn't tell you. | ||