The magnifying glass effect
The magnifying glass effect | ||
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Let's go back to that childhood fascination with magnifying glasses that set the scene for the How lens works in a camera? Take a closer look at the hot spot of light created by the magnifying glass. You'll see that the spot is actually a picture, a small image of the sun. Now, remember how you moved the magnifying glass back and forth until it was just the right distance to start a flame? You were focusing the picture the magnifier made on the leaf. In a camera, you have a lot of controls and sensors to help you focus, but basically you're still moving the lens back and forth. When light strikes an object—say, a race car—the car absorbs some of the light. The light that's left bounces off the car, scattering in all directions. At a point, A, the car absorbs all the light except orange rays. From that one point, billions of rays of faintly orange light spread out in a constantly expanding sphere. The farther the light travels, the more it thins out. Without a lens, all but one ray of light spreads out, away from the other point, B. The only ray of light from point A that can wind up at point B on the surface of an image sensor, or film, is the one traveling in a straight line from A to B. By itself, that single point of light is too faint for the sensor to register significantly. That changes when a camera's lens is put between A and B. Millions of other rays from A enter a camera's lens through the millions of points on the surface of the objective. The objective is the first of the half dozen or so simple lenses that make up the camera's compound lens. The simple, single lenses are distributed along the same line among separate groups called elements. The multiple lenses, which might mix positive and negative lenses, compensate for each other's defects in the way that two lenses fix chromatic aberration, which was shown in the previous illustration. Also, as you'll see, the compound lens design allows precise focusing and zooming. Although the rays of light from point A on the race car entered the objective at different points, after passing through— and being bent by—all the elements in the camera lens, the rays of light exit the lens, all of them aimed at the same focal point. The accumulated power of all those faintly orange rays of light creates a focal point bright enough for the digital image sensor to register. The same process occurs for every point on the car that can be 'seen' by the camera lens. For each point on the race car, there is a corresponding focal point for all those light rays that bounce off those points. All together, those focal points make up a focal plane. The plane is where you find the surface of a digital imager or a strip of film. Light that is focused on that plane produces a sharp, focused photo. | ||