How lens works in a camera?
How lens works in a camera? | ||
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You must remember boring, sunny summer days in your childhood when you took a magnifying glass outside looking for anything flammable. Having spied a dry leaf, you held your magnifying glass between the leaf and the summer sun. Moving the magnifying glass back and forth with the precision of a diamond cutter, you found the exact point where the magnifier's lens concentrated the sunlight to form an image of the sun on the hapless leaf. Within seconds, the concentrated rays of the sun caused a thin wisp of smoke to rise from the leaf. Then, suddenly, it burst into flame. You had just done the work involved in focusing a camera's lens, though with more dramatic results. To create an image inside a camera, it's not enough that light bends when it goes through glass or plastic. A camera also needs the bending to apply to each and every beam of light that passes through the lens. And it needs the bends to be precise, yet different, for each separate light ray. To make beams of light converge to a focal point like the hot spot on the leaf—the beams have to pass through the glass at different angles from each other. A positive, or converging, lens uses a smoothly curved surface that bulges so it's thick in the middle and thin at the edge, creating a convex surface. One beam of light that passes dead-on through the center, along the axis of the lens, doesn't bend because it doesn't enter the lens at an angle. All other rays of light traveling parallel to that center beam hit the surrounding curved surface at an angle and bend. The farther from the center that the light beams enter the lens, the more they are bent. A positive lens forces light rays to converge into a tighter pattern along the axis until they meet at a common focal point, where the energy in sunlight is concentrated enough to ignite a dry leaf. Positive lenses are also called magnifying lenses; they're used in reading glasses to magnify text. A negative, or diverging, lens does just the opposite. It has concave surfaces that make the lenses thicker on the edges and thinner in the middle. Glasses for people who are nearsighted (who can't see distant objects well) use negative lenses. Light waves passing through a negative lens spread away from the lens's axis. The light waves don't meet, so there is no focal point that a leaf need fear. There is, however, a virtual focal point on the front side of the lens where the bent rays of light would meet if they were extended backward. Focusing a clear image is a complex job that isn't done well by a single lens. Instead, most lenses consist of collections of different lenses grouped into elements. Together, they are able to focus objects at a multitude of distances from the lens with simple adjustments. Together, the elements determine the focal length of a lens. The focal length is the distance from the focal plane to the rear nodal point when the lens is focused at infinity. The rear nodal point isn't necessarily in the rear. In fact, it can be in front of the lens. | ||