Why does a camera have a convex lens?

Written by dennis costa
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Why does a camera have a convex lens?
Practically all camera lens configurations involve the use of at least one biconvex lens. (camera lens image by Albo from Fotolia.com)

For a camera to work, it needs two things: a light-sensitive surface to capture an image, and a lens that bends the light from a given scene and projects it onto the light-sensitive surface. The earliest known cameras, known as camera obscura, did not use lenses and employed just a hole in a wall, but the results were not that good. For truly great pictures, you need lenses, and the most important type of lens present in cameras is the convex lens.

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How Lenses Work

A lens is essentially a transparent object made of glass, plastic or other material that bends light through a process called refraction. Because it possess the characteristics of a wave, light essentially changes speed and direction whenever it passes from one environment to another; in this case, from gaseous air to solid glass or plastic. The light passing through a lens bends twice, once as it enters the glass or plastic and again as it leaves, according to the book "How Everything Works."

Types of Lenses

In a camera, lenses usually have the form of a disc and each side may be curved or flat. With this in mind, two main types of curved lenses exist. Convex lenses curve outward, and thus are thicker at the centre than at the edges. Concave lenses, on the other hand, curve inward and the centre part is thinner than at the edges, according to the book "The New Way Things Work." The most common type of convex lenses in cameras is a double convex or biconvex lens, in which both sides curve outward.

Biconvex Lenses

A useful characteristic of biconvex lenses is that when light rays pass through it, they converge. This means that light passing through the upper half is bent downward while light passing through the lower half is bent upward. This makes a biconvex lens a converging type of lens (you can also call it a collecting or positive lens). By contrast, a concave or biconcave lens is a divergent type of lens. A curious feature of concave lenses is that they make a scene or an object appear smaller to the human eye.

Forming the Image

The light that passes through a convergent lens must meet at a precise distance from the lens onto a screen to form an image. In cameras, this screen can be either a roll of film or a similar light-sensitive material, or in digital cameras it may represent an image sensor. No matter the medium in which the image is projected, the principle is the same: the light from the top of a lit candle, for example, is bent downward a bit when entering the convergent lens, then bends further downward when it exits the lens. The result on the screen is an inverted image that closely resembles the actual object, and which is made possible only by the use of convergent lenses like a biconvex.

Effect of Distance

Biconvex lenses can converge light and form an inverted image on a screen, film or image sensor, but the clarity and sharpness of the resulting image depends on the distance between the lens and the sensor. If the sensor is too near the lens, the image essentially doesn't have room to come together. Also, if the sensor is too far apart, light starts to break up before it reaches the sensor. Another factor is the object's distance to the lens. If this changes, the distance between the lens and the sensor must also change to keep the image in proper focus.

Compound Lenses

In theory, a biconvex lens should be everything you need to capture an image. However, in reality most cameras come with compound lenses to satisfy the demands of modern users, according to the book "Encyclopedia of Twentieth-Century Photography." What is commonly called a "lens" nowadays is actually a combined assemblage of concave and convex lenses mechanically calibrated to act in concert. The combination of convex and concave lenses comes in handy in zoom lenses that allow for narrow, magnified telephoto images as well as wide-angle pictures.

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