The Lens

The job of the lens is to form an image on a sensor so that it can be captured by the camera. If you have ever used a magnifying glass to form an image of the Sun on a piece of paper that’s exactly what is going on inside the camera.

It’s worth noting that you don’t want to focus the sun on your sensor. It can permanently damage it, as it would your eye.

If you play with a magnifying glass and the sun you will find that you need to hold the lens a fixed distance away from the paper to obtain a sharp image. We say that this sharp image is in focus. The distance of the lens from the paper when this occurs is the called focal length of the lens.

The size of the image (normally a circle) depends on the diameter of the lens. In a camera you need this image circle to be just big enough to cover the sensor. Within the image circle you might fit a landscape or just the head of one person. This depends on the focal length of the lens. A long-focus lens doesn’t fit very much, but subjects in the image are large. A short-focus lens fits lots more of the scene, but subjects in the scene are smaller.

The amount of a scene covered by the lens is called its field of view, and it’s measured as an angle. A long-focus lens has a narrow angle of view and a short-focus lens has a wide angle of view. This is why short-focus lenses are called wide-angle lenses. For some reason, we don’t call long-focus lenses narrow-angle.

For a traditional film camera, a lens with a 50mm focal length is referred to as a normal lens. Its field of view is not too wide, and not too narrow. It's supposed to approximate the field of view of the human eye. 28mm and 35mm are common wide-angle lenses. 85 and 135 are long ones. These are the common focal lengths, but there are lenses that are much longer or wider than this.

Phone cameras tend to have wide-angle angle lenses as this makes the camera design easier.

We talked about focussing an image of the Sun. The Sun is essentially infinitely far away. But as any subject gets closer to the camera the lens must be moved further away from the sensor to maintain the image in sharp focus.

So a camera lens is basically a tube that contains lens elements and a mechanism for moving the lens to focus it on the sensor.

When photographing sports or wildlife it’s common to use a lens of 400mm or more. But these lenses are not that physically long. They use special optics to make what is called a telephoto lens. This is a lens that is physically shorter than its focal length. This is why long-focus lenses are called telephoto lenses.

Nowadays zoom lenses are common. You can change their focal length using a ring on the lens, or a control on the camera. Their versatility has made them the standard lens when you buy a camera kit. They are so pervasive that normal fixed focal length lenses are often called primes, to distinguish them from the more common zooms.

For homework, you can try forming an image on a piece of paper using a lens. If you are indoors, you should easily be able to view the image of a window. Use a magnifying glass or a lens from an interchangeable lens camera. An old lens is the best bet as you should avoid exposing the rear element of any camera lens to dust.

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