CAMERA FACTS -
HOW LENSES WORK

LENS BASICS

Both digital and film cameras work by light from the target image being focused on some form of detector whether it be photographic film or the CCD sensor of a digital camera

Objects give off reflected rays of light all directions. Light for the target image enters the camera through a glass or plastic lens. This bends (refracts) the light. All the rays of light from a point on the target object will meet a certain point (focal point).

In a camera, by adjusting the distance of the lens (focusing) you can get the light from your subject to fall focused onto a plane where your sensor or film is (called the film plane)

Note how the image is focused upside down and with the left on the right side.

In film cameras the focused light falls on a photosensitive chemical emulsion coating a plastic film. In digital camera it falls onto a CCD receptor. Your eye works exactly in the same way with light being focused onto your retina

Obviously light falling from objects at different distances will have a different focal plane.

This explains why sometimes when you take a photo of something close up the background appears fuzzy (and vice versa)

There is an area to the front and rear of the item which you have focused on which appears to be in focus. Photographers call this depth of field. This is the size of this focusing field increases the further away the object is (the lens moves near to the film plane and light passes through with less and less refraction). The aperture of the camera also determines what depth of field you have (see later)

With all focusible cameras there comes a point when the lens is moved to a point as close as possible to the focal plane. Here light is most minimally refracted and light passes through almost straight. Therefore all objects at this focal length or beyond appear in focus.

This setting is called infinity and is not just useful for taking pictures of landscape.

FOCUSING SYSTEMS

Fixed focus camera makers exploit depth of field concepts by cleverly fix their lenses relatively closely to the film plane so that objects a few meters away are in focus but objects behind out to infinity will appear almost in focus

Other cameras have some form of focusing system. In some you manually move the lens back and forth to bring the target into view. More basic 35mm SLR camers are good examples of this. You move a ring on the lense round and this move the lense back and forth.

Other cameras employ autofocus systems. There are 2 common systems used.

Active systems for autofocus work a bit like radar or sonar. They are commonly seen on compact cameras. A beam of infra red light is emitted from the camera. This bounces of the target image and the length of time this "echo" takes to get back tells the camera how far away the target is. Many cameras have multipoint ( e.g. 7 point) systems where 7 beams of ligt are emitted thereby increasing chance of successful focusing.

Passive systems are seen in some autofocus SLR camers. Clever electronic circuitry exploit the fact if you have a sharply focused image it contains more energy than an out of focus image. Most autofocus SLR allow users to override these settings

Of course in both systems are only really effective if the are target correct and not on some object behind or in front of the target object

LENS LENGTHS

Cameras come with different lens lengths. What does this mean? And why can it matter?

When a camera focal length equals that of the maximal width of the film , it is said to appear normal. With 35mm film for example the largest width of the film is the diagonal at 43mm. Hence most lenses giving a normal angle of vision are described between 40-55mm (50mm being the commonest). The angle of vision with a 50mm lens is around 46^o which is similar to the human eye at 45-55^o (1)

So taking a picture with a 50mm lens would give an image with a similar perspective to that of the human eye.

Many compact camera have shorter focal lengths.

This is for several reasons. Firstly by bringing the lens nearer the camera you expand the angle of vision. This useful when in small spaces to fit more things in (e.g. you wouldn't need to stand as far back to take a group photo with a 35mm lens compared to a 50mm lens).

Secondly you might remember what I said about depth of field. Closer lens is to the film plane the greater this is. So wide angled lenses have a greater depth of field than longer lenses.

Lenses with a length greater than the focal length have a narrower angle of vision but will allow you to focus on distant objects and magnify them. They are often called telephoto lenses and good examples of extreme long focus lenses are the ones sports photographers use to get photos of footballers as if they were standing right beside them.

They are great for focusing on distant objects but have limitations on being focused on very near objects

The only problem with lenses with angles of vision bigger or smaller than the normal angle of vision is that they distort the perspective of the image

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