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How A Camera Works

Learn How A Camera Works With Light

How A Camera Works

Diagram of how the inside of a camera works with light to capture images.


Despite their visible differences, and varying use of electronics, all cameras essentially work in the same way. At the front is a lens, usually with a variable opening, which gathers light to create an image; at the rear is a light-sensitive sensor, or sheet of film, capable of making a permanent record of this image.

Camera obscura Today’s cameras are direct descendants of the camera obscura (meaning ‘dark chamber’), a much earlier device used as an artist’s drawing aid.

How a camera works

Cameras, from the most sophisticated to the most basic, share four common components: a lens, an aperture, a shutter mechanism and a viewfinder.

Light reflected from the subject is collected and focused by the lens. It then passes through an aperture – a hole, usually of variable size – so that the amount of light passing through can be controlled. The lightproof shutter, meanwhile, opens for a timed period to allow light from the lens to reach the imaging chip in a digital camera, or the film. The viewfinder is an aiming device, allowing you to preview the image before it is recorded.

Alternative viewing systems It is the difference in viewing systems that gives compacts and SLRs such different shapes.

Exposure Factors

Although photographs can be taken however bright or dark the conditions, the amount of light that reaches the film or sensor must be controlled. Three factors determine how a camera works with exposure:

  • the light sensitivity, or ‘speed’, of the film or imaging chip. This is normally denoted by an ‘ISO’ number
  • the size of the lens aperture
calibrated in f numbers)
  • the time the shutter remains
open (measured in fractions of 
a second)
  • Getting the right level of exposure used to be one of the most worrying aspects of photography. How a camera works in the modern day is with exposure systems that not only measure the exposure accurately in most situations, but automatically set aperture and shutter speed to suit. There are still situations, though, where even pros are unlikely to get the right exposure with a single shot.

    Aperture and shutter The smaller the f number, the larger the aperture (below). However, f22 at 1/60 sec, f16 at 1/125, and f11 at 1/250 will all give the same exposure.

    Aperture Diagram

    Each stop gathers one half the amount of light as the previous aperture.

    However, there is not usually just one combination of shutter speed and aperture that will give an effective exposure. If you choose a camera that allows you to see, and override, the settings chosen by the camera, you will have much more creative control over the result. Some form of manual control, or exposure override, is also useful in situations where the built-in metering system is likely to be misled, or when you want a darker or lighter result for artistic reasons.

    Aperture and speed

    Aperture sizes are denoted by f numbers. Confusingly, the smaller the number, the bigger the aperture opening. A typical sequence is f4, 5.6, 8, 11, 16, 22. How a camera works is, moving from one f number to the next smallest (f8 to 5.6, for example) increases the size of the aperture and lets in twice as much light.

    The shutter speed is indicated in fractions of a second. A typical sequence is 8 (meaning 1/8 sec), 15, 30, 60, 125, 250, 500, 1000, 2000, 4000. Moving the setting from, say, 250 to 125 means that the shutter will stay open for 1/125 sec instead of 1/250 sec – doubling the amount of light reaching the imaging sensor, or film. If the light meter indicates that a subject needs f8 at 1/125 sec, then using twice as much light (f5.6) for half as much time (1/250 sec) will give the same exposure.

    Pinhole image Although not as sharp as an image formed with a lens, a pinhole image (below) is surprisingly clear.
    How a Pinhole Camera Works

    Making a pinhole camera

    How a camera works can be demonstrated by making a camera from a shoebox. This basic camera does not use a lens, but instead forms an image using a pinhole. First, cut a good-sized hole in the side of the box and tape a piece of thick kitchen foil over it. Make sure no light can enter around the joins, Next, take a pin and make a neat, clean hole in the centre of the foil – it’s impossible to make a clean hole in the cardboard. In complete darkness, fix a length of ISO 100 negative film to the inside wall of the box, opposite the pinhole. Put the lid on and cover the box with a black cloth before switching on the light. Take the covered box out into bright sunlight, place it on a level surface so that it is pointed at a static subject, and gently uncover the pinhole. How a camera works with exposure is hard to judge, but after around 3 sec cover the hole with the cloth and, in complete darkness, remove the film for processing.

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