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At its simplest, a camera is a device for fixing shadows on some medium, so that the image of a scene is recorded.

Modern cameras have the following components:

  • A lens focuses the light so that a sharp, bright image can be recorded. Within the lens, a diaphragm controls the aperture, allowing more or less light through.
  • The shutter opens for a pre-determined length of time so that enough light reaches the sensor.
  • The sensor, which could be film or an electronic sensor, records the light that falls on it.
  • If film, the film is wound on to protect it from being re-exposed, if digital, the image from the sensor is processed by camera electronics and saved to a memory card, or transferred straight to a computer.
  • The viewfinder shows the image to the photographer prior to exposure. A viewfinder can be optical or electronic. If optical, it can be a rangefinder type, or a single lens reflex. A single lens reflex, such as any Nikon that has a number like D50, D3, D300, or F1, F2, allows the photographer to see exactly what will appear in the final image, allowing for accurate focusing and framing of the picture when using different types of lens. Some cameras, such as the D3, have both an optical and an electronic viewfinder option.

To control these components, every modern camera sets the following values, though many cameras allow some or all to be set automatically, and compact cameras often do not allow the user to set them.

  • ISO - the sensitivity of the sensor, set electronically in digital cameras, and by purchasing film of a particular speed for film cameras. Higher ISO values produce more noise, though the amount varies from camera to camera.
  • aperture - the degree to which the diaphragm is open. Aperture is measured in f stops, the lower the f number, eg, f 1.4, the more open the aperture, and the more light enters the camera. Aperture also sets depth of focus, which means that the range of distance from the camera which is in focus is greater when the aperture is less, ie, with higher f numbers. So, wide aperture = low f numbers = more light, narrow aperture = high f numbers = more depth of focus.
  • shutter speed - the length of time the shutter is open, measured in fractions of a second, such as 1/125. Shorter shutter speeds freeze the action, but allow less light into the camera, longer shutter speeds allow more light in, but can result in blur because the camera is not being held steadily enough, or because the subject is moving.
  • shutter release - the button you press to take the shot.

Almost all modern cameras include a light meter of some kind, which tells you when enough light is entering the camera for a successful exposure. When the light meter indicates too little light, opening the aperture, decreasing the shutter speed or raising the ISO will increase the amount of light, otherwise the image will be underexposed. When the light meter indicates too much light, reducing the ISO, increasing the shutter speed or decreasing the aperture will help to prevent the image being overexposed.

Digital cameras are often equipped with a histogram function, so you can view the exact distribution of light and dark after the image is taken. Not all scenes can be fully recorded, because of insufficient camera dynamic range. Any scene involving the sun will result either in blown highlights for the sun, or the rest of the picture being underexposed. The photographer must then choose which range to capture, or must bring in additional lights, such as the use of a flashgun to correctly expose dark parts of the picture.

Most cameras have a number of settings which automate some of these, based on preset ISO

Consumer cameras may have a number of other modes which set conditions for sports, portrait, landscape, and so on.

Additionally, some digital cameras can be set for auto ISO, which means that the sensor sensitivity is varied by the camera to ensure that the exposure is always correct, within the camera's capabilities. The function of this is most apparent in manual mode.

Further reading, Nikonians guide How does a digital camera work? by Digital Darrell [1]

  • This page was last modified on 31 December 2008, at 13:50.
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