Principle of camera operation

Apart from a few structural details which differ according to their type, the current cameras work on the same principle and are composed of the following basic elements:

  • A viewfinder
  • A shutter release
  • A lens
  • A diaphragm
  • A shutter
  • A film or a sensor

When the shutter release is pressed, the shutter opens to allow light to pass through the lens and provide a clear image of the subject to be photographed to the film or digital sensor. The amount of incoming light is determined by the amount of time the shutter stays open and the size of the aperture. At the end of the exposure, the shutter returns to its original closed position.

Lens and focal length

The lens is the optical system at the front of the camera. It is composed of lenses whose purpose is to form an image on the sensitive surface, film or sensor. Its choice is essential because it is responsible for the quality of the shot. It can be fixed or interchangeable.

Its fundamental characteristic is its focal length, which determines the magnification and the field of vision observed through the lens. More precisely, the focal length represents the distance in millimeters separating the film or the sensor from the optical center of the lens when the focus is made at infinity. The shorter the focal length, so the closer the lenses are to the plane on which the image is formed, the wider the field of view. Conversely, the longer the focal length, the smaller the field of view. A lens with a variable focal length is commonly called a zoom.

Diaphragm, shutter and depth of field

The diaphragm is a mechanism present on the lens, whose operation is similar to that of the iris of our eye. Composed of thin overlapping blades, it adjusts the amount of light passing through the lens. Its value is called aperture. Symbolized by the letter "f", it is the ratio of the useful diameter of the lens to its focal length (f/D). The most common values are 1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 ..., the amount of light is divided by two at each graduation. So the larger the aperture value, the more the diaphragm is closed.

The shutter can be located in the center of the lens or just in front of the film. It is composed of metal blades that overlap each other, whose purpose is to let the light pass or not. When the shutter button is pressed, the shutter opens and closes. The duration during which it remains open, called exposure time or shutter speed, can take the following values, expressed in seconds: ... 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, ... as for the aperture, the amount of light received at each graduation is divided by two.

The couple diaphragm/shutter allows to adjust the exposure, ie the amount of light received by the film or the sensor. We speak then of couple aperture / speed. Digital cameras generally offer four modes of exposure adjustment:

  • Automatic, where the amount of light is metered by the camera
  • Aperture priority, where the user can set the aperture of the diaphragm, the camera is responsible for adapting the shutter speed
  • Shutter priority, the opposite of the previous one, where the user chooses the shutter speed and the camera adjusts the aperture accordingly.
  • Program, where the user can choose one of the predefined modes among sport, indoor, night, portrait, landscape...

The depth of field is the distance between the closest point and the farthest point from which the camera provides a clear image. It is not equally distributed on both sides of the focusing distance: it is twice as large at the back of the subject than in front of it. The depth of field depends on:

  • the opening of the diaphragm: the more the diaphragm is closed and the greater the depth of field
  • the focal length of the lens: the shorter the focal length, the greater the depth of field
  • the distance camera/subject: the further away the subject is, the greater the depth of field

Film, sensor and sensitivity

In a silver camera, the image is formed on a transparent plastic film on which is deposited a photosensitive layer of silver grains. Chemically activated by the light in the camera, a latent image is formed. The resulting image is called a negative, because the tones of the photographed subject are reversed. In the development laboratory, the image is transferred to paper after being exposed under the enlarger. It is then revealed by the action of a chemical bath, called developer, which blackens the activated silver grains, the others remaining colorless. The paper is then passed through a stop bath to block the action of the developer. In order to avoid any further action of the light on the non-activated grains, another chemical bath is used to eliminate them, this is the fixative. The paper is then washed to remove all chemical residues. For the color, three photosensitive layers are superimposed and separated by colored filters. Different film supports exist, however they all have for common characteristic their sensitivity. Expressed in ISO, it is the amount of light required for the film to be properly exposed. Common values are 100, 200 and 400. Each time the sensitivity doubles, the amount of light required is divided by two.

On digital cameras, the film is replaced by a sensor. Composed of several million light-sensitive cells covered with a colored filter red, green or blue to restore the color, the CCD sensor can transform light energy into an electrical signal. An analog/digital converter then converts this signal into binary data. These data are then kept on the storage medium of the device, represented most often by a removable memory card. In order to facilitate the filing of the images, those are compressed. This compression mode can be configured before the shooting and will determine the number of images that can be stored on the card. As with a film camera, the ISO sensitivity can be modified, however the choice is made before the shooting, by selecting a parameter in one of the camera menus.

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Update: according to DZOptics, the normal focal length, the one that allows to get closer to human vision, is determined by the diagonal of the negative used: 43mm in 24x36 for example. In practice, we generally speak of a 50mm. Cameras with a smaller focal length are called wide angle, the others are telephoto lenses. The value of the minimum and maximum focal length is written on the front of the lens. These distances differ completely between a film camera and a digital camera, since the size of the sensor of the digital camera is relatively small. The lenses are then very close to the sensor. That's why we usually find the correspondence with a classic 35 mm that characterizes film cameras.

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