Architecture of the digital camera Internal circuitry

The true digital camera is similar in many ways to a conventional film camera in terms of shape, functions and features (Figure 8.16). There is a lens, a shutter and a viewfinder system, means of focusing and exposure determination, automation of features and a data display. There is a light sensitive array at the photoplane and an image recording system. There is a means of frame advance and a number of interfaces such as power in, video out and connection to a

Figure 8.16 Digital cameras. (a) Using a considerably modified 35 mm SLR camera body. (b) Design using an independently rotating lens and flash unit. (c) Design using a telescoping zoom lens unit and coupled finder

computer. The interior arrangements are very different. Unlike a film camera where the exposed or partexposed film cassette, cartridge or back is removed for subsequent 'wet' processing of the material (with some time delay unless self-developing materials are used), the digital camera has various electronic circuit elements to deal with the data from the photosensor array and provide means of display, playback, editing. storage and transfer. The various alternative organizations of these elements form the architecture of the camera. This can be shown in outline by a basic structure such as in Figure 8.17.

The optical image formed at the photoplane by the lens is transformed into detail and colour information

Figure 8.17 Example of a possible internal configuration or architecture of a digital camera

as pixels, whose small size corresponds to an image 'point' or circle of confusion in film cameras. Analysis into pixels is by a discrete number of photosensors in an array ('chip') giving an image format. The array properties are detailed above. Each photosensor (acting as an incident light meter) gives an analogue output corresponding to subject luminance (L) at that zone. A colour separation arrangement analyses incident light into its colour components. A very large amount of data is output as (x, y, L) for each pixel. This data is subjected to noise reduction processes and can also be used for exposure determination by controlling the integration time of an electronic shutter, i.e the total time the photosensors react to light and their output stored.

A clock circuit or oscillator drives a timing circuit to provide this read-out time and operate a mechanical shutter that may be linked to the electronic one. The clock also controls the read-out of the pixel data in sequential order. This analogue data is converted to digital form and the colour information processed as necessary. This data can be kept as the 'raw image' or compressed to various extents, then stored in a memory. The decompression is reversible. The colour image processing provides a format to supply the display in the camera with an image for user viewing and operational menu (interface) or via another interface to communicate with external devices such as personal computers (PC) or printers.

A variety of different arrangements are used or alternative approaches taken to give the variety of contemporary digital camera systems, each with advantages and limitations in operational use.

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100 Photography Tips

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