How Digital Cameras Work

Digital Cameras are very similar to conventional cameras in the way that they work. They both possess a lens to focus the image, a shutter to allow light inside the camera, and an aperture to govern the amount of light which enters the camera. The differences between digital and conventional photography take place after the light enters the camera. A conventional camera captures the images on film, while Digital Cameras capture the image on an image sensor.

Digital Cameras and Image Sensors

Image sensors are electronic devices made up of an array of electrodes or photo-sites which measure light intensity. The most common type of image sensor for Digital Cameras is the CCD - Charge Coupled Device although others such as CMOS and Foveon are occasionally used.

The number of photo-sites in the image sensor gives the digital camera its mega-pixel - millions of pixels rating. Each photo-site corresponds to a pixel in the final image, so a camera which is rated at six mega-pixels, for instance, has an image sensor which is 3008 pixels wide by 2000 pixels high.

When light hits the image sensor it is converted into electrical signals which are amplified and fed to an analog to digital (A/D) converter. The A/D converter changes the electrical analog signal into digital binary numbers which are processed by a computer chip housed in the camera body. Once the number crunching has been completed the resulting image is stored on a memory card in a digital format.

What about the Colour?

Photo-sites can only measure intensity of light and not colour. In order to generate a colour image, each photo-site must be covered with a coloured filter which can be red, blue, or green. These are the three primary colours which can be combined to produce any additional colour including white.

Because the human eye is twice as sensitive to green light, the photo-site coloured filters are arranged in a grid where there are twice as many green filters as there are blue and red. This arrangement of filters is called a Bayer pattern which consists of one row of red, green, red, green etc., and then the next row of blue, green, blue, green etc.

Since each photo site can only be covered with one coloured filter, computer processing is required to produce a full coloured image. This is done by analysing each and every pixel together with its immediate neighbours and producing a composite colour from the resulting calculations. For instance, if a brilliant red pixel is surrounded by brilliant green and brilliant blue pixels, the brilliant red pixel must in reality be white, because white is the combination of red, blue, and green. This technique is called demosaicing .

Once demosaicing has been completed, the image is adjusted according to the preset settings on your camera. These preset settings may include adjustments for brightness, contrast, and colour saturation. Some cameras may also apply a sharpening algorithm to enhance the clarity of the image.

Lets Save the Picture

Before saving the image on to the memory card it has to be compressed. The compression format favoured by most digital cameras is JPEG which reduces the file size by excluding surplus data. Once the this data has been removed it cannot be recovered hence the reason that JPEG is termed a lossy format.

Some cameras have the facility to save uncompressed images as TIFF files or raw data. Raw data is the original photo-site data before any image adjustments or demosaicing by the camera. Although the file size of raw data is that much greater, this data can be transferred to a computer for processing with special software that will accomplish all of the processing functions of the camera but with much greater control.