Digital cameras are quite similar to traditional cameras in their operation. They both have a lens to focus the image, a shutter to allow light inside the camera, and an aperture to control the amount of light which enters the camera.
The differences between digital and traditional photography occur after the light enters the camera. A traditional camera captures the images on film, while a digital camera captures the image on an image sensor.
Image sensors are electronic devices made up of an array of electrodes (or photosites) 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 sometimes used.
The number of photosites in the image sensor gives the digital camera its megapixel (millions of pixels) rating. Each photosite corresponds to a pixel in the final image, so a camera which is rated at six megapixels, for example, 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 signal into binary numbers which are processed by a computer housed in the camera body. Once the numbers have been processed the resulting image is stored on a memory card.
Photosites can only measure intensity of light — not colour. In order to produce a colour image, each photosite 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 other colour including white.
The coloured filters are arranged in a grid so that there are twice as many green filters as there are red or blue. This is because the human eye is twice as sensitive to green light. Filters are arranged in a pattern called the Bayer pattern – one row of red, green, red, green (etc.), and the next row of blue, green, blue, green (etc).
Since each photosite can only be covered with one coloured filter, computer processing is necessary to produce a full coloured image. This is done by analyzing each individual pixel and its immediate neighbors and producing a composite colour from these calculations. For example, if a bright red pixel is surrounded by bright green and bright blue pixels, the bright red pixel must actually be white, because white is the combination of red, blue, and green. This process is called demosaicing.
After demosaicing the image is adjusted according to the settings on your camera. Most cameras have settings for brightness, contrast, and colour saturation. After these adjustments are made some cameras may also apply a sharpening algorithm to make the image clearer.
The final step before saving the image on the memory card is to compress it. Most cameras use JPEG as a compression format. This reduces the size of the file by eliminating excess data. This data cannot be recovered, so JPEG is called a ‘lossy’ format.
Many cameras have the ability to save uncompressed images as TIFF files or raw data. Raw data is the original photosite data even before demosaicing. It can be transferred to a computer for processing with special software that will perform all of the processing functions of the camera but with much greater control.
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