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How are digital images captured?

Instead of film a digital camera has a CCD (charge-couple device) or a CMOS (complementary metal-oxide semiconductor) sensor, which consists of a grid of phototransistors to sense the intensity of the light across the plane of focus. CMOS sensors are slightly different from CCD sensors in that they generally use less power and have a different kind of light-sensing material.


On exposure, electrical charges are generated in proportion to how much light each pixel receives. A color filter array is placed on top of the sensor to capture red, green, and blue (RGB) parts of the light and, via the built-in analogue to digital converter or ADC, changes these into a stream of digital signals – each picture creating a ‘file’. The larger the number of pixels, the bigger the file size and the higher the resolution of detail in the image. Captured image files are the equivalent of exposed film frames.


Unlike analogue where you have a choice of color or black and white film, digital cameras only take pictures in RGB. You can however on some use a black and white setting (on several cameras you have additional effects including sepia) but the image is converted to greyscale via built-in software after you have shot it. This is useful when you wish to see the immediate result of the effect but you have far greater control over an eventual conversion in post-production.


You can view the pictures before, while and after shooting on a small LCD (liquid crystal display) screen, located on the back of the camera body. At this time too, any shots you have taken but don’t want may be deleted and their vacated file space used for new pictures, time and again.


The quantity of pictures a full memory card can contain depends upon the size of each file (higher resolution images contain more pixels) and the capacity of the card you have in your camera. At any convenient time, pictures can be downloaded from the camera or, more often, its memory card into a computer. You can then see them on a monitor, save your files prior to any necessary image manipulation, and print them out. Files can also be transmitted (e.g. by mobile phone or wirelessly from the camera itself) to a computer system located somewhere else, such as a newspaper picture editing desk.


What is the megapixel debate?


The output size and to some degree the quality of a digital camera is measured in megapixels. A megapixel is 1 million pixels and can be calculated as the number of horizontal pixels multiplied by the number of vertical pixels in an image – such as 4000×3000pixels=12,000,000pixels or 12megapixels.


Unlike film grain the pixels of a digital image are laid out in a rectangular grid. When

you magnify a digital image you can see each individual pixel clearly, which is not like the magnification of film where grain appears irregular, unless you’re shooting on a tabular grain film such as T-Max where the grain takes on a different shape. The human eye picks out regular patterns more readily than granular patterns, particularly since curved or diagonal lines in the image start to show jagged, staircase-like edges known as aliasing.


The number of megapixels your camera has dictates to a certain extent the output size of your final print. Prints carry a variety of output resolutions – a default of 300 dots per inch (dpi) is considered normal for a digital print to appear indistinguishable from prints from film. This assumes normal eyesight and average print viewing distance. It must be pointed out that 300 dpi is just a guideline and will vary on the type of printer you are using.


It should also be noted that the larger the image, the lower you can allow the resolution to go. For billboard-type prints, a dpi as low as 150 is often used. This has to do with the viewing
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