Digital Camera Picture File Size

All the information contained in the image is expressed in blocks called bytes. In technical terms, an image is an archive or document whose size is expressed in thousands (KB, kilobytes ) or millions (MB, megabytes ) of bytes. The larger the physical dimensions and/or the digital image's resolution, the greater the amount of bytes necessary to describe it. Also, an image made up of the same number of pixels gives way to a lesser or greater amount of information depending if the image is black and white or gray scale , were each pixel is described only by it's brightness; of RGB color , were each pixel details the proportions of red, yellow and blue; or of CMYK color , were the standard colors for graphic arts are used: cyan, magenta, yellow and black. The type of information the image contains is denominated mode . In grayscale mode we only have on color or channel (black), in RGB mode there are three channels, and in CMYK the image is composed of four channels. A digital photo made up of a million pixels contains approximately 1 MB of information if it is a grayscale image, around 3 MB if it is in RGB mode and almost 4 MB is in CMYK color. An image in RGB mode can be converted in an editing program to CMYK mode and vice versa; any of the two color modes can be converted to grayscale, in this case with a loss of chromatic information. An image in grayscale can be converted to RGB mode or CMYK -the image obtained will still be monochrome- as a step previous to adding color with an image treatment program.

Every channel registers a range of color that can also be variable. The range of gray or color is called color depth and is expressed in the number of different tones that the system can distinguish; and said number of tones is expressed in digital binaries or bits . If a digital sensor only distinguishes between black and white, we could express the values in in each point only with 1 in the case of white and 0 for black (1 bit). If it distinguishes four types of brightness we'd need 2 bits, if 8, 3 bits, etc. In cameras and scanners the color depth ranges between 8 and 16 bits per channel; some professional cameras offer up to 12 bits; the more sophisticated, digital backs and most scanners reach 16 bits per channel. The greater the color depth, the larger the file size will be. An image with a 16 bit color depth give a file size that is double the size of the same image with a 8 bit color depth. Normally, for color images, the depth is expressed having all the channels in account: an image in RGB mode with a depth of 8 bits per channel is defined as 24 bit RGB (= 8 x 3); if the image is in CMYK mode with the same depth it is denominated 32 bit CMYK (= 8 x 4), etc. The depth of color of an image can be set in most scanner and some cameras; with an image editing program we can convert the color depth of a file to larger or smaller depths.

In summary, the physical dimensions of a digital photograph and it's resolution give place to the number of pixels of the image; the mode and color depth influence on the amount of information necessary to detail each pixel. This final amount of information constitutes the file size . The amount of memory needed to store all this information depends on the format of the archive. In some cases the information contained in an image is recorded the same as in the camera, on the hard drive of our computer or on a diskette or compact disc (file format with no data compression) and in other cases the information is compressed to be saved (in some cases with some reduction in the image quality) and uncompressed when the file is reopened, which means saving a lot of memory space on the mentioned supports (data compression file formats).