PIXELS AND FILES

For over 170 years photos have been taken using film containing light sensitive chemicals. Digital photography replaces ‘film’ with a ‘sensor’ containing millions of tiny sensors, which convert light energy into electrical signals just like the human eye does.

PIXELS
The sensor screen in your camera is covered in millions of tiny light sensitive cells, like a honeycomb, but arranged in a rectangular grid. A sensor with 2550 cells in a row and 1950 rows on a sensor will have a total of 2550 x 1950 = 4,972,500 pixels. Camera manufacturers round this to 5,000,000 (5 million pixels) called 5 megapixels (5MP). Generally the more megapixels the better the definition of the photo, however pixels are not everything. There is no point on storing a perfect copy of a poor image from a cheap lens.

FILES
Each cell in the sensor records lots of information about the light hitting it. This information is not recorded by re-arranging atoms as film chemicals did, but as a series of ON or OFF information just like a computer.

These bits of ON or OFF information are called ‘BITS’! and 8 bits of information are called a BYTE. Enormous amounts of ON OFF information are recorded for each photo, and these are stored on a memory card in the camera as a file, hence each photo, or image is called a ‘FILE’.
COMPRESSION
Files are usually compressed to store more images on to a memory card. The camera can reduce pixels with a similar colour to a summary number to save space. The most common compression formula is JPEG (p.62) and it is also the most common for uploading to the web.
FILE REFERENCE NUMBERS
Each image file (photo) is given a reference number to identify it. A typical image file is given a number like DSC.1423.jpeg This means Digital Still Camera, photo number 1432 in a running sequence and the extension means it has been compressed to JPEG format.
FILE SIZES It is important that you understand the size of your photo file, otherwise you may send a massive email file (photo) which will clog the email, and computer screens do not need large file sizes anyway as they have low resolution. A typical photo from a compact camera could range from 1MB to 5MB, depending on the Megapixel size of your camera, the resolution selected on the camera and the chosen level of JPEG compression.