DSDN144 Digital Photographics
There are 2 different types of compression: LOSSY and LOSSLESS. LOSSY Compression: reduces file size by removing "unnecessary" data. The data is eliminated using a mathematical formula that takes the numerical colour value of one pixel, then groups it together with nearby pixels that are nearly the same, and remembers them as the same colour. Graduated tone is lost, which can give the image a 'blocky' look, however this can be controlled at the point of saving the file. JPEG's are the only type of file that uses LOSSY compression, and in Photoshop the amount of compression can be controlled.
This image was saved at value '12', or 'High'
This image was saved at a value of '0', or 'Low'
A good rule to follow is never to save a JPEG file below '5', or 'Medium' value. The important thing to remember is that every time you save an image as a JPEG file, you lose the same amount of information. So an image saved three times as a JPEG file will be three times more distorted than an image saved once. A good rule to follow is to save your images as a file type using lossless compression until you need to save it as a JPEG, then you will lose as little information as possible. Your images when they come off the digital camera card are saved as JPEGs, you should change this the next time you save them.
LOSSLESS Compression: is a type of compression algorithm that reduces file size without losing any data. Typically this is done by finding and eliminating redundant information. .BMP, .GIF, .TIFF, .PNG and the Photoshop file type .PSD, all use LOSSLESS compression. We advise saving your images as a PSD while you are working on it, this will save all your settings from Photoshop for the next time you work on your image. When you take your image to a shop to be printed, save the final file as a TIFF file, and make sure all your layers and paths from Photoshop have been deleted, this will ensure that the shop will be able to print your file easily.