Chapter 4 — Image Files
The Effects of JPEG Compression
One of the side effects of using the JPEG file format is that images are decompressed for display and editing and then re-compressed if any changes have been made (or if you choose Save As instead of Save). Because JPEG compression is the "lossy" type this means you lose some image compression with each save action. How much you lose depends on the Quality setting and varies with image content, but the loss is inevitable and cumulative, as these examples demonstrate.
500 saves at Photoshop Quality Setting 10 of a 100px x 100px image:
A larger image saved 600 times (quality setting unspecified:
The blotchy effects you see in broad areas of solid color and around high-contrast edges are called "JPEG Artifacts". Some small level of JPEG artifacts may be acceptable in web images, depending on their size and use, but they are definitely unacceptable in any master images you intend to save and use in the future. For this reason it's best to store all your archived images in a format that doesn't use lossy compression: PSD, TIFF and digital camera Raw formats are your best choices in this regard.
What is the BMP File Format?
Some Windows users will be familiar with Windows' own native file format "BMP", also known as "Windows Bitmap" format. It's not used very much any more, even it Windows, because the exponential increases in computing power that we've enjoyed in the past couple of decades have made JPEG compression/decompression into a trivial detail. Basically, BMP is a stripped down version of TIFF. It has no compression, no Layers and no color profile embedding. As such, they're not very useful of professional imaging purposes. If you even need to use one your best bet would be to embed a default color profile (sRGB) and then save it as a TIFF, PSD or JPEG.