A definition of Compression

Image compression can happen at both capture and editing stages, and has three advantages for photographer. Most importantly, compression leads to smaller file sizes, which means that many more images may be stored on a memory card. The further benefit of cost results from this, while a final advantage is that moving data from one place to another, using the likes of email or even just from memory cards or between hard drives, can happen more quickly if the file is smaller in size.
Compression is automatically applied to JPEG files in-camera, via a process known as lossy compression; this looks at areas where data may be discarded without it having much impact on the image's quality when viewed. Although the level of compression may be defined by the user, the resulting file will depend very much on its content; an image with plenty of unbroken blue sky, for example, may be compressed and uncompressed more efficiently than one which contains varied and intricate patterns.

The other method of compression is known as lossless compression, which seeks to reorganise the information within an image more effectively as opposed to discarding it. This option is available in TIFF, Raw and other formats, and as no information is removed its use results in better image quality. As such, it is best used when an image is being stored or edited in any way, where quality is paramount. The disadvantage of this method is that it fails to achieve anything near the small file sizes yielded from lossy compression.

Problems with compression concern the artefacts which can be introduced when this happens. JPEG compression, for example, works by dividing images up into blocks of pixels before each is compressed, which can translate into a ‘blocked' patterning once the file is uncompressed (particularly at higher levels of compression).