Lossy compression

Lossy compression reduces a file's size by removing or discarding some of the original data. The file still opens, plays or displays, but it no longer holds everything the original did, so some quality is lost — and that removed data can never be put back.

How much is thrown away?

Lossy compression usually gives a significant reduction in file size. You often choose the compression level yourself, and the more heavily a file is compressed, the more information is lost.

• Light compression keeps most of the detail, so the result usually looks or sounds the same as the original.
• Heavy compression discards much more, and the drop in quality becomes easy to spot.

Imagine a school orchestra recording a rehearsal on a phone to send to a group chat. At a light setting the strings and voices sound as they did; squeeze the same recording much harder and the cymbals smear and quiet passages go fuzzy. That copy is fine for a quick message, but not for selling the concert CD.

Digital artefacts

Heavy compression can cause digital artefacts: visible or audible damage that was not in the original. A picture may show pixelation, where smooth areas break into blocks and edges look jagged; audio can turn watery or metallic. At moderate settings the loss is normally hard to notice, which is why the method is so widely used.

When to use it

Lossy compression suits any situation where a smaller file size matters more than keeping every piece of original data or perfect quality, such as shrinking a photo before you send it so it uploads quickly over a weak connection. It is the wrong choice when the file must stay identical, like a spreadsheet of race times where every hundredth of a second counts. It removes detail inside a file; it never removes whole files or whole sections.