Ingrid Solberg

A compressed image cannot be judged from a file listing, so every page here shows the artefact rather than the byte count. What survives are the frequencies the eye resolves, which is why artefacts cluster around sharp edges and fine detail: text, hair, wire, foliage. The artefact that gives compression away fastest is banding across a smooth gradient, and it appears long before anybody notices blur. Lossy versus lossless is a choice about whether the file will be edited again. A file destined for print or for further editing should stay lossless. A file that goes onto a page and is never touched again should not. Resolution and quality are not the same setting, and confusing them wastes effort. Resolution decides how many pixels you have. Quality decides how the encoder spends them. A phone photograph at full resolution and low quality looks worse than a correctly sized image at high quality. Metadata is a real cost. A camera original carries a full sensor file with an embedded preview, and stripping to the embedded version is often a larger saving than any quality adjustment, with no visual change on a web page. For images on a page I finish with the format recommendation, because serving a format the browser supports natively beats compressing an older one to a comparable size. There is also a floor below which further compression is pointless, because the file stops shrinking at a rate that matters while the artefacts keep accumulating. Finding that floor for your own images is a short experiment and it saves guessing on every subsequent batch.