The exact quality thresholds and format choices that shrink image payloads by 70% or more with no visible artefacts.
Why uncompressed images hurt modern websites
Every time a visitor opens a page, the browser downloads each hero banner, product photo and inline illustration before the layout feels complete. Exports from cameras and design apps routinely weigh 1.5 MB to 5 MB per image, even though a properly compressed 180 KB version looks identical on screen.
Human vision is far more sensitive to brightness changes than to subtle colour shifts. Smart compression exploits this by discarding redundant chroma data without softening the edges and type that readers actually notice.
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Lossy versus lossless compression
Before moving a quality slider it helps to understand the two approaches:
- Lossless compression reorganises binary data without altering a single pixel value. It suits diagrams, archival masters and technical artwork, and typically achieves a 10-25% reduction.
- Perceptually lossless compression selectively quantises high-frequency colour detail in areas the eye cannot resolve. It achieves 60-85% reduction and is what web delivery needs.
The golden 75-82% quality band
In JPEG and WebP encoding, quality between 85% and 100% increases file size exponentially while adding almost nothing you can see. Dropping from 100% to 80% typically cuts two thirds of the weight with no artefacts at 1x or 2x display scaling.
Always resize the image to its display container first. Encoding millions of invisible pixels wastes bytes no quality setting can recover.