Field guide
Why quality 90 is not 90 percent
A JPEG or WebP quality number is an encoder setting, not a measurement. Ninety does not promise ninety percent of the original information, pixels or bytes.
01
Each encoder maps the scale to quantisation tables differently. Quality 80 in one browser or library may resemble quality 74 or 88 in another.
02
File size depends on image content. Flat sky compresses cheaply, while foliage, hair and sensor noise require more coefficients at the same setting.
03
Compare outputs at the size people will actually see, then inspect edges and gradients at one-to-one scale. A target byte count is useful only after visual limits are defined.
04
Repeated saves compound loss even when every save says 90. Return to the earliest available source for each delivery rather than recompressing yesterday's compressed output.