Lossy compression is image encoding that achieves smaller files by permanently discarding information the algorithm judges to be visually unimportant. The decoded image is an approximation of the original, not a bit-perfect reconstruction. JPEG, HEIC, WebP at default quality settings, and most consumer video codecs are lossy. The appeal is dramatic size reduction: a 50-megabyte 24-megapixel raw can become a 4-megabyte JPEG with quality losses most viewers will never notice on a phone screen.
Most lossy still-image schemes work by converting blocks of pixels into the frequency domain (typically with a discrete cosine transform), then quantizing the high-frequency coefficients more aggressively than the low-frequency ones. Because human vision is more sensitive to broad tonal gradients than to fine high-frequency detail, the discarded data is often invisible at normal viewing distance. Push quality settings too low and the suppressed coefficients reveal themselves as blocky 8×8 squares, ringing around edges, and other artifacts.
The key pitfall is that losses compound. Re-saving a JPEG repeatedly, especially with edits between saves, accumulates quantization errors that eventually become obvious as banding in skies, color shifts in skin tones, and degraded edges. This is why disciplined archival workflows preserve a raw or TIFF master and export fresh JPEGs from that master each time a new size, crop, or color treatment is needed, rather than editing the JPEG itself.
HEIC, introduced with iOS 11 in 2017, applies HEVC video compression techniques to still images and delivers roughly half the file size of JPEG at comparable quality, with optional 10-bit color and built-in support for image sequences. WebP and AVIF offer similar gains for web delivery. Despite these newer options, JPEG remains the lingua franca of photographic delivery because it is universally readable on every device, browser, and printing service.
Practical tips: export at the highest quality the destination tolerates, usually quality 80 to 95 for web JPEGs, knowing diminishing returns set in past that. Avoid intermediate JPEG steps inside an editing workflow; keep originals in raw or lossless form and treat JPEG strictly as a final delivery container. If a client requests edits to a JPEG-only file, request the source whenever possible. And recognize that what looks fine on screen at 1500 pixels wide may show compression damage when the same file is enlarged for print, where every artifact gets bigger along with the image.
Lossy is not inherently bad. For the vast majority of online use, social media, email attachments, and contact-sheet review, the size savings far outweigh imperceptible quality loss. Problems arise only when lossy formats are mistakenly used as masters or repeatedly edited and re-saved.