How to compress an image without losing quality

File size and visible quality do not move in lockstep

A smaller file is not automatically unsuitable, because encoders represent repeated or visually less prominent information efficiently. Results vary by image, format, encoder, settings, display, and viewer. No universal 70–80% saving or invisible difference applies to every photograph. Judge representative exports at the size and conditions in which they will be used.

Lossless and lossy answer different needs

Lossless encoding allows the same decoded pixel values to be recovered, although metadata or unsupported features may still change during conversion. PNG is lossless; JPG is lossy. Photographs often compress efficiently with lossy formats, but archival masters, scientific images, repeated editing, text, and evidence may require a lossless source.

A quality number is not a universal scale

Quality controls are encoder-specific; 75 in one tool is not equivalent to 75 in another format or library. File size and artifacts change nonlinearly with content. Start from a high setting, compare at the intended display size and at 100% zoom, and lower it only until the file meets its real requirement. Do not present 65–80 as a safe range for every photo.

Measured results from one photograph

• Physical dimensions. A photo resized down from 4000px wide to 1600px wide (still plenty for most web and screen use) will end up far smaller than the same photo merely re-compressed at full size.
• File format. Switching from PNG to JPG for a photograph alone often cuts the size significantly, independent of any quality slider.
• Metadata removal. EXIF, XMP, IPTC, thumbnails, profiles, and GPS fields can add bytes. Removing them may reduce size and improve privacy, but can also discard dates, color information, authorship, and workflow data. Check what the tool preserves.

Combining a modest quality reduction with an appropriately smaller pixel size is usually more effective, and produces a better-looking result, than pushing the quality slider down aggressively on a huge image.

A concrete example

In the measured Orisod example, one 2.0 MB image became about 950 KB at the tool's quality 80 and about 459 KB at quality 30. Those figures describe that image and encoder only. Compare edges, gradients, skin, text, and fine texture in your own export; a normal-size preview can hide artifacts that become visible when cropped, printed, or edited again.

How to compress properly

Work from the original, define the required dimensions, format, maximum bytes, transparency, and metadata, then make test exports. Compare the image at its final display or print size and inspect critical details at full resolution. Keep the source and record settings when repeatability matters.

Checks to make before replacing the source

Does repeated compression cause damage? Re-encoding with a lossy codec can accumulate artifacts, although the effect depends on settings and content. Export derivatives from the best available source instead of repeatedly recompressing a derivative.

How do compression and resizing differ? Encoding can change file size without changing dimensions; resizing changes the pixel grid and also requires resampling and usually re-encoding. Both can alter quality.

Is there one correct quality setting? No. Content, format, encoder, destination, later editing, and acceptable risk determine the setting. Use measured outputs, not a universal percentage.

The goal is the smallest file that still satisfies the actual visual, technical, and archival requirements. A derivative may be acceptable for sharing while the original remains necessary for editing, printing, or preservation.