Quick answer: Lossy compression makes files much smaller by permanently discarding detail that is hard to see or hear.
Lossy (Lossy compression) explained
Lossy compression makes files much smaller by permanently throwing away some detail. JPEG, HEIC, MP3, AAC, H.264 and HEVC are all lossy. They rely on how human eyes and ears work, removing fine detail most people will not notice.
The loss adds up. Each time a lossy file is edited and saved again, or re-compressed by an app, a little more quality goes. Heavy compression shows up as blockiness, blur or muffled sound.
Keep an original when you can, and compress only the copy you send. When the recipient needs the best quality, send the original file unchanged.
Worked example: why lossy compression exists
A 12-megapixel photo is 4,000 × 3,000 = 12,000,000 pixels. Stored without compression at 3 bytes per pixel (red, green and blue), that is 12,000,000 × 3 = 36 MB for a single photo.
A good-quality JPG of the same photo is typically just a few megabytes, and most people cannot tell the two apart on screen. Video is even more extreme: without lossy codecs, a minute of 4K would be far too large to store on a phone or stream over the internet.
Signs of too much compression
| File | What you notice |
|---|---|
| Photos | Blocky patches in skies, halos around edges, smeared fine detail like hair and leaves |
| Video | Blocks in dark scenes and fast motion, faces turning waxy |
| Audio | Swishy or underwater sound in cymbals and applause, a dull top end |
How to keep quality when sharing
- Keep the original file, from the camera or phone, somewhere safe.
- If you need a smaller copy, make it once from the original with a high quality setting.
- Avoid sending through apps that re-compress, especially for work that will be printed or edited.
- Send the original with a download link, which delivers it unchanged, up to 20 GB.
Lossy does not mean low quality. A phone video in HEVC or a high-quality JPG is lossy, yet looks excellent. Problems come from heavy settings and repeated re-compression.
Frequently asked questions
Can I undo lossy compression?
No. Discarded detail cannot be recovered. Converting a JPG back to PNG just stores the already reduced image in a bigger file.
Is lossy always bad?
No. At sensible settings it is hard to spot, and it makes streaming and sharing practical.
Why do my videos look worse after sending them on a chat app?
Many chat apps re-compress videos to save data. Sending the file as a document or with a download link keeps the original quality.
Should I use JPG or PNG for photos?
JPG for photos, because it is much smaller with little visible difference. PNG is better for screenshots, logos and images with sharp text.
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