How to tell if compressing a photo ruined it (JPEG artefacts explained)
You squeezed a photo down to a smaller file and it looks fine as a thumbnail, but you are not sure whether it will look rough once someone opens it full size.
Why compressed photos change at all
JPEG and lossy WebP save space by throwing away detail your eye is unlikely to miss. The picture is cut into small tiles (8×8 pixels in JPEG), the fine detail in each tile is rounded off, and colour is stored at lower resolution than brightness. Lower quality means more rounding. Done gently, nobody can tell; pushed too far, the rounding becomes visible as artefacts. More background in lossless vs lossy compression.
The five things to look for
| Artefact | What it looks like | Where to look |
|---|---|---|
| Blocking | A faint grid of 8×8 squares, most obvious when quality is very low | Shadows, smooth gradients, dark areas |
| Ringing (halos) | Fuzzy ripples or "mosquito noise" next to sharp edges | Text, logos, line drawings, horizons, branches against sky |
| Banding | A smooth gradient turns into visible steps or rings | Sky, sunsets, studio backdrops, painted walls |
| Colour bleeding | Strong colours smear past their edges, red text looks soft | Bright red or blue text and small coloured details |
| Smearing | Fine texture turns to mush, skin looks waxy | Hair, grass, fabric, faces |
Screenshots, diagrams and anything with text suffer far sooner than ordinary photos, because their sharp edges are exactly what lossy compression rounds off. For those, PNG or WebP at high quality is usually the better choice (see PNG vs JPEG).
How to compare properly
- Same size, same place. Two thumbnails next to each other hide almost everything. In the image compressor, drop the photo and use the Before / after slider: the original is on the left of the line and the compressed version on the right, drawn in the same frame, so you can sweep across one detail.
- Zoom to actual pixels. Press Zoom to actual pixels to see the compressed picture at its real size, with the original scaled to match. This is how it looks when someone opens it full screen on a laptop. Scroll to a face, a sign or the sky.
- Look where damage shows first (the table above), not at the busy middle of the picture, where detail hides artefacts well.
- Step the quality down until you see it, then go back up a notch. The preview redraws as you move the slider. The last setting where you could not tell is your answer for this photo.
- Judge at the size it will be seen. A photo for a 1280 px wide website, set with Max width, can usually take lower quality than one people will print or zoom into.
How low can you go?
There is no universal number, because quality scales differ between programs and photos differ wildly. As a rough guide for the browser encoders this tool uses:
- 90 to 100: visually identical, but files are much bigger than they need to be. Worth it only for photos that will be edited again.
- 75 to 85: the sweet spot for most photos. Usually indistinguishable at normal viewing size, often a half to a tenth of the original phone file.
- 55 to 70: fine for small web images and email; banding in skies and halos around text start to appear.
- Below 50: blocking and smearing are usually obvious full size. Shrinking the picture's dimensions is often a better trade than going this low.
When you have a hard limit such as "under 200 KB", type it as the target file size: the tool lowers quality first and only then trims the dimensions, and the slider shows you what that cost. If the result looks rough, a smaller width at better quality almost always looks better than a large picture at very low quality. See getting a photo under 100 KB.
Things that are not compression damage
- Sideways or upside-down photos come from a hidden orientation tag, not from compression; see why photos turn sideways.
- Slightly different colours on another device are often colour-profile handling, not quality loss.
- Re-saving repeatedly does add damage each time with JPEG ("generation loss"), so keep the original and always compress from it, never from an already-compressed copy.
Related
Phone camera blurry? Causes and fixes · Is pixelating or blurring a password safe? · Image compressor with before/after slider · Compress an image without uploading it · Get a photo under 100 KB for a form · Lossless vs lossy compression · Megapixels vs file size
Questions
How much can I compress a photo without losing quality?
With JPEG or WebP, a quality setting around 75 to 85 is usually impossible to tell apart from the original at normal size, while the file is often many times smaller. Check by comparing both versions at 100% zoom on sky, faces and text.
What are JPEG artefacts?
Visible side effects of lossy compression: a grid of 8×8 blocks, fuzzy halos around sharp edges, steps or bands in smooth gradients, smeared colour and loss of fine texture. They appear when quality is set too low.
Why does my compressed picture look blocky or banded?
The quality is too low for that picture. Smooth gradients like sky are the first to show steps. Raise the quality a little, or reduce the width and height instead of lowering quality further.
Is quality 80 the same in every app?
No. Each encoder has its own scale, so 80 in one program can look different from 80 in another. Judge by eye with a before/after comparison rather than trusting the number.
Can I compare the original and compressed image without uploading them?
Yes. The Tooltern image compressor runs in your browser and shows a before/after slider with a zoom to actual pixels. The photo never leaves your device.
Published · updated .