Compress JPEG files

What the JPEG quality number means

JPEG stores a photo in small blocks of 8 by 8 pixels and keeps less fine detail in each block the lower the quality goes. The number is not a percentage of anything you can measure: 85 does not mean 85% of the quality. Think of it as a dial from “keep almost everything” to “keep the outline only”.

The dial is uneven. Going from 100 to 90 often shrinks the file a lot while the photo barely changes, because the top of the scale stores detail nobody sees. Going from 60 to 40 saves less and shows more. Signs that you went too far:

• Blocks in smooth areas such as sky, walls and skin.
• Halos and specks around sharp edges and lettering.
• Smudged color along the edges of strong reds and blues.

In Make smaller the slider runs from 10 to 100 and starts at 85. In the Photo Editor's Export window it runs from 30 to 100, starts at 88 the first time, and remembers the quality you used last.

Pixels count more than quality

A 12-megapixel phone photo is 4032 × 3024 pixels. Set Longest side to 2048 and it becomes 2048 × 1536: about a quarter of the pixels, and the file shrinks roughly in step, before you touch the quality at all. On a phone or laptop screen the two look the same.

So when a JPG is too big, ask first whether it needs all its pixels. Prints and photos you will crop later do; pictures for a website, a chat or a form rarely do. Reduce the pixels, then use a quality of 80 to 90, and the file is usually small enough without any visible loss.

Longest side works for portrait and landscape photos alike, since it limits whichever side is longer. Export does the same job with its Size list of widths, and the image resizer page has a table of common ones.

Every save costs a little

JPEG loses something every time it is written. Open a JPG, save it, open that copy and save again, and each round trims a bit more detail, even if you changed nothing. A few rounds at high quality are hard to spot; many rounds at medium quality are not.

Octoolo writes a fresh JPG each time you export or compress, so plan around that:

• Make each smaller copy from the original, not from an earlier copy. Octoolo never changes the original, so it is always there to start from.
• Edit first, compress last. Crop, straighten and fix the light in the Photo Editor, then export once at the quality you need.
• Keep your work between sessions with File › Save. It writes an Octoolo project (.ophoto) that holds every layer exactly, with no JPEG loss at all.

When WebP or PNG is the better choice

Stay with JPG when the picture goes to people or places you do not control: email, forms, printers, older programs. Pick something else when:

• It is for a website or an app you run. WebP looks the same at a smaller size; see JPG to WebP.
• It is a screenshot, a chart or a scan of text. JPEG smudges sharp lines. Windows saves screenshots as PNG, and they are best left that way unless size really matters.
• It needs a clear background. JPEG has none, so clear areas turn white. Use PNG or WebP.

One thing that never helps: turning a JPG into a PNG to “improve” it. The PNG keeps the JPEG's blocks exactly, in a much bigger file.