Choosing image and document file formats

Photographs and graphics want different formats

Images with smooth colour gradients, like photographs, compress well with lossy methods. Logos, text and screenshots have hard edges, and lossy compression makes those edges messy. What the image contains is the first thing that decides the format. When one image mixes a photo with text, favour keeping the text crisp: save as PNG, or use a lossy format at a high quality setting.

• Photographs: JPEG or WebP
• Logos, icons, screenshots: PNG
• Shapes and text that must scale cleanly: SVG
• Short moving images: WebP or video rather than GIF

Do you need a transparent background?

JPEG cannot represent transparency, so saving with a transparent background fills it with white or black instead. To place a logo over another background you need PNG or WebP. Backgrounds turning unexpectedly black in a conversion tool almost always trace back to this. GIF can only show fully transparent or fully opaque pixels, so logos with soft edges or semi-transparent shadows end up with rough outlines.

Quality drops each time you re-save

Lossy formats lose a little more quality every time you open, edit and save again. Keeping one copy of the original and editing from it each time avoids the accumulation. Lossless formats stay identical however many times you re-save. Messengers and social platforms often recompress on upload as well, so re-posting a photo you received drops its quality another step.

Which format to send a document in

If the other person needs to edit it, send an editable format; if they only need to read it and the layout must hold, send a PDF. Document files can carry author information and revision history, so it is worth checking before sending anything outside. For a document several people must edit, working together in one shared document avoids the version tangle of passing files back and forth.

The order for shrinking a file

Reducing dimensions before changing format has the larger effect. People often use full-resolution images where a screen-sized one would do, and resizing alone cuts the file sharply. Adjust quality next, and change format last. For photos viewed on screen, resolution far beyond the display only adds size with almost no visible difference.

Phone photos and HEIC

iPhones save photos as HEIC by default. It is usually smaller than JPEG at the same quality, but some programs and websites cannot open it or refuse the upload. In that case, convert to JPEG before sending, or switch the camera setting to the more compatible format. Keeping originals in HEIC and converting only when sending also works well.

Formats at a glance

Here are the formats you meet most, in brief. None is simply better; they suit different jobs, and whether the recipient can open the file is the final test.

• JPEG: lossy, for photos, no transparency, the widest compatibility
• PNG: lossless, supports transparency, large when holding photos
• WebP: a web format supporting lossy, lossless, transparency and animation
• SVG: stores shapes as coordinates and formulas, sharp at any size
• PDF: fixed-layout documents, suited to printing and submission

Common mistakes

Most format trouble comes from a few habits. The most common misunderstanding is that renaming the extension changes the format. Changing the end of a file name from jpg to png leaves the content untouched and can stop some programs opening it. To change format, use a converter or the save-as option in an editing program.

• Renaming the extension and assuming it was converted
• Expecting lost quality to return when turning a JPEG into PNG
• Saving text-heavy screenshots as JPEG so the letters blur
• Deleting the original and keeping only a compressed copy