Why choose PNG for an iPhone photo
HEIC is the format iPhones have used for photos by default since iOS 11. It is based on HEIF, the High Efficiency Image Format, and it keeps photos compact through lossy compression. That efficiency comes with a catch: many programs outside Apple devices cannot open the files at all.
PNG solves the compatibility problem in a particular way. It is lossless: once the HEIC has been decoded, every pixel is written to the PNG exactly, and it stays that way however many times you open, edit and save the file. That makes PNG a sound working format when a photo is about to be:
- retouched or annotated over several sessions in an image editor;
- cut out, for example to remove the background, since PNG can store the transparency you create;
- placed in a design tool or upload form that lists PNG but not HEIC;
- combined with screenshots, text or graphics that need to stay crisp.
The trade-off: much larger files
PNG is the heavy option for photographs. Lossless compression has to record every speck of grain and texture, so a PNG made from an iPhone photo is usually several times larger than a JPG of the same picture, and larger still compared with the original HEIC. A folder of PNG photos can fill a disk or hit an attachment limit quickly.
If you simply want photos to open on a PC, send them to someone or print them, HEIC to JPG is the better choice: the files stay compact and the quality difference is usually hard to see. Choose PNG when you will keep working on the image, then export a compact copy once the edits are finished, for example with PNG to JPG.
Reducing the dimensions is the most effective way to keep PNGs manageable. Settings → Size can scale photos by a percentage or fit them within a maximum width and height, without distorting them and without ever enlarging them.
What carries over, and what does not
- Pixels: stored exactly as decoded. Browsers work with 8 bits per channel, so that is the precision of the PNG.
- Colour and HDR: output is in sRGB, and HDR photos are converted to standard dynamic range. Photos captured in Display P3 may look slightly less saturated than on the iPhone.
- Orientation: applied to the pixels, so the PNG is upright in every program.
- Metadata: EXIF data such as GPS location, capture date and camera model is not copied. That protects your location when you share the PNG, but caption and copyright fields are dropped too.
- Live Photos and bursts: only the still image is converted. The Live Photo video is not included, and for files with several images, the primary image is used.
- Transparency: ordinary camera photos have no transparent areas, so the PNG starts out fully opaque. It can hold any transparency you add later in an editor.
HEIC and PNG compared
| Aspect | HEIC | PNG |
|---|---|---|
| Compression | Lossy, very efficient | Lossless |
| Size of a photo | Small | Usually several times larger than a JPG of it |
| Editing and re-saving | Each lossy save loses a little | No further loss |
| Images per file | One or several (for example bursts) | One |
| Transparency | Rare in photos | Full alpha channel |
| Opens on | Apple devices; elsewhere not always | Practically everywhere |
| Best for | Storing photos on an iPhone | Editing, design work, exact pixels |
Decoding happens in your browser
Safari can read HEIC on its own. Chrome, Edge and Firefox cannot, so the first time you add a HEIC file in one of them, Convertisio downloads a built-in WebAssembly decoder of about 2 MB. It is the decoder that travels over the network, not your photos: decoding and PNG encoding both happen on your device.
Each photo can be up to 50 MB and up to 100 megapixels on a desktop computer (50 megapixels on iPhone, iPad and devices that report low memory). Safari on iPhone and iPad also limits canvas size to about 16.7 megapixels, so larger photos are scaled down there automatically, with a notice. Up to 10 files are converted per batch, one at a time.
Converting HEIC photos all the time? On the iPhone, Settings → Camera → Formats → Most Compatible makes the camera save new photos as JPG instead, at the cost of more storage space on the phone.