When a PNG copy of a HEIC photo makes sense
HEIC is a lossy format, and JPG is too. If you convert a photo to JPG, edit it, and save it as JPG again, it is compressed a second and a third time, and each round adds a little damage. A PNG stores the decoded picture exactly, so you can edit it and save it again as often as you like without further loss until the final export.
- Photo editors and design tools that cannot open HEIC, but where you do not want JPG artefacts in a file you will edit.
- Cut-outs and compositing, where PNG's lossless pixels keep clean edges for layers you mask later.
- A working copy before a large batch of edits. Make the PNG once, then export the finished pictures as JPG or WebP.
If the photo is only going to be viewed or shared, a JPG is a better fit, and HEIC to JPG makes one that is a fraction of the PNG's size.
What the PNG contains
The PNG holds the picture as the HEIC decoder returns it, converted to sRGB at 8 bits per colour. It is not the original: HEIC had already discarded detail when the phone saved the photo, and that detail is gone. The PNG just stops any further loss.
- No transparency. A camera photo has no see-through areas, so the PNG is fully opaque.
- No metadata. EXIF data cannot be copied from a HEIC original, so the date, camera and location are not in the PNG.
- One picture. The 48 tiles in the iPhone file we examined are assembled into the main photo. Its thumbnail and extra greyscale picture are ignored.
- Complete files only. A HEIC cut short by an interrupted transfer is refused with a message, instead of giving a PNG with green blocks where tiles are missing.
How big the PNG gets
A PNG keeps every pixel, so a photograph is large. Our 12 megapixel iPhone photo, 4032 by 3024 pixels, was 2.7 MB as a HEIC and 15.7 MB as a PNG. macOS's own PNG export of the same photo was 19.5 MB, so ours is about a fifth smaller. In our test the PNG was nearly six times the size of the HEIC, and macOS's export was more than seven times.
Above 16 megapixels the optimiser is skipped to keep things fast, and the list says so on that file. Pictures above 100 megapixels are refused. Phones and small laptops have less memory than desktops, so convert large photos a few at a time there.
How the colours were checked
Camera photos use Display P3, and a decoder that returns the raw numbers makes them look washed out. To test the conversion we took a normal sRGB photo, converted it to Display P3, saved it as a HEIC, and compared the PNG from this page with the original photo. Measured as PSNR, where a higher number means closer to the original, our PNG scored 39.0 dB and macOS's own conversion of the same HEIC scored 38.6 dB. The two conversions differ from each other by about one level out of 255 on average, and most of what separates either of them from the original is the HEIC compression itself.
In plain words: colours come out the same as Apple's own export, not duller.