HEIC - High Efficiency Image File Format

A HEIC file is a modern photo container, and what an iPhone produces by default. The underlying standard is HEIF, the High Efficiency Image File Format, published by MPEG, the same standards group behind MP3 audio and MP4 video. HEIC is the flavour of HEIF whose pictures are compressed with HEVC, also called H.265, a video compression method. Apple adopted it to store comparable photo quality in a much smaller file, and to hold the extras a modern camera produces, such as depth information, bursts and reversible edits, which a single flat image cannot express.

How a HEIC stores its picture

HEIF borrows its structure from MP4: the file is a tree of labelled boxes, one listing the items it contains, one recording where each item's data begins, one holding properties, and one holding the data itself. The pictures inside are coded with HEVC using its still-image techniques: predicting each block from its neighbours, transforming the difference into frequency values and rounding them off. That is normally lossy, which is where the size saving comes from; lossless coding is permitted but rare.

The container is the more interesting half, because a HEIC is not necessarily one image. One file can hold:

• The main photo, often stored as a grid of tiles assembled when it is displayed.
• Thumbnails and other reduced previews.
• Auxiliary images not meant for viewing, such as a depth map recording how far away each part of the scene was, or a transparency mask.
• Several related stills, such as a burst or a sequence.
• An untouched original plus instructions describing an edit, so the change can be undone later.

Those extras are why portrait-mode background blur can be adjusted after the fact, and why exporting to JPG produces one flat picture and silently leaves everything else behind.

What metadata a HEIC carries

Everything a JPG carries, stored differently, plus some things only a container can express. Metadata means the descriptive information kept alongside the pixels.

• EXIF, the camera metadata standard, sits as an item inside the file's metadata box rather than as a marker at the start. The fields are the familiar ones: Make, Model, LensModel, DateTimeOriginal, ExposureTime, FNumber, ISOSpeedRatings and FocalLength.
• GPS tags, part of EXIF, give latitude, longitude, altitude and a timestamp in UTC, and phone photos very often have them.
• Maker notes, the manufacturer's private block, which on Apple devices records a good deal about how the shot was captured and processed.
• XMP, Adobe's XML metadata, can be stored as an item too, carrying creator, description, keywords and ratings.
• Structural properties that behave like metadata: a rotation property and a mirroring property, applied instead of re-encoding the pixels; a cropping property that narrows the visible area; a colour-information property holding an ICC profile or colour-space codes; and a pixel aspect ratio.

Because one file can hold several images, a report on a HEIC may show several sets of fields, one per item, rather than a single tidy list. Two properties are worth understanding: a rotation property means the picture is stored one way and displayed another, and a cropping property means pixels you cannot see are still inside the file. On the personal side, a HEIC from a phone is one of the most revealing image files there is: location, device model, capture time, and the identifier pairing a still with its Live Photo video all sit in it by default.

When to use HEIC

HEIC is a good format for keeping photos. The files are smaller than equivalent JPGs at a comparable quality, the format supports 10 bits per colour channel for smoother skies and gradients, and it preserves the depth maps and edit instructions that make later adjustments possible.

It is a poor format for sending things to other people. Support outside Apple's ecosystem is uneven: some browsers, older phones, printers, upload forms and office applications will simply refuse it, and the HEVC compression inside carries patent licensing that has slowed adoption. The usual compromise is to keep the HEIC and share a JPG, accepting that the copy loses the depth map and the other images. WebP and AVIF address the same size problem with fewer licensing complications.