History Lesson: What Is a .cr3 File? Canon’s RAW Photo in an MP4-Style Container

What is a .cr3 file?

A .cr3 file is the third generation of Canon’s proprietary camera RAW. RAW means the data the sensor recorded with little processing — a “digital negative” you develop later — rather than a finished JPEG. Canon Europe’s file-format guide says Canon cameras saved RAW as .CR2 until the DIGIC 8 image processor arrived with the EOS M50, which enabled the .CR3 format. Canon announced that camera on 26 February 2018, describing it as the first to feature the new CR3 14-bit RAW format and a C-RAW option.

C-RAW stands for Compact RAW; Canon’s US training pages stress that it is not “compressed RAW.” It keeps the full pixel resolution but uses lossy compression — some image information is discarded — to shrink the file. Canon’s materials do not agree on how much: the M50 launch announcement said about 30–40% smaller, while Canon Europe’s guide says 35–55%. Canon’s US pages add that its engineers have not disclosed exactly how C-RAW achieves this.

What is inside a .cr3 file?

Open a CR3 in a hex viewer and the first thing you meet is not a photo header but a box. In the ISO base media file format (ISOBMFF, standardized as ISO/IEC 14496-12), all data lives in boxes, and each box starts with a 4-byte size and a 4-byte type. The first box is ftyp (file type). Its brand — the label naming which specification the file follows — is “crx ” (three letters and a space) in a CR3, and that byte pattern is how registries such as the UK National Archives’ PRONOM recognize Canon RAW 3.0 files.

After ftyp comes moov, the box that describes the file’s contents, then Canon-specific uuid boxes, then mdat, where the picture data sits. In Laurent Clévy’s reverse-engineering notes, Canon’s metadata boxes (named CMT1 to CMT4) hold TIFF-style structures with the Exif camera data, Canon’s MakerNotes (manufacturer-specific tags), and GPS data. The image data is split into tracks — the same word video files use for streams. On an EOS R, his notes list a full-size JPEG preview, a small RAW image, and the full-resolution RAW image, plus Canon’s timed metadata and, on some cameras, a second image for Dual Pixel data.

The compression is Canon’s own crx codec. Clévy’s notes describe it as a mix of ideas from JPEG-LS (Golomb-Rice and run-length coding) for lossless RAW and from JPEG 2000 (a wavelet transform, which splits an image into coarse and fine detail layers, plus quantization) for lossy C-RAW. Wikipedia lists CR3 as reverse-engineered: Alexey Danilchenko worked out the codec, FastRawViewer 1.5.1 implemented it, and the LibRaw project’s decoder source followed in October 2019, more than a year and a half after the M50 was announced.

A cinema codec, and more than one picture

The crx codec did not debut in a photo format. Canon announced its EOS C200 cinema camera on 31 May 2017, almost nine months before the M50, and it records CRM (Canon RAW Movie) files; Canon’s manual calls CRM a Canon proprietary file format for Cinema RAW Light. Clévy describes the CRM layout as very similar to CR3, with the same “crx ” brand and the same codec. Only a version string inside Canon’s metadata box says “CanonCRM” instead of “CanonCR3.”

One CR3 can also hold more than one picture. Clévy documents burst “roll” files named CSI_*.CR3 that can contain up to 70 pictures, on cameras such as the EOS M6 Mark II and the PowerShot G7 X Mark II. A box-and-track container can hold a sequence as easily as a single frame.

Why CR3 files still sit on disks in 2026

Because the format was new, software had to catch up. Canon Europe’s guide notes that RAW files differ by camera model even within CR2 or CR3, which is why converters are updated for each new body. Adobe forum answers from spring 2018 point users to Camera Raw 10.3, released in April 2018, or newer — or to Adobe’s DNG Converter 10.3 or newer — for M50 files.

By 2026, a camera that wrote its first CR3 in 2018 is eight years old. Clévy’s notes list CR3 files from many Canon bodies, including the EOS R, EOS RP, EOS 90D, EOS 250D, and EOS M6 Mark II, so card dumps, Lightroom catalogs, and “old camera in the drawer” backups hold them. Windows can show a blank icon until a RAW decoder is installed. These are legacy files in the everyday sense: valuable captures that need software that understands Canon’s box layout and crx codec.

Legacy File Converter converts CR3 and other listed camera RAW extensions (CR2, DNG, NEF, ARW, and related family members on Supported File Types) locally on Windows to JPG or PNG. Nothing is uploaded. This article stays the history; for practical open-and-convert steps, see Opening RAW Camera Files Without the Camera Software. For the format CR3 replaced, see History Lesson: What Is a .cr2 File? Canon RAW.