The X3F file (.x3f)

X3F is the raw format of cameras with a Foveon X3 sensor. Foveon, a Californian company, announced the sensor in February 2002, and the first camera to use it was Sigma's SD9 digital SLR. Sigma went on to use Foveon sensors in all its SD-series SLRs and DP-series compacts, and bought Foveon in 2008.

When one of these cameras saves a raw photo, it writes an X3F file: the values from all three layers of the sensor, previews, the camera settings and the data needed to turn them into colours. Sigma's free Sigma Photo Pro software is the reference way to process them.

X3F is a niche format. Foveon sensors work differently from almost every other camera sensor, so the usual raw processing steps don't apply, and only a handful of applications read the files. Sigma's newer cameras with conventional sensors, the fp, fp L and BF, save DNG instead.

In a normal camera sensor, each photosite sits under a red, green or blue filter (a Bayer pattern) and records one colour. Software then estimates the two missing colours at every pixel from its neighbours. This step is called demosaicing, and every raw converter is built around it.

A Foveon sensor has no colour filters. It stacks three layers of photodetectors in the silicon and relies on how deep each wavelength of light travels: blue is absorbed near the surface, green deeper and red deepest. Every pixel position records three values, so there's nothing to demosaic. Sigma says the sensor also needs no low-pass filter, because it doesn't produce colour moiré.

• The values aren't red, green and blue. Each layer responds to a broad, overlapping range of colours. Turning the three readings into accurate colour takes heavy processing based on calibration data the camera writes into the file.
• That calibration data is protected. The US National Archives notes that Sigma encrypts data needed to process the raw file, and libopenraw describes the raw data as encrypted. Third-party developers have had to reverse-engineer it.
• Raw converters can't reuse their pipelines. Software designed around demosaicing a Bayer pattern needs a separate path for Foveon data. Many projects never added one.

The file's version number changed along the way. ExifTool reads version 2.x headers one way and version 4.0 and later another way, and warns about versions it hasn't seen. Files from all generations still start with FOVb, so identification works the same.

The sd Quattro H's DNG option shows the trade-off. In tests published in 2017, diglloyd found its DNG files took about 150 MB each against about 68 MB for X3F, while holding 12-bit values instead of 14-bit, and the Sigma Photo Pro of the time couldn't process those DNGs.

• Adobe: Camera Raw and Lightroom read X3F files from the earlier Sigma cameras, but never added the Merrill and Quattro generations. A feature request for Merrill support, posted on Adobe's forum in December 2013, is still marked "Not Prioritized". Adobe's DNG Converter can't help with files Adobe can't read.
• Apple: Apple's list of supported raw cameras includes Sigma's fp, fp L and BF, which record DNG, and no X3F camera, so Photos and Preview don't decode X3F files.
• LibRaw: the open-source library behind many tools, including Microsoft's Raw Image Extension, has carried a Foveon decoder based on Roland Karlsson's X3F tools since 2013. Since LibRaw 0.20 (July 2020) it's only compiled in when a build sets USE_X3FTOOLS, so whether an application built on LibRaw opens X3F depends on how it was built.
• Other converters: darktable doesn't open X3F files; RawTherapee opens some. Colours can differ from Sigma Photo Pro's, since each project had to work out the processing on its own.

• Sigma Photo Pro (Windows and macOS, free): supports all of Sigma's SD, DP, sd Quattro and dp Quattro cameras, plus the fp series' DNG files. Version 6.8.5 came out in February 2024. It exports JPEG and 16-bit TIFF.
• Affinity Photo: develops X3F files from the SD and DP cameras, including the Merrill and Quattro models.
• Photoshop: Sigma released a free plug-in in 2018 that imports X3F files into Photoshop CC.
• Just a look: the JPEG preview inside the file is enough for browsing, and any tool that reads it, or ExifTool, can show it.

# camera, lens, settings and the file version
exiftool photo.x3f

# extract the full-size JPEG preview (not on SD9 and SD10 files)
exiftool -b -JpgFromRaw photo.x3f > preview.jpg

For long-term storage, the US National Archives recommends converting X3F files to DNG. In practice, many Sigma owners keep the X3F originals alongside 16-bit TIFF exports from Sigma Photo Pro, which any image editor can open.

• "Unsupported file" in Lightroom or Photoshop. The file is probably from a Merrill or Quattro camera. Use Sigma Photo Pro or Affinity Photo, or Sigma's Photoshop plug-in.
• Only a small or low-quality image appears. The viewer is showing an embedded preview. Use a real X3F converter to get the full image.
• Strange colours or magenta casts in third-party software. Foveon colour processing depends on calibration data that other converters only partly understand. Compare with Sigma Photo Pro.
• An X3I file won't open anywhere. These files come from the Quattro cameras and need Sigma Photo Pro.
• Metadata edits don't stick. ExifTool can only write to the JPEG preview inside an X3F, not to the properties section.

X3F files can't run code, but they do contain the camera's serial number and the date and time, so check them with exiftool before publishing originals. Like any parser, X3F decoders have had security bugs: LibRaw fixed an out-of-bounds read and a possible stack overrun in its X3F parser in 2018 (CVE-2018-10529 and CVE-2018-10528), and now limits the X3F decoder's memory use. Keep raw software up to date.