Reference Formats
X3F
X3F is Sigma's proprietary raw format built specifically for Foveon X3 sensors, which capture full red, green, and blue color at every pixel site instead of using a Bayer mosaic.
Extension
.x3f
Full name
Sigma X3F (Foveon X3 RAW)
Introduced
2002, with the Sigma SD9, the first camera to use a Foveon X3 sensor
Vendor
Sigma / Foveon
What is inside the container
X3F uses a proprietary multi-section container storing three stacked color planes per pixel (captured via Foveon's layered-photodiode sensor, which exploits silicon's wavelength-dependent light absorption) rather than the single-value-per-pixel Bayer data most raw formats hold, compressed with a Huffman-based scheme, plus an embedded JPEG preview and Sigma processing metadata. Because there is no demosaicing step, X3F decoding logic differs fundamentally from Bayer raw decoders, and Sigma's own color-processing pipeline in Sigma Photo Pro remains the closest thing to a reference implementation.
How to open it
For archives
X3F carries real long-term risk for a DAM: Sigma is a niche manufacturer, Foveon's stacked-sensor color science is unusually vendor-specific, and full-fidelity processing still depends on Sigma Photo Pro rather than any broadly adopted decoder. Convert to DNG for cataloging, search, and cross-tool preview generation, but retain the original X3F master — some Foveon color nuances and Super-Fine Detail modes cannot be reproduced outside Sigma's own software.
X3F arrived with the Sigma SD9 in 2002, the first production camera built around Foveon’s X3 sensor. Where a conventional Bayer sensor records only one color value per pixel and relies on interpolation (demosaicing) to reconstruct the other two, the Foveon X3 sensor stacks three photodiode layers at every pixel site, using the fact that red, green, and blue light penetrate silicon to different depths. The result is a sensor that captures full color at every pixel with no demosaicing required — a fundamentally different kind of raw data from any Bayer-based format in this reference series.
That sensor design is baked directly into X3F’s container structure: the format stores three complete color planes per capture, Huffman-compressed, alongside an embedded JPEG preview and Sigma’s own processing parameters. Sigma Photo Pro, the company’s dedicated raw software, remains the tool most closely aligned with how the sensor’s color data is meant to be interpreted, and it is required for Sigma’s Super-Fine Detail multi-shot mode, which produces the related .X3I files.
For a DAM, X3F sits at the higher end of format risk covered here. Sigma is a small manufacturer relative to Canon, Nikon, or Sony, third-party decoders (Adobe, darktable, RawTherapee) offer only partial parity with Sigma’s own color processing, and the sensor technology itself is unusual enough that generic raw-handling code cannot simply be adapted from Bayer logic. Keeping the original X3F alongside a DNG-converted access copy is the safer approach when the archive needs to preserve the option of reprocessing through Sigma Photo Pro later.
Frequently asked
Why is X3F decoding fundamentally different from decoding a Bayer raw format?
Foveon X3 sensors capture full red, green, and blue color at every pixel site via stacked photodiodes rather than a Bayer mosaic, so there is no demosaicing step, and X3F decoding logic differs fundamentally from Bayer raw decoders.
Can any third-party tool fully replicate Sigma's own X3F color processing?
No. Sigma Photo Pro remains the only tool that fully implements Foveon-specific processing, including Super-Fine Detail X3I files; other tools like darktable, RawTherapee, and Adobe Camera Raw offer support but color rendering can differ.
Should a DAM discard the original X3F after generating a DNG?
No. A DAM should keep the X3F master alongside any DNG derivative rather than replace it. DNG conversion tools (including Adobe's own DNG Converter) approximate Foveon color science but cannot fully reproduce Sigma Photo Pro's stacked-sensor processing or Super-Fine Detail (X3I) reconstruction. Discarding the X3F would permanently lock the asset to whatever color rendering the DNG conversion happened to capture, with no path back to native reprocessing.
What compression does X3F use?
X3F applies a lossless Huffman-based coding scheme to the raw sensor data, which stores three 12-bit color planes per pixel captured by the stacked Foveon photodiodes rather than a single mosaiced channel. This differs from the lossless-JPEG compression used by most Bayer raw formats. Alongside the compressed sensor data, the container carries an embedded JPEG preview for fast thumbnailing and a block of Sigma-specific processing metadata.
When was X3F introduced?
X3F was introduced in 2002 with the Sigma SD9, the first digital camera built around a Foveon X3 sensor. The same container format carried forward through the SD10, the DP compact series starting with the DP1, and later Merrill and Quattro-generation sensors, meaning a DAM may encounter X3F files spanning more than two decades of Sigma camera generations with varying internal sensor characteristics.
Why does X3F carry particular long-term risk for a DAM?
Compared to Canon, Nikon, or Sony raw formats, X3F sits behind a much smaller install base, so fewer independent tools have reverse-engineered it fully, and third-party decoders like ACR, darktable, and RawTherapee only approximate Sigma's own color rendering. Because true fidelity depends on Sigma Photo Pro, a single vendor's software, a DAM's long-term access to properly processed color from these files hinges on that application continuing to run and be maintained.
Does the Sigma Photoshop plugin cover all X3F variants?
No. The Sigma X3F Plug-in for Photoshop opens standard X3F raw files from cameras such as the DP Merrill, DP Quattro, SD1 Merrill, and sd Quattro series directly inside Photoshop's Camera Raw workflow. It does not process X3I files, the multi-shot Super-Fine Detail output; those still require Sigma Photo Pro for reconstruction, so a DAM cannot rely on the plugin alone to cover every Foveon capture mode.
What library gives open-source tools X3F support?
darktable relies on the RawSpeed library for low-level X3F container parsing and camera identification, though full non-Bayer Foveon decoding has been a persistent limitation for both projects. RawTherapee ships its own dedicated X3F reader, built up gradually since early Foveon models like the SD9/SD10/SD15, but coverage of later Merrill- and Quattro-generation X3F files has historically lagged behind Sigma Photo Pro. Neither open-source tool reproduces Sigma's proprietary color processing exactly.
Sources
- The Sigma SD9 was the first production camera to use the Foveon X3 image sensor, launched in 2002 checked 2026-08-07 — DPReview, Sigma SD9 coverage
- The Foveon X3 sensor stacks three photodiode layers per pixel to capture full RGB color at every pixel site without demosaicing checked 2026-08-07 — Wikipedia, Foveon X3 sensor
- Sigma Photo Pro is the original X3F development software and is required for processing Super-Fine Detail (X3I) files, which the Sigma Photoshop plug-in does not support checked 2026-08-07 — Sigma Global, SIGMA Photo Pro support page
- darktable supports X3F and other niche manufacturer raw formats via the RawSpeed decoding library checked 2026-08-07 — darktable-org/rawspeed GitHub issue tracker