Sigma X3F MIME Type

What does image/x-sigma-x3f mean?

The image/x-sigma-x3f media type identifies a Sigma or Foveon X3F raw photograph, normally stored with the .x3f extension. An X3F can hold raw image data, rendered previews and thumbnails, and descriptive properties for one photograph. Its purpose is to retain the camera capture for later development rather than bake every color, white-balance, contrast, and sharpening decision into a finished image.¹

image/x-sigma-x3f is a conventional media type, not an IANA registration. The identifier does not appear in IANA's image media-type registry.² That distinction matters when configuring a server or upload filter: software may recognize .x3f without agreeing on a formally registered Content-Type.

How can you identify an X3F file?

An X3F begins with the four ASCII characters FOVb. In hexadecimal, the signature is:

46 4F 56 62

The public FOVb specification places a version value after that signature. It describes a little-endian container with a header, data subsections, a directory near the end, and a final pointer to that directory. Individual data sections can carry images or text properties. The format was also designed so that later information could be appended without rewriting earlier bytes.¹

The signature is more dependable than the filename alone. Renaming a JPEG or DNG to .x3f does not turn it into a Sigma raw file. Likewise, changing .x3f to .jpg does not make a browser able to decode the container.

X3F files commonly include a camera-rendered preview. ExifTool documents PreviewImage and JpgFromRaw resources, plus header and property fields such as dimensions, rotation, white balance, camera model, exposure, lens information, and timestamps.³ A quick preview can therefore look different from a fresh raw development, because the preview reflects rendering choices made earlier by the camera or software.

When was X3F introduced?

The Sigma SD9 brought the X3F workflow to photographers in 2002. Sigma records the SD9's debut in October 2002, and the camera manual shows that every captured image used an .X3F filename.⁴⁵ The SD9 paired Sigma's camera system with Foveon's three-layer X3 sensor and recorded losslessly compressed 12-bit raw data.⁶

Foveon's surviving external specification starts its public version history in January 2003. That document deliberately covers enough of the format to read previews, thumbnails, and image properties, while omitting some image encodings from the public description.¹ X3F is consequently documented enough for identification and partial parsing, but it does not have a complete public specification comparable to DNG.

How to open an .x3f file

Use a raw processor that supports the camera that created the file:

• SIGMA Photo Pro is Sigma's application for displaying, adjusting, and developing raw images from Sigma digital cameras. Current releases continue to read X3F and save developed images as JPEG.⁷
• Adobe Camera Raw and Lightroom support X3F from listed Sigma models. Adobe's camera table gives the minimum compatible version for each model.⁸
• Photoshop with Sigma's X3F plug-in supports selected Merrill and Quattro cameras. Sigma directs owners of several earlier cameras to Adobe Camera Raw instead, so the plug-in is not a universal X3F decoder.⁹

Chrome, Edge, Firefox, Opera, and Safari do not natively display X3F as a web image. MDN's browser image guide lists formats such as JPEG, PNG, WebP, AVIF, GIF, SVG, and APNG, but not X3F.¹⁰ A website should publish a rendered web image and offer the X3F separately when visitors need the raw original.

Support in Poly

Poly gives X3F photos practical support for browsing and discovery:

• Thumbnails and an in-app image preview make the photograph easy to browse.
• Filename, color, and visual-similarity search help you find it again.
• The Poly agent can inspect and reason about the visible photograph.

Poly shows a rendered representation rather than a raw-development workspace. Use SIGMA Photo Pro or another compatible editor when you need to adjust white balance, exposure, noise reduction, sharpening, or camera-specific color from the sensor data. Text photographed in an X3F is not indexed as text.

Converting X3F for editing or sharing

Raw conversion interprets the capture and creates a new file. Simply changing the extension is not conversion.

Sigma camera manuals describe JPEG and TIFF as developed outputs from X3F raw data.¹¹ Choose settings such as white balance and exposure before export, because a finished derivative cannot recover every option available from the original capture.

Privacy and file safety

An X3F can contain more than visible pixels. The public specification describes a per-image identifier derived partly from camera identity and capture time, while ExifTool documents camera, lens, exposure, owner, serial, and GPS-related fields that may be present depending on the camera and workflow.¹³

Review metadata before distributing an original file. Exporting a derivative may reduce what is shared, but do not assume it strips every field. Also treat malformed raw files as untrusted input and open them with maintained software; the signature proves the container type, not that every offset, section, or embedded preview is valid.

Is X3F the same as DNG?

No. Both are raw-oriented image containers, but X3F was created for the Sigma and Foveon ecosystem, while DNG is Adobe's publicly documented Digital Negative format.

Some Sigma cameras and software also work with DNG, but that does not make existing X3F files interchangeable with DNG. A conversion must decode and rewrite the capture, and camera-specific behavior should be checked before replacing an archival original.