A file with the extension .X3F is associated with a RAW image file taken with a digital cameras that incorporate the Foveon X3 direct image sensor.

A 'RAW' image file contains only minimally processed data taken via the sensor of the digital camera.

The purpose of this is that settings such as: exposure, white balance, etc. can then be modified later as desired.

Foveon X3 direct image sensors are (currently) the only image sensors that directly capture color in three layers in the same manner as colour film does.

The .X3F file extension is known to be supported by the following models:

Sigma SD9 DSLR
Sigma SD10 DSLR
Polaroid x530 point-and-shoot digital camera.