50 Free HDRI Panoramas

The library now features over 50 HDRIs at 4,096 x 2,048px resolution, including both skies and outdoor environments, shot in a range of weather conditions.

The HDRIs are said to be generated using 24-36 Exposure Values (EVs). The files are available in EXR format. You can use them for non-commercial projects or pay a $15 fee at the online checkout to use them for commercial projects.

You can also buy high-resolution HDRIs and photo reference packs or get higher resolutions of the HDRIs (15K and 30K versions) if needed. 

What is more, the library features a number of commercial packs of reference photos for matte painting and concept art.