Fabien Christin released a great video explaining HDRI.
He uses Photoshop CC to create the HDRI files because there is slightly better quality.
This is a video for the photographer addon for Blender.
Fabien Christin Sample HDRIs from THETA SC
Unity presentation on HDRI
THETA SC lit by sunlight
Pro-Tip: Never use Roughness 0
Note: the red is negatives. He wants the HDRI to have more dynamic range in bright sunlight
Possible lens cap filter. Untested.