He used an 8-stop neutral density filter to darken the lens (1/256th the amount of light that would otherwise be let through the lens, if I did my math correctly), a circular polarizer filter to minimize reflections, and set the aperture to f/9.
Usually an ND 8 is a 3 stop (2x2x2=8), at least in my experience.
When I get home I'll post a real HDR, not that toxic crap. I found an algorithm that doesn't clip the highlights, and a nice tone map that just helps me keep some of the shadow detail. That one was a joke
Clock, wish I could +1 you for that one. Great shot! Were you staying near that barn?
^ What aperture was shot using?