One of my friends jumped the Samyang 8mm for a short while, awesome lens, brilliant pictures.......
However, after maybe 30 seconds in freefall he'd sometimes get a steamed up circle right at the centre of the picture, nothing he tried would stop it. This was in Spain in the summer so nice and warm, not very humid, maybe the odd bit of cloud kicking around here and there, no idea why! He's now swapped it for the tokina 10-17mm which seems to do the trick perfectly and is slightly more versatile for using on the ground too.