I haven't short with a polarizing filter since I don't remember when, but I thought I would try out my recently acquired Rokinon f2 16mm with one (part of a cheap set of 77mm filters & got to try with the lens). Of the two test shots hurriedly tried just before lunch today, this one seemed to turn out best. I cropped it, partly to emphasize the horizon line, but also to take away most of the open blue sky above the cloud bank, which was decidedly blacker on the left side. I wasn't really looking to see in the viewfinder whether the sky deepening effect of the polarizer was even across the sky--I was mainly looking for what setting of the CPL brought out the clouds the most. That is something to definitely watch for the next time, since the risk of not getting an even effect across the field with a UWA is much greater than with a normal lens, what I have mainly used polarizers on in the past. There is a little fine-tuning of the exposure level and highlight brightness, in addition to the crop.
Rokinon/Samyang f2 16mm, at f5.6
K10D, iso 100, 1/250 sec
Pro Optic CPL