"wow!! flaming tree"

"You can correct the colour cast, but not any interference patterns. I gave up on this and stick to the ND110 from B&W."

"i'm interesting in two polarized variable nd approach, did it work with you or always make that unwanted color casting, or only after a certain amount of darkness?? some people say it would work till some level of darkness i mean the angle between the two filters"

"Hi David, the logic was in cropping the sigma you upped the iso to app 800 should a large print be required surely this would result in more noise appearing in the print than the lower iso derived from the use of the 500mm mirror. Sorry I am not in the market for any lenses at the moment, when I made the switch to digital from my cannon ae I went for a compact not quite the same as a dslr or as good I agree but it suits my needs, enjoyed looking at some of your photos."

"Thanks for the comment, not sure about the logic of that, but glad you like the 500. Let me know if you would like to buy the 500mm mirror as I am planning to pass it on."