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It's not the lens, as it would be a waste of optical material.
It's a colour graduated filter like these:
http://www.camerafilters.com/pages/halfcolor.aspx
http://www.2filter.com/tiffen/tiffenfilterinfo.html

037f02.jpg

ColorGrad_Blue_140.jpg
 
Shooting video with a grad filter on like that might be the silliest thing I've seen, especially in a world of color grading in post. I can see using a grad for like a still shot or something, but that video is horrid. You have the orange tinge of the grad filter all on the top of the screen it's just silly. Magic Bullet Looks and Apple Color are your friends.
 
It's almost always a very bad idea to mess with video before it's captured, especially in a way that is not uniform and cannot be altered later. Capture video without color filters, preferably with a low-contrast preset if your camera offers that functionality, and do all of the color work you want in post-production through computer software -- a good option for color grading on Mac is Magic Bullet Looks, as mentioned above.
 
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