The math doesn't quite work out though. DJI advertises a FOV of 120 degrees. With a 6.17mm X 4.55mm sensor and a 5mm focal length the FOV is only 75 degrees. With a sensor that small, you'd need a focal length of closer to 2.5mm to actually capture 120 degrees Field of View.

That's because, (in case you didn't notice the barrel distortion the Phantom has a non-rectilinear lens. The formula you are using assumes the lens is rectiliniar. The spec I found says 110 degrees. Regardless the wider FoV is achieved through the fisheye distortion. http://www.dji.com/product/phantom-2-vision-plus/spec

Point taken.... First off, you are correct about 110 degrees....looking closer I see I accidentally took the specs form the standard Vision 2, not the Plus. Secondly.... Yes, I'll grant that a fisheye lens can give a much wider Field of View than a Rectilinear lens of the same focal length. Although using the most common formulas for fisheye lenses with a 6.17mm X 4.55mmm sensor and 5mm focal length still comes out to closer to 90 degrees. With that being said, I'm not convinced that it's a true "fisheye" lens, as much as a rectilinear lens with strong distortion. As a professional photographer the fisheye lenses I've used have much stronger distortion than this...and I've got some rectilinear lenses with distortion in this same ball-park (albeit made for a much larger sensor) It kind of reminds me of those cheap wide angle adapters that you can buy for camera lenses. You put it over the front element of your rectilinear lens and it gives you a much wider angle of view by distorting the image coming in through the front element....while still maintaining lines that are reasonably straight at least towards the center of the image. I guess it's really not important....to most people. This kind of math does interest me, because I develop Android and iOS apps for photography...so I need to understand the math...and when numbers don't quite look right...it bothers me. Anyway...sorry for taking this thread off on a "tangent"