Aspect Ratios and HD
HD video's native aspect ratio is 16:9 (1.78:1) which was chosen by SMPTE as the standard because it's a happy middle ground between the old television standard, 4:3, and the common American Academy motion picture aspect ratio of 1.85. HD video cameras are native 16:9. Digital Motion Picture cameras such as the Arri D-21, Phantom, Red One, and others aren't really video cameras and can window their sensors to output a variety of aspect ratios. It's great to have a giant image that you can pan around and re-compose which is really very similar to working with 4 perf Super 35. On the video side if you want a wider than 16:9 presentation such as the common Cinemascope format, 2.35:1, you can easily matte your footage down to that size. There are literally dozens of different motion picture formats that have been used over the years - http://en.wikipedia.org/wiki/Aspect_ratio_(image) - 1.33, 1.5, 1.66, 1.78, 2, 2.4, 2.76, 4, etc. Recently for kicks I've been playing around with putting HD footage into different format masks noting the aesthetic differences.
I have all the test images I made as well as comments up on my Flickr at: http://www.flickr.com/photos/negativespaces/sets/72157604313911086/
Frames from "Scaredy Cats" Director: Bossi Baker
Native 16:9 HD image right out of the camera (Panasonic HVX200)
Down converted and matted to 1.37:1 (4:3) via Pan and Scan. Center Cut down converts for SDTV are fare more common and less expensive than manually re-positioned each and every shot.
Matted down to 1.85:1 American Academy widescreen standard. Really not much wider than the native HD aspect ratio.
2.35:1 Cinemascope. Native aspect ratio of widescreen anamorphic images. Love this shape!
A 4 Perf Super 35 frame is a good point of reference to see how these various aspect ratios are extracted.