The Canon 550d is an excellent photo camera with its 18MP(5176x3456 pixels) sensor, but the main reason that I purchased one was for its HD video modes. It is capable of shooting 1920x1080p at 24, 25 & 30fps as well as 1280x720p at 60 & 50 fps. In order to produce these frame sizes/rates while still using the full sensor area the camera "cheats" by skipping lines. When shooting 1080p for example, i believe that the camera is skipping every third line. The 550d has another video mode which doesn't use this cheat. It is called Movie Crop Mode which is interesting but in most cases not very useful. Check out this diagram:
The part of the image outline in red represents the entire usable pixel area of the 550d. Outlined in green is the 16x9 part of the sensor used in the HD modes with gray lines in half the frame to illustrate showing the line skipping occurs. The little blue box in the middle is the part of the sensor used in Movie Crop mode. The camera is cropping out the standard definition-sized center of the sensor to make videos of 640x480 resolution. This is essentially the same as taking a photo from the camera and cropping it to 640x480, except that it is 30fps video. What does this mean? The most apparent thing you notice when using this mode is whatever lens you have on is effectively 7 times longer. A 50mm lens becomes a 350mm lens. A 200mm becomes a 1400mm. It also means you have some incredible macro capabilities because the minimum focus distance of the lens doesn't change. Finally, it actually has less aliasing artifacts than the HD modes in the camera. As interesting as all of this is, it is not HD so it's actual applications are very limited. Here are some real examples:
iPhone/iPad compatible Youtube Link
A few notes about this video: The camera has a normal 640x480 mode which is not included in the diagram at the top of the article. It works essentially the same as the HD modes by line-skipping. I included two HD clips at 100% to show that the Movie Crop really does resolve more information than the HD mode. The footage of the green bug was taken with an 80mm lens, which really shows off the macro abilities of Movie Crop. The baby foxes were shot at 3200 ISO, it is amazing that one can film in such lighting conditions even if it is grainy.
Movie Crop is a fun feature to have, but the lack of HD makes it mostly useless. It would have been nice for Canon to include a 1080p Movie Crop function which would give non-line-skipped footage with a crop factor. Maybe this is something that will be included in professional Canon HDSLRs in the future.
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