Clips recorded by the Canon EOS 5D Mark II use a variable bitrate codec. Basically, it means the compression used for the clip adapts itself according to what it needs to process. Also, the camera stops recording once the file has reached the maximum file size (around 4Gig) dictated by the FAT16 file system used on the memory card. This is why you can’t tell in advance the exact maximum duration of a clip, it depends of what you are shooting.

NOTE (8/2012): This article was written not long after the Canon EOS 5D Mark II was released. Newer cameras like the Canon EOS 5D Mark III and the Canon EOS-1D X have a maximum recording length of 29:59 (going over 30 minutes means a 30% increase in the taxes required in Europe as it would classify as a video camera). The camera still has the 4gb file size limitation so they simply split the recording over multiple clips and you put them together in your editor.

Test of the edge cases

To prove the point, we did a few tests using extreme test cases: super over exposed (all white) frame and fully under exposed (black), both clip shot at 24p, ISO 4000 and no sound. While the over exposed image is perfectly white with no details, the black video is not perfectly black: there is some camera generated noise which requires a lot of data to compress.

Hence the reason between the file size difference. Would the clip have been perfectly black, both would have been the same file size and duration but now we can use the underexposed clip as a highly detailed sample.

Test Size Size Duration
Over exposed 815Mb 29:59
Under exposed 4.28Gb 15:12

Does frame rate have an effect?

Since clips are limited by file size and a higher frame rate stores more images, each second of clip should take more space, resulting in a shorter clip. Here are the results of the same test as above but done at 30p.

Test Size Size Duration
Over exposed Mb 29:59
Under exposed 4.28Gb 12:15

Notice how the under exposed clip lasted around 20% less than its 30 frame per second version. Which is the same ratio as the frame rate difference.

Conclusion

Looking at extreme cases which should never happen in real shooting situations, it gives us a better idea about how clips are stored and processed. Based on this, we can guess that Canon claims of 5DMrkII clips having a maximum duration of 12 minutes is just an approximation based on a heavily detailed scene to give an idea of the minimum duration someone can expect.

We also saw how shooting with a lower frame rate increases clip duration. So if you are worried about clip duration, you should shoot at 24p without hesitation. You have everything to gain: cinematic look + smaller files.

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planetMitch

chief astronomer at planet5D LLC
Mitch "planetMitch" Aunger is the creator and mastermind behind planet5D.com — which covers every aspect of HDSLR cameras (and a whole lot of other cameras as well). A lover of photography since his early days, he graduated to selling stock photos to make a little bit of spare cash. It wasn’t much, but it allowed him to upgrade to better cameras as well as computers, and it wasn’t taking money away from feeding his family.
He loves the stories that the still image can tell. In the summer of 2008, he read a story written by Arnold Kim called “I Quit My Job” — Arn is the owner of macrumors.com. He started macrumors in 2000 as a hobby, and by 2008 he was making more money with the site than he was on his day job, so he quit. The surprise to the story is that he was a doctor making more than six figures! Right then and there, planetMitch dreamed about doing the same thing.
September 2008 rolled around, and Canon announced this new version of the incredible 5D — the Canon EOS 5D Mark II. It was the first Canon DSLR to shoot full HD video and the earth moved! (It really was a huge deal in the photography world.) planetMitch knew if there was ever a chance to ride a wave of something he was really passionate about, this was it — and the blog at planet5D.com was born. It is now one of the most popular HDSLR blogs on the planet, and he is making a full-time living from blogging. He couldn’t be happier!
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