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Maybe you know this wonderful video:

Can someone help me to understand how the LED trail effect has been done?

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There are two ways this can be accomplished. The easiest is by using a slow shutter speed. While most DSLRs may be limited to 30FPS and faster, it is possible through the use of third party firmwares to lower that arbitrarily further. As you can see from the details on Vimeo, they were using GH2s with the EOSHD Vanilla hack. This removes many of the limitations that would otherwise be on the system. It is also possible with most Canon hardware using MagicLantern's FPS Override.

The other way to do it is to artificially do it in post production (though the result often isn't as good) by additive combining each frame with previous frames. This results in a trail that can animate on and animate off and maintain the original frame rate, but it is likely to introduce gaps in the trails and either requires a pre-existing plugin or some creative layering in After Effects. Layer each video one from apart and then apply blending until you get something that looks like you want.

It is hard to tell which technique is being used for sure without a downloadable copy that I can go through frame by frame, though my initial impression (particularly based on the hacked firmware and the general feel of the video) is that they are using the lower frame rate option in the hacked GH2.

Update: I managed to dig up my old Youtube downloader and got the video in to Premiere. I can confirm that it is in fact a lowered frame rate (around 1/12 or 1/15 for most of the trail shots).

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do you have any idea about software to be used? I have 550D 5D GoPro and EOS-M and NOT a GH2 :-( –  Roberto Alfieri Jan 8 at 0:10
    
@RobertoAlfieri - most of the 5Ds and 550D both support MagicLantern which will do the slow shutter for you. I highly HIGHLY recommend it if you are doing videography with your DSLRs. I actually prefer Magic Lantern to most of the GH2 hacks, though the GH2 hack is hard to beat in terms of value for the cost of the camera. I shoot Canon bodies but a buddy of mine that I work with a couple times a year shoots GH2s and GH3s and we always compare notes. –  AJ Henderson Jan 8 at 0:19
    
i actually HAVE MagicLantern on my 550D, but it does not go under 30... #ohwait! –  Roberto Alfieri Jan 8 at 0:24
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AJ, here is another one for your repertoire: There are specialized frame buffers configured to reiterate images as illustrated in the example. I credit Ed Tannenbaum for developing this device as far back as 1980 when it debuted at the Exploratorium in San Francisco. For more information Google with key words "Ed Tannenbaum Exploratorium". Ed is a colleague of mine and he told me that his inspiration for this device was from watching Norman McLaren's 1968 film "Pas de Deux". –  filzilla Jan 9 at 18:49
    
@filzilla - good to know, can this easily be accomplished on non-specialized hardware though? If not, I'm not sure it is particularly helpful for this answer, though it certainly is interesting information from a historical context. (Really, it is what the After Effects approach is emulating if I understand you correctly, it just does it with a specialized frame buffer.) –  AJ Henderson Jan 9 at 18:50
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