If you pop into the menu of the Canon 5D Mark III (or 1DX when available), you can change the shutter button to Start and Stop video recording. This feature wasn't available for previous Canon DSLRs (unless you have Magic Lantern). To start and stop video on previous Canon cameras, you were required to use an infrared remote, or more expensive USB remotes.
Now that this feature is available in the new Canon cameras, you can use a very basic corded shutter remote to initiate video recording. If you're looking for a small and inexpensive trigger to place next to the handle of your rig or along the pan bar of your fluid head, something like these basic shutter remotes will do. They are much smaller than my bulky time lapse remotes (seen below), so I thought i'd grab one of the smaller basic remotes just to place next to the handle of my DSLR rig.
(Above) Bulky Timer Remote - Intervalometer
Canon's RS-80N3 Remote Switch isn't very different, and will run you about $45 dollars. These cheap ones are also handy for doing long exposure photos so that you don't shake the camera and they only run under $4.00 dollars on eBay (click here).
Canon 5D Mark III Shutter Remote Start / Stop Video
There's also a similar version available on Amazon for $4.68 + Free Shipping
Remote Shutter Release Cord for Canon EOS 5D, 50D, 40D, 20D, & 10D Digital SLR Cameras
Pre Order Canon 5D Mark III
This might not be such a big deal for some people, but it's always been a huge inconvenience with all current Canon DSLR cameras. The latest 1Dx camera from Canon showed that Video mode could be started from the same shutter button used to take photos. What does this mean? Well, this means that your most basic and most inexpensive wireless (not just infrared) shutter trigger systems will be able to start video mode.
The question I had was will the 5D Mark III have the same, and Mitch from Planet5D.com was able to get the information from Canon stating 'Yes, the basic shutter will start video mode'. There's also a constantly updated Blog article about all the main questions coming in about the 5D Mark III found here: http://blog.planet5d.com/2012/03/questions-answered-for-the-canon-eos-5d-mark-iii-hdmi-video/. [Thanks Mitch]
I'm not saying that's a reason to upgrade from a Canon 5D Mark II, but that was one very annoying feature Canon seems to have finally fixed. Guess what? That also means if you purchased a set of Yongnuo RF-602 or RF-603 triggers for your Speedlite flashes, those also can be used to start and stop video with your new Canon camera.
Yongnuo RF-603 Wireless Flash Triggers
Some of you might already own a set, but ideally these very cheap Wireless trigger systems should now be able to start and stop video mode on all your 1Dx, Canon 5D Mark III, and hopefully all future Canon DSLRs.
Wireless Shutter Remote Trigger For Canon Cameras
Looking to start or stop video on your camera remotely? For Canon cameras, this can only be done via compatible infrared remote. It's a nice addition to stick on the handle of your rig so that you don't get those few seconds of unusable video. There's plenty of DIY solutions to route the infrared signal to the camera if the remote needs to be positioned elsewhere. One of those popular methods is by using an Optical Audio cable (like home stereo stuff). These optical cables run the light from the remote to the front of the Canon camera. I just ordered a remote and verified that it does indeed work on both the Canon 5D Mark II and Canon 7D. On the T2i it can fire off photos (no video mode). To initiate video, you'll need to use the S2 button. Total cost was $2.66 + 0.50 cents shipping. Found below (click here).
Wireless Shutter Release Remote for Canon EOS