How do you record audio when using a 7d, 5d mark 2 or other HDSLR?
There are several different ways to do this, and which method you choose will depend on the type of sound quality you need and your budget.
Option 1: Get a sound recordist to do it. This is easily the best option, I nearly always use a sound recordist when shooting HDSLR. All you need to make sure is that the Sound Operator owns a digital recorder so sound files can be recorded onto his or her hard drive and then synched with the HDSLR footage later.
This synching can be done quickly and easily with a programme called Plural Eyes. Plural Eyes picks out the audio levels on the internal camera mic and the sound recordists files and puts them together. For this to work YOU MUST ENSURE AUDIO IS SWITCHED ON IN CAMERA, you won't use this audio, but it is needed as a guide.
If the Sound Recordist is standing along way from the camera, the two sound sources may be different and therefore Plural Eyes will struggle to synch them. There are a couple of ways around this. Firstly you can send signal from the Sound Recordist to the camera via radio mic. A radio mic receiver will sit on your camera hot shoe and feed the audio recording into the camera. You might ask, well why not just use this audio? The reason is that the HDSLRs have their own preamps which mean you aren't recording high quality audio in camera, and with the 7d it only records audio on auto. It is best just to use this as a guide track for Plural Eyes to Synch with.
If your soundie doesn't have a spare radio mic to do this, then you can synch simply with a clapper board, or even a hand clap at the start or end of each take. Likely hood is, Plural Eyes will pick out this clap and synch the clips automatically at the edit stage, if not at least the editor will have something to help the process along manually.
Another option to aid synching is to film the sound recordists timecode. If you lean over and get a shot inside the bag you will see the timecode and this will allow the editor to workout fairly accurately what bit of sound this image relates to. Some sound recordist even hook up ipods which can display the timecode coming off their recording device, which is easier to film as it has a bigger screen.
Option 2: If you don't have budget for a soundie, and if the sound recording is very simple - say a sit down interview - then you might be able to use your own external recorder. For this I would recommend the Zoom H4n The great thing about this device is it has it's own pretty decent mic. ideal for atmos, and it has XLR inputs with phantom power so you can plug in your 416 mic, radio mic whatever and record that audio to an SD card. It is a decent quality product and very easy to use. There are 2 XLR channels, and a clear read out that is well lit and easy to read audio levels from.
A word of warning: if you do go this route, get plenty of batteries, this thing eats them. (2 x AAs will just about last 1 day shooting if you are lucky.) If you are powering a mic that needs phantom power, such as a 416, as the power in your battery gets close to low, the sound level will start to fall, so make sure you top up the batteries. (It does come with plug in power, but I can't imagine using that much.) Another thing you can do to help the power output is put batteries in the mics that require phantom power, such as a 416 or ECM mic.
Option 3: If you don't really need to much audio and just want to get a bit of atmos only, then you could just get a Rode mic and plug it into the camera. The are a few issues with this touched on earlier. The 5d mark 2 has controllable audio levels with a firmware release, but the 7d does not, so you will constantly be on auto with the 7d. As this is just for atmos it may not matter to you. The other issue is both cameras have low quality pre amps, so it is not going to be the best audio quality in the world.
If you can, get a Sound Recordist, there is enough for the cameraman to do on an HDSLR shoot without having to faff about with audio issues.