I am videotaping a presentation later today and afterwards they are having an audience q&a session. Anyone have any tips for capturing good audio? I am putting a lav on the presenter and plan on giving my client a hand-held wireless to pass to audience participants.
That's a good way to go. Otherwise, you could have the HH on a stand in the center of the room (in an aisle) where folks can walk up to ask questions.
If you have a second audio recording device available, it would be helpful to have it running with mics for the room, to catch general crowd reax like applause and laughter. Record these mics separately so that you have control over the mix in post.
Although it's not highly creative work, I am asked to record quite a few sessions like that at the school where I work. Your approach is pretty much right on -- a lavalier for the presenter, and a wireless handheld for audience questions. Alex H's suggestion is also good. You could even do both if you have the mics and mixer/recorder channels available.
When there is more than one presenter, usually a panel discussion kind of setting where everyone is at a table, I might use a number of (usually) Shure SM94 cardioid mics on desk stands wired into a mixer, but that doesn't sound like it's your situation this time.