Just to be clear, I wouldn't recommend making a film with an iPhone either. At the very least you want to be able to mount your camera on a tripod. But if you're going to spend $500 I'd recommend getting some good wireless mics over a DSLR.
Ok, I am defintiely interested in recording interviews (videoing them is a plus) - and at the risk of HN wrath, here goes with the hardware questions:
what makes a good microphone,
why go wireless (I can guess) and
I would assume I just plug into the 3.5mm jack on the
side of my laptop - and hit record - why is that a bad
idea?
That depends a lot on what kind of film you're making, but there are two that are particularly useful for documentaries: lapel mics and directional shotgun mics. Lapel mics give you better sound quality, but shotguns are good for impromptu situations where you don't have time to wire your subject.
Wireless is good for the obvious reasons. Wired mics are really good only for sit-down interviews unless you can also put a recorder on your subject (which is possible -- iPod nanos make dandy audio recorders). Don't worry too much about quality if all you're recording is people talking. I'd avoid mics that are selling for $1.99 on eBay, but you can find excellent lapel mics for low 10s of dollars. (Shotguns cost more.)
Yes, you can just plug a mic into the jack on your laptop. You'll even get decent quality. But 1) your laptop is probably much bigger and more power hungry than a dedicated recorder and 2) it probably only has one mic jack and you'll almost certainly want at least two no matter what you're doing.
If you're using an audio recorder that's separate from your camera, don't forget to put an audio marker on each clip so you can synchronize the sound later in post. You don't need a slate. You can just clap your hands (make sure you're in front of the camera and in range of all the mics). It looks goofy, but it works.
First, rent your microphones, you get what you pay for, and cheap microphones sound cheap. Better yet, hire a sound guy, who will know what he's doing. At the no budget level you can basically hire someone for the price of their gear rental, which is about 5% of the book value per day. To put that in perspective, I used to turn up to a shoot with $4-5000 worth of gear. My shotgun alone costs about $1500 to buy. If you do do it yourself - and that's more acceptable for documentary because nobody minds seeing a microphone - at least buy a book and do some practice in your spare time. You'd be amazed how noisy a refrigerator sounds when you're trying to record someone, or how loud a typical city street it.
But the reason I praise the Zoom above is because it's no larger than a typical camcorder. It fits in your pocket, won't attract attention on the street, and you can record up to 4 tracks with it. You won't always have a table for a laptop.
> you get what you pay for... cheap microphones sound cheap
That is certainly true. But a cheap sound today is a lot better than a cheap sound ten years ago, and it might be good enough for you. The only way to tell is to try it. On which note...
+1 on doing practice runs. Much more important than mic quality is placement and setting your levels properly. A cheap mic well placed will get you much better results than the best mic in a bad location.
