The ad doesn't even say which Audio Technica mic it is, there are a few in that price range. The picture looks like the AT897, which can be had for about $230 on other sites.
The further away the mic is from the actors, the higher the volume needs to be. The higher the volume, the more “noise” you get. Your goal is to have a very high signal to noise ratio - more signal (the dialogue) less noise (the background).
What directorik is referring to is the
Inverse Square Law. In simple terms, this means that a sound twice as far away is only one-fourth as loud. A sound four times as far away is only one-sixteenth as loud. So a human voice that records with adequate volume three feet away is just one-sixteenth as loud at twelve feet. This will force you to increase the gain on the audio recording device thereby increasing the recorded level of the self noise of the mic. Yes, mics create their own noise and, generally, the cheaper the mic the more self noise it generates. This is especially true with cameras; their focus - pardon the pun - is to capture a great picture, so the audio components, even in expensive cameras, get short shrift. They have noisy pre-amps and minimal audio implementation.
Even a very good, expensive Sennheiser mounted on the camera will pick up a lot of background ambiance because it’s far away from the actors.
The way the mic is handled is just as - or even more - important as the mic itself. Shotgun mics have terrific off-axis rejection (or are supposed to). First example; if you want to shoot the actors face you don't put him/her in profile, the camera can't "see around corners;" a mic can't either. Second example; you are 20 feet away from the actor and are using a zoom lens to focus on his/her mouth. If you pan the camera two inches to the left you are now shooting the tree over his/her left shoulder. A shotgun mic works the same way. If you are off by just a little bit you are recording the traffic behind the actor rather than the dialog. Yeah, you will hear the dialog, but the ambient sound will be louder than it should be.
So to improve your audio quality you want the mic as close to the actors as you can get it. You will need a boom pole.
Believe it or not, the boom-op has one of the hardest jobs on the set. S/he has to aim a mic at the end of a long boom pole directly at the actors mouths, has to noiselessly switch aim from actor to actor and noiselessly follow them around the set while keeping the boom out of the shot and prevent it from casting shadows. Sound like something you want a random inexperienced PA doing?
One more thing - a shotgun mic is not always the best choice for shooting indoors. Because of a shotgun mics off-axis rejection it only records what is in front of it. It picks up the dialog, but it also picks up all of the sound reflections bouncing off of the hard surfaces behind the talent. This is what causes the well known "roomy" sound prevalent in many indie projects. A non-shotgun cardioid condenser mic is preferable for indoor shoots.
