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Boom ALWAYS Needed?

Must I always use a boom or can i use a shotgun mic when shooting a dialogue scene when the actors are relatively close to the camera?

Also whats the maximum distance the mic (boom or shotgun) can be from the actor to have "hearable sound"?
 
Just to clarify: Some people are confused about a boom mic. The
"boom" is a piece of equipment - most often a pole of some kind -
that the microphone is attached to. 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). 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.

If the actors are close enough to the camera and you want to keep
the mic on the camera then that can work. You lose flexibility - if the
actor turns slightly away from the camera, the audio isn't picked up
as well. A boom operator can move the pole as the actor to turns
away - the camera operator cannot.

The maximum distance depends on many factors so there is no, one
answer (no more then 5 feet) that will work. This is why it's good to
have a boom op with head phones during the rehearsals. They can
listen carefully and decide where the mic needs to be.
 
A couple things to keep in mind, in addition to what Rik has said:

A shotgun mic is super-cardioid, meaning it has a very narrow field of pickup, and is designed to reject noise coming from the sides and rear. It can be mounted on a hand grip and aimed at the actor's mouth by a person standing off camera, in which case you would not need a boom.

What you need to keep in mind, however, is that there may be other sounds occurring behind the actor - such as automobile traffic, for example - which the mic will also pick up. This will reduce your signal/noise ratio as described by Rik.

One of the many reasons mic's are mounted on boom poles is because they can be held above the actor and aimed downward. That way, the only background noise in its field of pickup is whatever is going on below the actor, usually limited to footsteps and other such noises -- much easier to deal with than traffic hiss.

For tight shots, the boom can also be held below the frame with the mic pointed upward. This works fine until an aircraft passes overhead, in which case the engine noise will dominate the track.
 
One more note that occurred to me:

Say you're shooting a dialog scene at an outdoor cafe and the mic is mounted on the camera. First you shoot an angle on Actor 1, who is seated with his back to the cafe. Then you swing the camera around for the angle on Actor 2, seated with her back to the street.

In post, when you try and cut the conversation together, the background sound will change noticeably each time you switch speakers. The only way to disguise it will be to layer wild cafe hubbub and street noise over the entire scene, leaving you with a very cluttered sound track.

If an overhead cardioid mic on a boom was used to record the scene, the background noise would be nearly identical for both angles, making life infinitely simpler for your sound editor.
 
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