When you see a boom mike in a movie, it's usually the projectionist's fault. Film is manufactured to a certain standard size and aspect ratio (ratio of width to height of image), but directors often want to use a different aspect ratio for artistic reasons. To achieve this, the projectionist is instructed to mask the top and bottom of the image. Assuming the projectionist will do his job correctly, the film editors don't need to worry about images that appear in the part of the film that is supposed to be hidden by the mask.

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