What exactly does a compendium (matte box) do?

[Vespasian]

Does a compendium really change things and if it does what exactly does it do?

[Greg Crawford]

It's an adjustable Matt box that will work with a variety of lens.

The purpose of a Matt box is to shield the lens from unwanted flair. Most have a two or more stage filter holder.

The original purpose was for in camera effects holding back light from part of the frame. So you could go back and re-expose the matted area with a new image component. What with digital matting blue or green screen the days of using a Matt box for its original purpose is over. All though the name live's on.

The frame shape front end of a Matt box will give the lens better flair protection than a round lens shade.

[Vespasian]

so I should probably use one? Will it make the film look better. I know what you mean by unwanted flair. It is better to shoot with one right?