In this scene from Flags of Our Fathers, the top and bottom parts of the image are a little darker when compared to the middle portion of the scene. (I marked it with yellow lines)
What is the name of this effect?
Later, the word came to be used for a photographic portrait which is clear in the center, and fades off at the edges. A similar effect occurs when photographing projected images or movies off a projection screen, resulting in a so-called "hotspot" effect.
Vignetting is often an unintended and undesired effect caused by camera settings or lens limitations. However, it is sometimes deliberately introduced for creative effect, such as to draw attention to the center of the frame. A photographer may deliberately choose a lens which is known to produce vignetting to obtain the effect, or it may be introduced with the use of special filters or post-processing procedures.