Most of the time the shape of the flare is close to a circle, but recently I saw a movie where the lens flare was elliptically shaped. The stills below, taken from the movie Die Hard, show the effect I am referring to.
What in the lens design is responsible for this elliptical shape?
The elliptical flares were probably from an anamorphic lens. These lenses squash the image horizontally in order to get a widescreen picture on a standard width film strip. The anamorphic elements are usually on the front of the lens so the lens barrel appears elliptical to the camera.
Lens flares are just reflections so can take the shape of any lens component but the most common source of reflections are the glass elements so flares tend to be either circular or take the shape of the aperture.
Movies typically use an aspect ratio that's wider than the frame on the film, so lenses are used to compress the image in the horizontal dimension. You're probably familiar with some of the brand names Cinemascope and Panavision -- these are lens systems that were/are used to record a wide image onto standard film, and then to project it again. This may explain the elliptical lens flare in the image. I'm not fully convinced of this myself -- one would think that a circular lens flare in the camera would be converted to a vertical ellipse on the film and then back to a circle upon projection, but who knows how the light bounced around in the lens?
Another example of something that seems similar is the holes that used to be punched in films to tell the projectionist to switch to the next reel. The holes were circular, but they appeared elliptical on the screen due to the lenses.
