How it works:
The flare is generally due to the presence of a protective filter in the camera, after the lens. It also works however with some lens without this
Anyway, the flare is a mirror ghost of the original bright object, with the image center serving as a point of symmetry (or
Black arrows indicate the light rays of a distant bright light source that form a regular image point on the film (1). Values for the reflectance of
undeveloped photographic film vary from 15% to 40% [see sources 1,2], which makes the film a much stronger reflector than any optical component in the
So, a significant percentage of the light is reflected off the film, partly specular and partly diffuse. (For convenience, we will consider that paths
of the reflected light are the same and thus are already drawn for the incident light).
Thus, the blue arrows
indicate light reflected from the film. This light encounters the filter, which specularly reflects a
small fraction (red arrows
). The red rays are parallel and consequently focused onto a point on the film. (2)
The virtual source of the mirror point is traced by the dashed black lines. Note that the blue rays reflected by the film seem odd from the viewpoint
of specular reflection; they merely illustrate the fact that all light rays that originate from a single point on the film, and which are collected by
the lens, emerge parallel at the filter.
Not all light reflected off the film makes it back to the mirror point. The presence of an aperture stop further reduces the number of rays allowed to
return to the film.
The risk of being confronted with filter flare reduces with a smaller focal length, a smaller aperture (larger F-number), an increased separation of
highlights from the image center, and with a better filter quality. However, by far the most secure way to avoid this type of flare is to omit the
People who want to try their luck with UFOs may improve their chances by using a tele lens at a large aperture. I guess that, instead of a filter, a
lens element with a flat face could also give rise to mirror ghosts.
 SPSE handbook of photographic science and engineering, edited by Woodlief Thomas Jr., John Whiley & sons, p. 204 (1973).
 Sidney F. Ray, Applied photographic optics, 3rd ed., Focal Press, p. 139 (2002)