Definitions for "Spot Filter"
an area in the center of the lens which greatly reduces the amount of light passing through it
A small filter placed in the center of one of the elements of a lens to increase the high end of the f stop to the lens from f64 to f1000 or more. These filters are “neutral density” type which do not affect the color rendition of the lens.
A supplement to the iris which allows the lens to have a larger aperture opening than is physically possible with the iris only. These usually range from F/88 to F/1600. This allows very sensitive cameras to view bright scenes easily. The iris of a lens without a spot filter would not be able to close down enough in bright light without creating an image degradation caused by refraction.