1.  Field of the Invention
This invention relates to filter devices, and more particularly to a filter device for use in conjunction with single-lens reflex cameras with through-the-lens light metering facilities, to enable use of these meters as incident light meters in a method so convenient and accurate as to assure it the popular acceptance which other incident-light methods have been denied. 
2.  Description of Prior Art
It is believed that prior art related to the subject matter of this invention is classified in class 350, sub-classes 255 and 266; and class 256, sub-classes 213, 221 and 234.  A search of these classes and sub-classes has revealed the existence of U.S. patents as follows: U.S. Pat.  Nos.  2,803,162; 2,824,696; 2,879,690; 2,930,281; 2,972,930; 2,983,186 and 3,112,684.  Reference is also made to allowed pending application Ser.  No.  151,391 filed May 19, 1980, now U.S. Pat.  No.  4,381,890, by the inventor herein, and the patents cited therein. 
The above listed patents, with the exception of U.S. Pat.  No.  3,112,684, relate to light meters equipped with translucent domes which work in conjunctuon with the light measuring circuits incorporated in the light meter, and thus permit a photographer to take readings or determinations from the light meter that can then be transferred as appropriate settings to a camera.  The exception, U.S. Pat.  No.  3,112,684, discloses the broad concept of a translucent dome in conjunction with a camera in a permanent relationship that complicates the use of the camera in a conventional manner without the use of the translucent dome.  To develop a better understanding of the problems involved with focusing single-lens reflex cameras equipped with through-the-lens metering facilities, it should be understood that there are two basic methods of employing light meters to aid in the determination of camera exposure settings for optimum exposure of a subject being photographed.