Color Spot Special Effects Filter... A colored glass filter with a hole in its center. Available in yellow, green, red and gray, alowing you to change your background color.
The central image stands out clearly while the background appears the color of the filter.
The filter effect varies with the f-stop chosen and the focal length. The wider the aperature the softer the effect.
The size of the clear circular area increases relatively with the selection of longer focal lengths, the longer the focal length the larger the center clear spot.
For exposure, either determine the exposure without the filter on, or if you have spot metering capabilty , measure the center clear area.