Hoya Color-spot filters consist of a clear circle in the center which allows the portrayal of an object in a different colored surrounding. The filter effect can be directly influenced by the f-stop and the focal length. The transformation is softer with wider apertures. The size of the clear circular area increases relatively with the selection of longer focal lengths.
This filter consists of a gray outer area. The portion outside of the image center will be reproduced darker than the center.
The exposure determination should be done without the filter. If the camera has the capability of spot-metering, then one can measure with the filter in place.