When using the Hoya Center Spot Filter, only the circular middle section of the image is reproduced sharply. The rest of the image out to the edge becomes blurred. When photographing people (portraits), this filter can be invaluable in giving a flattering edge to your subject. Since the eye moves towards the brightest and sharpest part of the photo, the viewer will directed towards the center. The degree of the blurred effect varies with lens focal length and aperture opening.
72mm Center Spot Glass Filter