I ordered this K&F Concept 82mm Spiral Halo Camera Lens Filter to use for special effects.

Basically, this filter consists of a large and heavy cone shaped piece of glass that introduces distortions. The center is a circle of clear glass that is distortion-free. This lets you take an image where the center is normal but the surrounding portion of the image is distorted and "swirly." This sort of effect can be useful for portraiture and certain landscapes when you really want to separate your subject from the background.

This filter is heavy. Because of its shape, you cannot attach other filters to it. If you want to use any other filters, you will have to attach them to your camera and then screw this one in. You cannot use any sort of lens cap with this filter as it projects out of the rim.

Construction is good. The filter screws into and out of my lenses smoothly. The sizing is spot on. Whiile you can expect some decreased sharpness in the center, I did not find it to be objectionable; this is not a filter you would use when critical sharpness is required. The convex shape of the filter also means that flaring can be a problem, especially when shooting outdoors under bright sunlight.

This is most definitely a special purpose filter with limited utility. That said, it does produce excellent results when used with an appropriate subject, aperture, and focal length.