It works quite well. It bends the reflecting flare outside film area (so it gets lost in the blacking of the mirror box), because the surface is not planar. Otherwise it's just an UV. The difference is that with bright lights inside frame, and lenses with flatter front elements, the lights reflect back from the the lens onto the rear filter surface and back onto film, creating a secondary, often sharp, image of the light. The curved UV filter negates this.
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