Over the past few years, I've ordered a number of filters from various K&F series filter lines, and with only VERY few, I've been happy with all of them. This "K&F Concept 72mm Circular Polarizer Lens Filter [CPL]" is another plus product from them. I ordered this 72mm filter as an experiment to see how it would work along with my Sigma 15mm f/1.28 EX DG Fisheye lens. However, since this lens contains a large bubble for the front element, I wanted to try screwing it into the extended lens cap. Then tested if the CPL still worked. After screwing it into the extended cap, the filter rotated smoothly. Photographically, while the CPL filter did a good job on both the sky color and water reflections, the lens's barrel effect, exacerbated by the extended cap's distance from the lens element, made it not worth reprinting the images here. Despite this result, I will keep the lens rather than return it for 72mm camera lenses that I will undoubtedly purchase in the future. All in all, I think this “K&F Concept 72mm Circular Polarizer Lens Filter” is a good buy considering its mid-range cost and above-average performance. I think it is a solid 4-star product that should be considered by anyone in the market for such a CPL filter.