tested the K&F Concept Nano-X PRO Series 67mm Polarizing Filter with a Sony A7RII and a Sony SEL-20F18G. I use polarizing filters 99% of the time for landscape photography. Since the effect is practically impossible to replicate in post-processing, a polarizing filter, along with neutral density filters, is one of the only physical filters I regularly use. The K&F is a slim filter. This can prevent vignetting at extreme wide-angles, but the narrow mount makes handling significantly more difficult. This is the case with almost all manufacturers due to their design. At least the filter has a threaded mount at the front, allowing multiple filters to be screwed on one behind the other. A lens hood and lens cap can also be attached, depending on the type. The filter comes in a high-quality, sturdy case with a magnetic closure. As a little gimmick, the manufacturer also includes a microfiber cloth for cleaning. The thread screws on neatly. The rotating mount operates smoothly and smoothly. The slim design makes handling very difficult at times. On the one hand, you have to be careful not to screw the filter onto the lens too tightly, otherwise it will be difficult to remove by hand. On the other hand, when turning the filter counterclockwise, there is a risk of accidentally loosening the filter if it is not screwed on tightly enough. With the lens hood attached, it is also no longer possible to operate it. The lenses are from the Japanese company Asahi Glass and appear to be of high quality. I did not notice any loss of sharpness even at extreme magnification. I did notice a minimal reddish color shift, but this is hardly noticeable. Since my lens is not susceptible to lens flare, I was pleased that no deterioration occurred with the filter attached. The filter absorbs almost exactly one f-stop of light, which is a very good value. The filtering effect is good, but I honestly didn't expect anything less in this price range. In my attached pictures you can see the result in strong backlight, with and without a filter. Here I could not detect any differences in quality or lens flare. The filter effect is clearly visible on the leaves in the other two photos. Here the sunlight falls on the bushes at an angle of approximately 90 degrees. The polarization effect is clear. The white balance was set manually to daylight for all photos. Here you can hardly notice the very slight color shift. I hadn't really considered K&F Concept in this price range before. The results of the filter are first-class and do not reduce the quality of my high-quality lenses. Unfortunately you have to live with the somewhat difficult handling with this design; K&F isn't reinventing the wheel here. However, I give it a full five stars for the optical performance.