Having used regular/fixed K&F Concept ND filters previously with great results, I was invited to try out this variable ND filter and was provided one for review.My usual use for NDs is taking photos of moving water, mostly waterfalls, using the ND to smooth out the water to give it a misty/milky look.Often cheap filters are made from plastic and resin, but these K&F filters are metal frame with glass elements which is a great bonus, they feel nice and solid.The filter was slightly larger than expected as it's advertised as 'slim' - but there was no vignetting evident. The action of turning the filter to adjust the strength was nice and smooth with the right amount of resistance.I was happy with the results from the test shots I took of a local waterfall, the ND did the correct job. There was very little colour cast that I could see, nothing that would be noticeable to the eye for most people, I shoot RAW so it was an easy fix in post processing.Full strength did produce a stronger cast, but i would suggest buying a dedicated 10 stop if you need this level of ND darkness.The only reason I've dropped a star from this review is because of the lack of a scale on the filter, this makes it difficult to measure and repeat settings when moving around, or shooting at a different time. An element of trial and error is needed - which can add time to getting the right shot. Hopefully K&F will rectify this in the near future as it's an easy update.Overall, this is a great piece of kit for the price if you want to experiment with slowing the shutter down in daylight without needing multiple filters or paying out for high end equipment straight away.**I received a discounted product in return for this review, but the above review is open, honest and unbiased**