Full disclosure: I received this filter system at substantially reduced cost in exchange for my unbiased review.

I just picked up a new Canon RF 100-400 lens that's got some pretty good glass but isn't L quality. Although I have a good B&W CPL filter that fits this lens I've never been very happy with the color cast. I've tried other K&F Concept glass before and was very impressed with the neutral color cast so decided to try out this polarizer kit.

I really like the engineering in this kit. This kit comes assembled as a compact sandwich with front and back machined metal caps. All of the caps and filters are clearly marked with filter size. There's a little storage pouch to help protect the filters and your other stuff when you toss this in your bag.

I tried another K&F filter previously and while I was very impressed with the glass I was far less impressed with the machined aluminum frame. Unlike that previous filter these attach to my lenses easily and don't bind. In practice the knurling on the filters make removal quite easy and in the case of the CPL adjustment is very smooth. There is perceptible resistance but no hint of binding or free movement that you get with cheaper filters. The feel of this filter is quite nice.

The thin profile UV filter is MRC coated to reduce glare and improve cleaning. This filter imparts practically no color cast at all. When held against a white background I can't discern any difference between filtered and unfiltered white. I don't ordinarily use UV filters since most of my lenses have better front elements than any filter I own, but in the case of the RF100-400 I will probably leave the UV filter on the lens since it doesn't have the same quality coatings as my other lenses.

The CPL is quite a bit thicker, but not so thick that I'm going to worry about vignetting since I'm mainly using these with medium to long telephoto lenses. All CPLs will affect color to some degree. Depending on the amount of polarization and the angle of the sun they will typically darken skies and change the color of water, sometimes dramatically, so it's difficult with a CPL to determine what color cast may be present. Using the same white background test I can discern an ever so slight cast versus the adjacent white background that seems to have a bit of a brownish tinge to it but could simply be the result of the light loss through the CPL. In test photos I could discern no perceptible cast at all. It's close enough to neutral that I don't think it's going to have much of an impact on color correction in post.

All in all really nice filters and a pretty good value to boot. Recommended.