Objectively speaking, you might ask yourself what exactly justifies this price: a filter that many photographers only use as "lens protection" and whose effect is usually not even visible in the photos. But this line of thought falls short in my opinion.

The Chinese manufacturer K&F (or Kentfaith) is no longer a cheap provider. The product range and thus the expertise includes not only UV filters but also other filters (CPL, NDF, etc.). This filter therefore comes from a group that puts a lot of thought into how to build even better filters. And you can tell that from the packaging of this X-Pro filter:

This filter came elaborately packaged suitable for gift giving. It also looks great with a gold colored inlay showing its pedigree. Please see the image that’s part of this review for a look at part of the gold colored lettering. It sure does look the part. I tried some test shots with the filter on and off the lens and then did the best pixel peeking I could noting no difference in exposure or color cast. To be fair, I’ve never noticed this with any high quality filter. I’ve never bothered with bargain basement filters because I have no interest in Lomography or its related ilk. The manufacturer says this is thin and it is. Please see the image where I compare this filter with another UV that costs less than half as much. Both seem thin to me. I don’t see how thickness makes that much of a difference because adding any filter will require the removal of this one or you’ll get image quality robbing refractions from between the two filters. I considered using a micrometer to measure the thickness of the lens itself but gave up the idea because I had no way to assure that I’d not scratch either this or another filter I’d use for comparison. In sum, this does work well as a UV filter and it does not in any way alter the images when compared to the same lens without this filter attached.

I ordered the K&F Concept Nano-X PRO series UV filter to use with the Pentax HD DA★ 11-18mm F2.8 ED DC AW lens. I usually use my high-quality lenses without a UV filter. Ultimately, any additional glass in the optical path degrades the quality. But in dusty weather or when it's raining, using a UV filter is definitely a good idea, as you can then rub the filter dry with a cloth or remove dust. If the filter gets scratched, the front lens of the lens is still undamaged.

Now for the filter. The K&F Concept Nano-X PRO series UV filter comes in quite elaborate packaging. The filter is in a pouch, parts of which are even made of real leather. The closure is magnetic. There is a strap on the side of the pouch. If you pull this, the filter slides out of the pouch.
A cleaning cloth is also included in the delivery, and a cardboard stand (?). I don't really understand the point of this. Perhaps it is intended for presentation purposes for dealers?
The filter is "Made in China". There is an importer's address from Germany on the box.

The filter can be easily screwed onto my Pentax HD DA★ 11-18mm F2.8 ED DC AW lens. There is nothing to complain about. The Pentax HD DA★ 11-18mm F2.8 ED DC AW lens is not shaded by the filter. The filter's workmanship is also really good.
I took a few test pictures with and without the filter, and could not see any negative effects. Under normal conditions, I don't think the image result will really deteriorate. Perhaps there are problems with light reflections in backlit pictures when the sun is at the right angle. But that would be a general problem with filters on lenses.

Conclusion:
The filter is really good. The current price of €149 for the 82mm filter is certainly anything but a bargain. Ultimately, it doesn't make sense to buy a lens for around €1500 and then screw a cheap filter onto the lens, which then leads to poorer image quality. I give the filter 5 stars.

If I'm honest, I've always relied on established brand manufacturers such as B&W or Rodenstock for filters. I do have a few filters from Kase and K&F in my inventory, but I hadn't really considered Asian manufacturers when it came to filters.
And that's exactly what K&F wants to change with the Nano-X Pro series

This new line of pro grade filters from K&F Concept is truly fantastic. These filters are super slim, with smooth threads that attach easily to my lens. The UV filter doesn't have any color cast or produce any optical aberrations in general. Just a crystal clear filter to keep your lens safe. Comes with a padded case for safe storage.

K&F’s build quality has been second to none in the last several years. They’ve created a series of extremely high-quality filters using very slim precision machined brass rings ( brass is a much better material to use for this purpose than aluminum. It’s much less likely to strip threads or to cross thread) and excellent optical glass. This filter is no exception. It has absolutely no optical distortion whatsoever. It filters UV and pretty much nothing else I think it has about a 99.9% visible light transmission. Most people use UV filters to protect their lenses but it’s also helpful to filter out UV light for photographic reasons. It can help to increase the life of your camera sensor and prevent distortion in the images. The anti-reflection coding on this is extraordinary. This competes well with my expensive high end German filters that cost up to four times as much money. This filter is slim enough that it should not interfere with the image or create any vignetting even with wide-angle lenses. This is my review for the UV filter but honestly every K&F filter I have purchased has been extraordinary. Look at the photo and see if you can tell any difference.

its hard to believe, but this is the first UV filter that I have had that actually makes my images BETTER! Most UV filters I have used over the years have had no real real visible effect on my images, and some have actually had negative effects because of poor quality. This filter is seriously amazing to me. It better be for the price, right?! Lol. its pretty expensive, but my images are actually coming out better with it, so I guess its worth it. Loving everything that is coming out of K&F Concepts lately. they have really stepped up their game.

This new line of pro grade filters from K&F Concept is truly fantastic.

These filters are super slim, with smooth threads that attach easily to my lens.

The UV filter doesn't have any color cast or produce any optical aberrations in general.

Just a crystal clear filter to keep your lens safe.

Comes with a padded case for safe storage.

The lens looks and feels nice. It seems to be of good quality and would last longer. The storage case looks elegant and comes with a wipe to clean the lens. The few pictures I have taken seem reasonable, and the lens doesn't cause any issues with the quality.

Got this to combo with my Canon R5 and the nice all-around RF 24-105 4 L lens. Fits nicely, and the lens cap is secured. Its UV protection is a nice feature to have. It keeps my pictures free and clear from those pesky UV rays when taking pictures outdoors, especially at the beach. My kitty ended up being the celebrity for the day while we were getting some sunshine hehe. Definitely worth getting, especially for anyone doing outdoor photography with the 70-200 2.8 L lens or 24-105 4 L lens (77 mm diameter).