I tested this K&F Concept 52mm UV Filter
Here's what I found...

UV blocking ilters protect lenses from dirt and damage (and stop colour shifts if you still shoot film)- but they can also reduce image quality - so I test all new filters and reject some.

How did I test it?
[1] Colour shift - I used a set white balance and looked for colour shifts with and without the filter in place on a Nikon DSLR with a prime lens set to its highest quality setting (100asa and F8) both inside with artificial light and outside in sunlight.
[2] vignetting - I looked at the edge of the image for any additional shadows with and without the filter
[3] flare and contrast- I photographed lightbulbs and the sun between tree branches with and without the filter
[4] distortion - I photographed a newspaper page with and without the filter

Pros ✅ Viewed on a retina laptop screen I could see no differences except in contrast. This turned out to be the effect of NOT using the correct lens hood. Basically - and side lighting can catch glass and cause reflections. Internal reflections are what photographer call "veiling flare" and this kills contrast. As the filter makes the first glass surface of the lens much less recessed (and so more visible) it risks vcausing eiling flare.
This happens with any lens - and the solution is to fit and use a lens hood whenever possible.

Cons ❌ none in "real world use". As with all filters, when I went to 800% image size there was a slight loss of resolution, but for my standard HDTV use or printing to 8x6inch size this was "invisible".

The only difference I could see over the classic "Hoya" filters was that the filter ring is thinner and so better to use on a wide-angle lens. There is a cost to this however, as the light weight of this filter tells me it is made of aluminium and so is easier to bend than say the brass ring filters from B&W (which cost way, way more!) - so the moral of this is to leave the filter on the lens - or protect it in a rigid lens case if you take it off. And there you get a good snap closing filter box in the pack - along with some useful additional accessories.

OVERALL - I like this - and would be happy to get it again. Unless you are ready to pay the £50 plus for the brass-made B&W filters, these seem to be as good as any other but at a lower price.

Delivery was quick - it came the next day which is great service.