Filter up to my expectations, consistent with the description, no cross!!
This is very good value for money! I can only recommend.

***Update***

I have now also bought the 72mm version in the flash offer and am just as happy with it. I now use the variable ND filters as a "quick" filter when I'm on the go. If I want to expose more professionally, I use my K&F square filters. These are also highly recommended, by the way!
The 72mm round filter has similar good properties to its big 77mm brother. It has slight color casts in the maximum range, but impresses with its consistently good image quality and workmanship.

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The filter is very high quality and comes with an extra cover. The filter is made of metal and glass. The two rings can be rotated very smoothly against each other.

The variable ND function works very well for me. Both discs are perfectly matched to each other. I could not see any loss in image quality in my test images. The design automatically results in vignetting and sometimes a color cast. But nothing that can't be quickly worked out with image editing software!

A great tool for filmmakers and photographers to be able to do a variety of creative work with just one filter.
The price of around €50 is basically justified, but the price here could be closer to €45.

All in all, a recommended purchase.

Amazon delivered very quickly, which meant I was able to test the ND filter in no time.
The first impression was positive, the packaging including the plastic case protected the filter during transport to me.
The plastic case looks sturdy and has a simple yet sturdy closure, so you can even store it loose in your camera backpack.
The adjustment aid is marked (non-linearly) and has two end stops. The first attempts with the ND filter were quite okay, when the weather is better I'll try it on the next river or in the city and will post the results later if necessary.
The ND filter from K&F Concept is absolutely top-notch for 48 euros, recently it was even available for a good 40 euros in a flash offer. I would definitely recommend buying it!

Bought this filter for my friend as a birthday present. She loved this filter. She is starting as a photographer. She said it is very easy to use, easy to install and easy to clean.

It works fine with my normal spectrum camera even at ND 128 there is no cast to be seen as in shot 3. The vignetting is due to a DX lens being on a FX camera and is nothing to do with the filter.
Shot 1 is ND 128 on a full spectrum camera, shot 2 is the view from a full spectrum camera without the filter. ND 8 is just a paler version of shot 1.

I particularly like the fact that this filter is variable which makes it very practical while shooting at low light since you can lower the intensity without having to remove or change filters.

I've tested some cheap variable ND filters and this one is by far the best for this really small price. The main feature is that this filter really does not deliver any cross image errors, unfortunately this is a problem that all cheap filters have but the one from K&F delivers a flawless image! The color shift is also only minimal and provides decent sharpness! I use this filter on a BMPCC 6K to shoot in daylight with the aperture completely open. If you can't spend more than 100 € on a good filter, this is pretty good for you.

After extensive testing, I can absolutely recommend the filter. It shows no annoying vignetting, the glass is very well made.
The packaging of the filter is also nice, the enclosed box saves space and is great for transport.
Due to the variability, it can be used in almost any lighting condition.

Der Filter tut einfach, was er soll. Die Verarbeitung ist tadellos. Ich hatte bis jetzt ND Filter von Hama, Hoya und Walimex. Farbneutral war keiner. Dies ist mein erster variabler. Er macht das Bild etwas wärmer und gefällt mir am besten. Der Farbton bleibt auch bei allen Einstellungen gleich. Getestet mit Sony a7III. Ob der Weißabgleich bei anderen Herstellern das gleiche Ergebnis liefert, kann ich nicht sagen. Die Belichtungsmessung muß ich +1 korrigieren, um ein gleich hell belichtetes Bild zu erhalten. Die beigefügten Bilder sind in LR identisch entwickelt. Auch bei 24mm habe ich kein dunkles X. Die Schärfe leidet auch nicht.

I can highly recommend this article. The delivery was really fast. The first view was nice, because it looks quite strong and it feels that I will not break so fast. Which is good because I take most of my picture while I am hiking.

This filter is really good also for filming and long exposures, so I don't have to change filters. I can use this one for many different situations. The prize is also really good and in the postproduction I saw no problem while I was editing in lightroom.

So if you want to have a good filter which is not to expensive then this is a good is good choice!