From 1-5 stops, there will be a slight X cross for lens under 24mm. An X cross will appear at the highest stop and can be removed by rotating the filter ring, which is a normal situation instead of a quality issue. 

Tested with Nikon Z 7 and Nikor 14-30mm Sline, dark corners are not obvious for 14mm and no dark corners for 16mm. 

As K&F Concept ND2-ND400 filters adjusts the amount of light that can pass through with polarizing glass, a colour cast can't be avoided for all the variable ND filters on the market. However, K&F Concept variable ND filters are made by the best coating technology, which reduces the effect of the polarizing film on image color. *Based on our professional test, the colour cast of K&F Concept ND2-ND400 filters is around 290k while the industry standard is 500k. 

To avoid dark corners, K&F Concept ND2-ND400 filters are designed in ultra-wide construction so the front side of the ND filter is one size larger than the camera lens. It is recommended to use a lens hood of a larger size. 

First, K&F Concept ND2-ND400 lens filter allows changing shutter speed based on the effect you want with the best sensibility and the same aperture. Second, K&F Concept ND2-ND400 lens filter allows creating a blurred background with a large aperture while the shutter speed remains the same. Third, K&F Concept ND2-ND400 lens filter is designed to prevent overexposure and maintain image purity and quality at the same time. *Like a zoom lens, the variable nd filters are suitable for multiple shooting situations. If you're looking for perfect image quality, a fixed ND filter is recommended. 

K&F Concept ND2-ND400 lens filter controls the amount of the light that enters into camera lens with two sets of polarizing glass of high transparency.