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. 

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. 

so it should be 55mm. 

I always prefer to buy filters for my wider lens and then use step ups. Quick tip: if you’re using an wide angle lens like 20mm and less I advise you to use always a filter wider then your lens to avoid vignetting. My wider lens is 77mm so I always buy filter of 82mm. I hope it helps you. Peace! 

Yes 

You need to use a Retractable Rubber Hood with an internal and external screw thread. You mount the hood first, then you screw the filter to the hood. You retract the hood then you turn the filter to desired ND adjustment. I can reach and turn it without retracting the hood.