There is a slight cross-hatching at maximum aperture with ultra-wide angle lenses up to 24mm at 1-5 stops of reduction, the longer the focal length used the less the cross-hatching will affect. At the maximum stop of reduction there will be a noticeable cross-hatching, which will disappear naturally if you continue to rotate the lens, this is a physical phenomenon and not a product problem. 

Due to the use of a polarising film to control the amount of incoming light, colour cast is a common phenomenon on all commercially available adjustable ND's. KF has minimised the effect of the polarising film on the colour cast by using a mature manufacturing process and perfecting its coating technology. *According to laboratory data, the ND2-400 has a bias value of around 290k, which is below the industry average of 500k. Does the ND2-400 have dark corners? Tested with Nikon Z 7 and Nikor 14-30mm Sline, dark corners are not noticeable at 14mm, no dark corners at 16mm. 

1. The shutter speed can be freely varied to suit the desired effect without changing the aperture, while maintaining the optimum iso. 2. A large aperture exposure can be used to create a bokeh background effect while maintaining the same shutter speed 3. Even with a shutter speed of 1/8000 second, a maximum aperture of F1.4 or F1.2 can lead to overexposure. The ND2-400 can easily eliminate overexposure in such cases, preserving highlight detail. *As with zoom lenses, the adjustable ND has a wider range of applications and is suitable for a wider range of scenarios, so if you are looking for the ultimate in image quality it is recommended that you choose a fixed value ND. 

Two sets of polarising filters with high transparency are used to control the amount of light entering the lens through the filters. 

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.