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. 

It is a continuous spin without stops. 

This one is high quality materials, and contains both a variable ND filter + a polarizing filter. Also - most variable Nd filters show and “x” pattern when they’re turned to the max, essentially making them useless at full power. This one shows no “x” pattern. I’m happy I bought it! Very high quality and sturdy. 

"X" cross on images is a common issue with wide angle lenses and variable ND filters. Physics limit the capabilities of variable ND filters on wide angle lenses.
When you enter the edge of the max range,the "X" issue may appear on your image. Because you are using it beyond its capability according to the laws of optical physics,which will inevitably cause this phenomenon.
 

No, the lens cap can only be used with K&F Concept variable ND filters.