It can soften the entire image, reduce the contrast of the highlights, and produce a halo, making the image soft. 

Answer: By design we can avoid the use of the filter in non-wide angle lens will not have dark corners, if your focal length is less than 24mm still may back to produce. 

Black soft filters can be used to reduce blemishes and wrinkles in people or objects by reducing the resolution of the image. They can also be used to increase contrast and create a halo or glow effect for specular light sources in a scene. 1. soften the light effect of the actress's facial details, 2. It can effectively reduce some wrinkles and spots on the face of the actor, but at the same time maintain the details and clarity of the character's hair and eyes. 3. It also plays an important role in film shooting. Excellent black mist filter does not affect the color temperature, the use of dim scenes with a hazy age will create the atmosphere of the scene well. 

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. 

By design we can avoid the use of the filter in non-wide angle lens will not have dark corners, if your focal length is less than 24mm may still return to produce.