Tiffen Solid Neutral Density Filters

Tiffen ND filters reduce the amount of light passing through the camera lens without changing the colour of the scene. They are especially useful in bright light conditions to help prevent overexposure. Neutral Density filters also allow proper exposure at a wider lens opening for reduced depth-of-field to highlight a key subject by making the foreground and/or background out of focus

A fixed ND are measured in increments of Stops. An illustrative chart can be found below.

And so on.