Neutral Density (ND) filters are created for an equal transmission reduction across a section of an exact spectrum. It equally modifies or decreases the intensity of all colors or wavelengths of light without changing the color of the scene. These are commonly determined by their Optical Density (OD) which defines the amount of energy-restricted by the filter. Moreover, low optical density means high transmission while high optical density means very low transmission.
There are two types of filters: absorptive and reflective. Absorptive ND filters use glass substrate at a specific percentage in absorbing light. It’s useful in controlling and measuring exposure in imaging. Reflective ND filters utilize glass substrate which contains thin film optical coatings that are usually metallic. The coatings can be adjusted on the desired wavelength spectrum such as near-infrared (NIR) or ultraviolet-visible (UV-VIS). Proper care should be considered so that the system will not be interfered by the reflected light.
Neutral Density (ND) filters are especially useful to prevent overexposure caused by bright light conditions. It allows correct exposure at a broader lens opening for a concentrated depth-of-field by highlighting the main subject. The focus for foreground or background is adjusted. This is the best filter for nature and landscape photography.