MECO Neutral Density Filter . The following models of ND filters are listed by the number of aperture stops it reduces and the amount of light during a specific time it let in. Bigger number means less amount of light and lower light density. Note: The ND filters on the market are by no means perfect . And they can’t reduce the density of light of all wavelengths equally, which might cause color cast of pictures, especially with inexpensive filters. More importantly, most of the ND filters are only specified over the visible region of spectrum, and do not proportionally block all wavelength of far infrared ray and ultraviolet ray. Thus using ND filters to observe these ray sources such as the sun, incandescent metal or glass can be dangerous. These sources emit intense non-visible radiation that can cause damage to the eye even though they don’t look bright when viewed through the filter. Special filters must be used if such sources are to be safely viewed. The typical usage of ND filters includes the following: Blurring water motion (e.g. waterfalls, rivers and oceans) Reducing depth of field in very bright light (e.g. daylight ) Reducing the visibility of moving objects Adding motion blur to subjects The ND filters are used also on the catadioptric telescope camera, in which there is no aperture device that van seriously affect the imaging quality.