What is the ‘Natural Night’ filter?

The Natural Night filter is a filter which blocks wavelengths of light that commonly cause light pollution.  These are usually oranges and yellows emitted from mercury and sodium vapour lamps.  It is best used for night cityscapes or for astrophotography, particularly within the vicinity of towns causing troublesome light pollution.  It also blocks approximately 1 to 1.5 stops of light.  Here are some examples of the Natural Night in use: