The Heliopan 28mm #29 Dark Red Filter is a basic contrast filter, primarily for use with black and white film, that completely absorbs blue and green tones, enhances reds, and increases contrast for outdoor photography. It is suitable for use in tri-color separations and for photographing faded documents. There is an approximate 8-12x filter factor with this filter, requiring an increase of 3 to 3.5 stops for normal exposure.
The front and rear surfaces of the filter both feature a single anti-reflection coating. Each coating helps to increase light transmission by reducing reflections, scattered light, and ghosting. Additionally, they offer protection to the filter substrate while helping to keep the glass clean for longer than uncoated glass.
This Heliopan filter is constructed from Schott glass for increased optical clarity and color fidelity. It features a brass filter ring for durability as well as jamming prevention.