The Heliopan 27mm #15 Dark Yellow Filter is a basic contrast filter, primarily for use with black and white film, that very strongly darkens skies and blues, increases contrast, and is often used for outdoor photography to emphasize contrast and shadows. There is an approximate 4x filter factor with this filter, requiring an increase of 2 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.