The Heliopan 27mm Light Yellow Filter is a basic contrast filter, primarily for use with black and white film, that is primarily used as a haze filter to subtly darken blue tones and increase overall contrast. There is an approximate 1.5-2x filter factor with this filter, requiring an increase of 0.5 to 1 stop for normal exposure. For all outdoor photographs. Small correction to blue sky, improved contrast in distance photographs.