The most popular family of contrast filters for black and white photography, they create dramatic effects by brightening red and orange while darkening blue and blue-green in sky, water and architectural scenes.
With underexposure, red filters may be used to simulate a moonlit scene at midday.
With infrared film, red filters give extreme contrast to skies, turning foliage white and cutting through fog and haze.
Red filters are available in most sizes and in light #23, medium/dark#25 (standard) and dark#29. As with all colored filters the darkest absorbes the most light and gives the most dramatic effect.