Commonly used to raise contrast slightly with black and white films. Especially useful for clear contrast between blue sky with clouds and foreground. It is also suitable for portraits, landscapes and nature scenes.
Most Kodak gelatin filters are available in 3 x 3" (75 x 75mm), 4 x 4" (100 x 100mm), 6 x 6" (150 x 150mm), and 14 x 18" (350 x 450mm).
Filter factor is 1.5. (+.5 stop)