Red filters, such as the Hoya 25A, are used primarily for increasing contrast in black-and-white photography. They can give dramatic effects in landscapes where a blue sky will be darkened and white clouds will stand out more clearly. Greens, water and shadow areas also tend to darken. A range of shades and densities of red filter are available from various manufacturers.
In general, red filters lighten reds and yellows; and darken blues. Blue skies are recorded as black, particularly when a camera is directed away from the sun. The sky close to the horizon tends to record as a paler tone. They also cut through heavy haze and are excellent for recording light architecture against blue sky.
Red filters are sometimes used creatively with colour film, and for infrared photography.
Filter factors are in the range 6 x to 10 x.