The other day I wrote about Black and White photography (Black & White) and the very next day, I got an email about the red filters. It seems that they are some of the least understood filters now a days. Instagram users apply the red filter at times to improve their selfies (a term which I fail to understand). I am talking here of the true red glass that fits over the lens. They are also denoted by the Wratten numbers from 23A to 29.
What are these red filter and how are they used? First let me clarify a few things. Instead of calling them red filter, they should ideally be called blue/green-inhibiting or blue/green-subtraction filter. That is exactly what they do.
In the above mentioned article, I have also included a photograph with dark red filter in use. The dark red filter reduces the amount of blue light that can pass through and darkens the sky. It can be used instead of polarizers in monochrome pictures. The polarizers darken a portion of the sky, depending on how the light is scattered in the atmosphere (more on polarizers – Polarizing Filters). The red filter on the other hand can darken the sky very smoothly when used with wide angle lenses.
(Zorki 4 with Red filter)
Red filters are also used in underwater photography to remove the color cast and to bring out the natural colors of the underwater world.
One of the non-conventional use of the red filter is to visualize the scenes directly through the filter and get an idea of how it would look in monochrome. This works only for a few seconds and then our eyes adapt to the red filter and start seeing colors again.
Very very deep red filters which are almost near opaque are used in infra-red photography. You guessed it right, they permit infra-red light to pass through and subtract or inhibit most of the visible spectrum.
Other color filters-
The principle behind the use of color filters in Black and White photography is the same as that with the red filters. It all comes down to understanding the color theory and color wheel.
The color wheel is an old concept where all the colors are plotted on a ring. The colors situated next to each other are called ‘Analogous Colors’ and the ones on the opposite sides of the ring are called ‘Complementary Colors’.
All the color filters reduce the amount of light passing through of the complementary colors with a little spill over on the adjoining colors of this complementary side. Very specific true color filters (without any spill-over to adjoining colors) are very expensive and used only for specialized applications like in medicine.
Going by the color wheel, try to imagine what kind of effect a blue-green filter will have. The complementary colors are in the range of red. It can darken the red lips of a model or the red rose against a green background. It will also darken blond hair. The effect will be similar to the orthochromatic films of the yesteryear. A magenta colored filter will help darken the greens. A green filter on the other hand will darken the colors in the magenta region.
The same glass filters are available in their virtual form in most of the image editing programs. If you are converting your images to Black and White, use them. I do suggest buying a true red filter (Wratten 26 or 29) if you like landscape photography and want to try Black and White.
(A dark red glass filter was used in front of the lens to capture the image. Notice the sky! A polarizer would not have given such a smooth darkening of the sky. Nikon Df with Zeiss 135mm at f/5.6, 1/500 sec shutter speed, ISO 100)
The digital filters in post-processing work well but using a glass filter on the lens is a better option in many ways including getting the correct exposure. Switch to center-weighted metering while using dark colored filters. Read about metering modes – Metering Modes