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The RED filter will only let RED light pass through it making red look whiter or lighter.

It will absorb BLUE and GREEN making those colors appear darker.

Filters for Black and White

The black and white photograper still must be aware of colors. Color in light and the color of our objects will effect the grey tones in our final photo. Notice how the photo at the left has the red can and the blue cover of the book similar in shade. To change that we must use the creative control of a filter.

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White light when it hits a prism or water in the sky is divided into its primary PARTS or colors. White is made up of EQUAL parts of Red, Green and Blue. These colors ADD together to make white. We call this RGB color and is what TV sets work with.

An object we see appears red to us because it absorbs blue and green and reflects back only RED.

A colored filter that is RED will absorb green and blue and let ONLY RED light get through to our film.  It will thus make red things look like they were white. Things that were blue and green will look blacker or darker because they did not get through.

In the center photo on the left a yellow filter was used. It will reduce blue while only slightly reducing the red and green colors. A yellow filter is used to make colors appear near normal in shade on black and white film. 

The bottom photo was taken using a RED filter. Notice how it made the red can appear so light in grey that the letters rubber cement don't show up ver well now. The blue was completely absorbed by the filter causing it to look black in the photo. This is a dramatic or CREATIVE effect when used like this.

The photographer would use a RED filter whenever they want to make red things look light grey or when they want blue things to look darker. Red is used to make clouds from a storm look nicer. The folds in a cloud are blue in color and photograph very low in grey in black and white. The red filter will make them darker and thus more dominant.

 

Because a filter absorbs some of the light the light meter must be adjusted for proper exposure. Early photographers had to memorize a filter factor or number to multiply the exposure by to get a proper exposure. Today the modern Through The Lens meter will respond to the change in light and adjust the exposure to let in more light.  The red filter requires 8X more light which means that the basic exposure must have 3 stops (either more aperture or more shutter) more light to compensate. The yellow changes only one stop from the basic.  Each shutter speed or aperture changes the exposure by a factor of 2X so one stop is 2x, two stops is 4x, three stops is 8x and four stops is 16x difference. 

SPECIAL FILTERS

In color photography colored filters are only used to correct lighting color or lighting temperature errors as they are called. For example, indoors using outdoor type color film under a normal lamp the subject will turn yellow or with a fluorescent lamp the subject will look green. To correct this a CC or Color Correction filter is used.  To darken the sky in a photo or to remove reflections from shinny objects a Polarized filter can be used. This filter acts like sun glasses and removes the glare and it usually will darken the blue sky.  Finally in the bag of accessories comes the neutral density filter that is just a neutral gray dark filter that is used to reduce the quantity of light like sun classes. This filter is used to allow for longer exposure times by reducing the amount of light that gets through the lens.