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Filters for Black and
White Photography Most general purpose black and white films are panchromatic, which means that they record all the wavelengths of the visible light spectrum (400 - 700 nanometers, or violet through red) as shades of gray. Because your eyes and black and white film "see" a color scene differently, filters are often used to enhance the range of contrast of the subjects in the image either for clarity or for dramatic purposes. (For example, film is more sensitive to ultraviolet and blue wavelengths than your eye, so the sky in a black and white photograph can often look too light, the perfect time to use a filter...) Because filters absorb different amounts of light (depending on their color), the film will require more exposure than it normally would without a filter. The "filter factor" will tell you how much more exposure the film requires. Fro example a filter factor of 2 means that the exposure should be increase by 2 times (double) or one stop. How Filters Work: Colors similar to the hue of the filter will pass through the filter while colors that are the opposite (complementary colors) of the filter will be mostly absorbed by the filter. Think about film exposure and how that relates to print density. Colored filters allow you to change the relative lightness and darkness of the tones in a black and white photograph. Colors similar to the hue of the filter will pass through the filter and expose the film, leading to a buildup of density in the negative, which will result in a lighter value in the print. Colors that are opposite of the filter will be mostly absorbed by the filter, leading to less exposure on the negative and a darker value in the print. For Example: A green filter will absorb much of it’s opposite color, red, but allow green to pass through. So if you had a red apple surrounded by green leaves, using a green filter would make the green leaves appear lighter and the red apple would appear darker. Test: Even though your in-camera meter measures the light passing through the lens and should be able to compensate for the reduction in light caused by your filter, it is possible that some in-camera meters will underestimate the change needed. For example, a deep red #29 filter requires 4 stops additional exposure, but some cameras may only suggest a 2 1/2 stop increase. You can do a test yourself to see how much your camera meter differs from the exposure compensation suggested by the filter factor. First meter an average scene without a filter. Write down the aperture and shutter speed suggested. Then meter the scene with the filter on the lens. See if the amount of light change suggested by your camera meter agrees with the recommended filter factor. Go with the change recommended by the filter factor for this filter even if your camera says otherwise.
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