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How colored filters work


Solid-color filters come in a wide range of colors and densities.
Solid-color filters come in a wide range of colors and densities.

In black and white photography, a colored filter lightens its own color and darkens its complementary color. For example, green foliage is rendered lighter - as a vary pale, almost-white shade of gray - when using a green filter, but becomes darker when using a magenta filter. You may wonder why.

WHY DOES A COLORED FILTER LIGHTEN ITS OWN COLOR?

Hold a solid-colored filter to your eye and look at a scene illuminated by daylight. The entire scene you see through the filter takes on the overall color cast of the filter. If it is a green filter, everything you see has a green tinge. But, you will notice that objects are not blocked from your vision; they are simply more green. The filter allows green light to be transmitted through it, and blocks red and blue light by absorbing it.


So, when you take a black and white picture with a green filter on your lens, the picture is made almost exclusively from green light. Since red and blue light don’t make it past the filter, the overall amount of light striking the film is reduced, and your exposure will not be correct. There won’t be enough light. You must make up for the lost red and blue light by letting in more green light. You have to increase the exposure to properly-expose the film.

But, when you increase the exposure, even more light from objects that are normally colored green passes through to the film, because green light is not blocked by the filter. This means that green objects become over-exposed. This is why they appear paler than objects of other colors.

Objects that aren’t pure green, but that contain some green, also become paler for the same reason. The green portion of their reflected light is over-exposed, and therefore the object appears paler. Yellow light, for example, is made up of equal parts of red and green light, and therefore yellow objects become paler in a black and white picture when a green filter is used.

A green filter causes a picture to be taken with mainly green light.
A green filter causes a picture to be taken with mainly green light.

Held a few inches in front of the lens, a green filter shows how much darker it renders part of the scene, making it necessary to increase exposure when using it.
Held a few inches in front of the lens, a green filter shows how much darker it renders part of the scene, making it necessary to increase exposure when using it.

FILTER DENSITY

Filters made with more or less of a solid color are said to be more or less dense. Color filters range from dark to light. By selecting different filter densities, you can control the degree to which filtration affects your picture. A lightly-tinged green filter will absorb some, but not all the red and blue light, whereas a dark green filter will only permit the smallest amount of red and blue to reach the film.

FILTER EFFICIENCY

When we said above that red and blue light don’t make it past a green filter, that was a bit of an exaggeration. No green filter totally blocks all red and blue light. You will notice when you hold a green filter to your eye that a red object will still appear red, although its tone will have changed. In the mostly-green scene on the left, a small red rose still shows its color in spite of being seen through a green filter. In fact, no filter of any color perfectly absorbs its complementary colors or totally transmits its own color. Nonetheless, colored filters do function as described above.