Learning Event 4: Explain the Theory of Filters

Photographic Filters and Techniques
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Learning Event 4:

EXPLAIN THE THEORY OF FILTERS

Now that you know what filters are made of, how do they work?

Remember the red apple against the green foliage that looked so dull because

both the apple and the leaves looked the same shade of gray? With filters,

you can now have it either of two ways. If you want to make the apple look

dark against relatively light leaves, try using a green filter. Leaves look

green because they absorb a lot of the red and blue parts of the light, but

reflect most of the green part. The green light passes right on through the

filter, and the exposure of the leaves is hardly affected at all, because

almost all the light reaches the film.

But the apple, which looks red

because it absorbs blue and green and reflects only the red part of the

light striking it, has most of its light blocked.

The film receives much

less exposure from the apple and as a result it looks darker than the

foliage, standing out clearly in the print. On the other hand, if you were

to change to a red filter, the opposite effect would happen.

The green

leaves, reflecting very little red light, would be darkened considerably,

while most of the red light passes through the filter to the film.

The

result would be a light apple against dark leaves.

Figure 2-7.

Red apple and green leaves

The girl is wearing a blue blouse and yellow dress; how would you

handle a situation like that? Using a yellow filter will let most of the

yellow light from her dress through to the film, making it look relatively

light, while the blue light from the blouse will be stopped by the filter

and appear darker than if no filter is used.

White clouds in a blue sky is a problem that has many solutions. If

you were to use a yellow filter, the blue light from the sky will be blocked

and the sky will be darkened. The white clouds will also be darkened but

since they are white and have a lot of green and red light coming from them

they won't be darkened as much. Now, a light yellow filter will block blue,

but not entirely, and the effect will be somewhat modest.

Using

progressively stronger filters, each more efficient at cutting out blue

light, will strengthen the effect. Finally, by using a red filter, the blue

sky can be made very dark, while the white clouds stay white.