Figure 2-5 might help make this a little clearer.
In general, when using
filters for black and white photography, thinking subtractively is more
useful. This is because filters, without exception, subtract something from
the light that is there in front of you; they never add to light. How could
they, anyway?
Figure 2-5. "Yellow light" can be thought of either as
"Red plus green" or as "White minus blue"
Learning Event 2:
DESCRIBE THE COLOR SENSITIVITY OF FILM
1.
Even though black and white film doesn't record color, it does react
to it - but not the same way the human eye does. The wizards at the film
manufacturers have been trying for a long time to make black and white film
react in human fashion.
2.
The very first photographic emulsions certainly didn't work like the
eye; in fact, they responded only to ultraviolet and blue light. For this
reason, they were called colorblind (also sometimes monochromatic)
emulsions.
It was almost 40 years before chemists discovered dyes that
could be added to a film emulsion which extended its sensitivity range to
include green light. They were so elated by this dramatic improvement that
they called the film orthochromatic (correct color).
This was an
exaggeration, of course, because the emulsions were still blind to red
light.
It wasn't until the 1890's that further dyes were found which
extended the emulsion's range to include the red portion of the spectrum.
They couldn't call the new emulsion orthochromatic because that word was
already taken, so they came up with panchromatic (all color).
Most black
and white films used in cameras today are panchromatic (called "pan" for
short), but orthochromatic (ortho) and colorblind emulsions still have uses
in portraiture and copy work, and especially in scientific, technical, and
astronomical applications.
They also are quite common in printing papers
and motion picture print films. In fact, all three types of emulsions, in
specialized form, are used in making color film (fig 2-6).
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