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While digital and film photography are world’s apart, the
basic concept of color recording is similar. To get a sense of how they, in
many ways, are similar, let’s first take a brief look at color film recording
and draw parallels as we go.
Film and digital photography work in an RGB (red, green, blue) color space. The capture element (the sensor in digital and the film itself) blends the three to create every variation of color and tone.
Film is composed of light-sensitive grains, known as silver
halides, suspended in an emulsion that is coated onto a clear plastic base. The
important thing about the halide grains is their efficiency of light capture,
which determines the film's light sensitivity, known as "speed" and
expressed as an ISO number. Generally, the faster the film the larger the size
of the light-gathering crystals. In a given type of film, faster films will always
have crystals with greater area than slower films.
Being an electronic-based medium, the digital sensor has a
“native” speed, or ISO. Greater sensitivity is achieved by choosing a higher
ISO, which programs the camera to apply a higher electronic charge across the
sensor, known as “gain.” So for film it’s grain and for digital it’s gain.
The result of working with larger film grain, or the
salt-and-pepper pattern in the image, is greater visibility of grain that manifests
when enlargements are made. The result of using higher ISO settings in
electronic imaging is “noise,” a kind of static in the image. Excessive noise
is less of a concern today than in the past, since noise reduction algorithms
do a good job of suppressing the ill effects of gain, although this can result
in some loss of sharpness. Its generally agreed that noise gets in the way of
image quality, although emphasizing this grainy look can be used for creative
ends.
Color film is constructed with various layers, including
those that respond to different wavelengths of light. While it is more
intricate than that, in the way the color in the scene is mixed onto the
resultant image, the three basic layers are responsive to red, green, and blue
light. Film records color by adding proportional density in the appropriate
color recording layer during exposure.
Digital records color by separating the image into three "channels" which are then blended by the processor in the camera to create a full color image. Digital can create millions of color variations that result from the blend of both color and its density in the recorded image. Likewise, film creates color layers that are blended when printed or, in slide film, during a reversal process that takes place when the film is processed. Both rely on this RGB scheme to create a full-color image.
The more blue in a scene, the greater the density in the
blue recording layer after exposure and development. Film captures
all the colors of the rainbow, and blends the colors in subtle ways depending
on the mix of color captured. During development, the silver density is swapped
for dyes that correspond to the color and intensity of the light in the scene.
These dyes are what create the color image. Both negative and slide film begin
as a “negative” image (reverse of the colors): when enlarged, negative film
causes the colors to reverse; with slide films the reversal occurs during
processing and emulates what happens when negative film is printed.
Digital sensors also have red, green and blue recording
layers, although, being electronic, the light is filtered accordingly and an
electronic signal is produced, a code that reads out as a specific color in the
image. The same blending and nuances of color are produced as with film,
although the profile and color space can be altered in the camera, with
so-called Picture Styles. In short, both systems operate in an RGB (red, green
and blue) system. In film, that record is “baked in” while in the digital world
the processing, in-camera and when editing in the computer later, can be
highly variable according to the taste and intentions of the photographer.
Next post: Saturation and color enhancement
