Apple Aperture Digital Photography Fundamentals - Manual (Page 31)

Chapter 2 How Digital Images Are Displayed 31
With the invention of color film came a whole new set of considerations. In addition to
correctly exposing the image, photographers had to take into account the various color
tints different light sources cast across their film emulsion. Film manufacturers
improved the situation by developing film emulsions rated for daylight and tungsten
lamp color temperature ranges. Camera manufacturers also jumped in and developed
color filters, attached to the camera’s lens, to help photographers shoot outside the
temperature range of the film. However, these solutions didn’t completely eliminate
the problem because images shot in unforeseen and adverse lighting conditions
remained irreparable during the printing stage.
How White Balance Establishes Color Temperature
When you take a photograph with a digital camera, the color temperature of the scene
is not taken into account until the image is processed by the camera’s processor. The
camera refers to its white balance setting when it processes the image. When the
camera’s white balance is set to auto, the camera assumes the brightest value is white
and adjusts all other colors in the image accordingly. If the brightest value is white, the
colors in the image are rendered correctly. If the brightest color is yellow, the camera
still assumes that value is white, and shifts all the colors out of balance.
However, you can adjust the color temperature of a digital image. White balance is a
mathematical process that calculates an image’s color temperature and applies the
effects to the color values in the image after the RAW image is stored. That color
temperature data is stored as metadata in the image. The digital data that makes up
the original RAW file is unchanged. So, no matter what white balance or color
temperature setting was applied at the time the image was shot, the color temperature
of the image can always be corrected after the fact. Digital cameras’ RAW files solved the
problem of color temperature flexibility that the chemistry of film never could.
Measuring the Intensity of Light
In order to shoot an image with the correct exposure, you have to know the correct
value of the intensity of light. Photographers use light meters to measure the intensity
of the reflective light in a scene. Digital cameras have built-in light meters that are very
sophisticated and incredibly accurate. However, their accuracy is subjective. The
recommended aperture and shutter values are determined by how light falls in the
scene and by how the light meter is set. The camera’s light meter may recommend an
aperture and shutter combination that offers a decent exposure. However, it may not
give you the perfect exposure because it doesn’t know what you’re photographing.
Light meters can’t evaluate colors or contrast. They only see luminance, which is the
brightness of the reflected light in a scene.