Graphics Programs Reference
In-Depth Information
What is dynamic range?
Dynamic range is basically the range of input a device can capture.
For example, the range of light under which the human eye can see extends
from the brightest sunlight to the dimmest moonlight, and this range
of extremes dei nes the dynamic range of the human eye. The range of
human vision far exceeds that of most cameras and computers. Because the
dynamic range of i lm and digital capture is more limited than the human eye
can see, photographers must be selective about what is important in a scene
and expose with this scale of limitations in mind.
Lost highlight detail
Like transparency i lm, if a part of a digital image receives too much light or
overexposure that portion falls beyond the capability of the sensor (or i lm)
to record image content and the result is rendered as pure white. Once that
portion records as pure white, the information is ef ectively “blown out” or
no longer holds any image data, which translates photographically as no
image detail in the highlighted areas of the image. The same is true if a part
of the image receives too little light, the capture data will fall beyond that
which the sensor can record on the other end of the scale. The digital sensor
will therefore underexpose the corresponding image information as pure
black. A recognizable image is only recorded if the light hitting the i lm, or the
digital sensor, falls within a range of about i ve f-stops. This is the approximate
dynamic range of digital capture.
Lost shadow detail
Preserved highlight and shadow
information
Exposing for digital capture is somewhat dif erent than exposing for
i lm, as digital cameras behave quite dif erently than i lm cameras do
in the way they handle highlight and shadow information. With digital
capture, extreme overexposure ef ectively saturates highlight information
beyond recovery, and underexposure pushes shadow information and
detail into noise, making quality rendering almost impossible. How sensors
respond at the saturation limits at either end of the dynamic range limitations
are also dif erent from i lm, and how to deal with these characteristics are
all embedded into the raw conversion and subsequent post-processing
procedures. Although digital cameras today have a much greater exposure
latitude than most 35 mm i lms do, it is unfortunately still much easier to
blow out the highlights on a digital sensor than it is with i lm. The digital
camera's imaging sensor is very similar to color transparency i lm when it
comes to its sensitivity to light. For this reason, the best exposure strategy
with digital is to expose for the highlights, breaking the rules of the i lm
lessons that taught us to expose for the shadows and print for the highlights.
Exposing for digital capture
Proper exposure with i lm or digital is dii cult to dei ne, as the best exposure
for any given image will depend on the image itself and corresponding
circumstances. There is really no such thing as a perfect exposure, as artistic
preferences should drive exposure to vary depending on the tones of an image,
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