Graphics Programs Reference
In-Depth Information
and the resulting mood or impact the photographer creatively wishes to achieve.
Predominantly, however, our general goal with exposure is to simply place the
tonal values found in the scene most appropriately within the dynamic range of
the camera's imaging sensor. “Most appropriately” means that the midtones found
in the image fall roughly halfway between the darkest and the brightest values,
while taking into account the importance of shadow and highlight information
within the image. If a subject is exposed too far into either extreme, the limitations
of the sensor will become more obvious. If the values fall too close to 0 (absolute
black), there will not be an image at all, or the image will appear very dark and
noisy. If the values fall too close to 255 (absolute white), the pixels will appear
oversaturated with no image information or detail. The best exposure strategy for
digital capture, therefore, is to keep highlights from reaching the maximum output
value of the sensor, except for specular highlights that do not detract from the
image if they are blown out.
Histogram with all three channels combined and
averaged (Red, Green and Blue)
Histogram displaying each color channel separately so
that you can monitor each channel individually
Once you have captured an image in the i eld, bring up the histogram on the back
of the camera. The histogram data should indicate how the image data falls within
the dynamic range of the camera at the time of exposure. Using that information,
the photographer can evaluate what is important in the image, like highlight or
shadow information, and which takes precedence, and re-expose the image based
on creative interpretation of the data. You will have to consult your camera manual
to i nd out how to turn this valuable camera feature on. Some cameras just have
one histogram which is all three channels combined and averaged (Red, Green and
Blue). Other more advanced cameras will show you the histogram for each color
channel separately so that you can monitor each channel individually.
Reading and Interpreting Histogram Data
Clipping
Isolated lines or spikes or a pile at either edge of the data box is an indication of
clipping or data loss within an image. Clipping indicates that improper exposure
or image manipulation in the digital darkroom has caused some parts of the
image to move beyond the maximum brightness level within the dynamic
range of capture. Such a histogram shows that image data has been recorded as
 
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