SIGMA DP3 Quattro - Manual (Page 98)

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HISTOGRAM
The histogram is a graph depicting the distribution of brightness values in
the image for each of the three color channels (Red, Green, and Blue) (The
histogram displayed in the Shooting Mode is the distribution of tones in the
image). The horizontal axis shows the brightness level, with darker pixels
towards the left side and brighter pixels toward the right. The vertical axis
shows the proportion of pixels at each level of brightness.
By examining the histogram of the full image, you can gauge the overall
exposure of that image. You can also use the histogram to determine if
zoomed-in sections of an image are over or under exposed.
This histogram shows that the higher pixel
values are never used in the image,
indicating that the image is underexposed,
and therefore will appear dark. In general,
when the histogram is higher on the left
side, the image is mostly composed of
dark pixels, causing it to appear
dark—either because the image is
underexposed or because it is a dark
scene such as night shot or sunset.
This histogram shows a fairly even
distribution of pixel values, indicating that
the image is well exposed, with good
contrast. However, the histogram
distribution of a well exposed image will
vary greatly depending on the subject.
This histogram shows that many of the
pixel values are at their brightness limit,
indicating that some areas of the image
may be blown out. In general, when the
histogram runs off the right side, the image
will have many white pixels—either
because the image is overexposed or
because it is a bright scene such as the
beach or snow.