How To Read And Correct A Histogram - Microtek ScanMaker IIG User Manual

How to read and correct a histogram

A histogram shows how the brightness and
darkness levels are distributed in an image.
The darkest pixels are at the left, and the
lightest pixels are at the right.
An image with good contrast will have a
histogram with vertical lines spread across
the scale from left to right. Here, the
histogram is heavily skewed to the left,
where the darkest pixels are, indicating a
dark image.
To change the histogram (and thus the
image), use the three triangles below the
histogram.
In the original histogram, the pixels are
mostly to the left where the black triangle is,
indicating a dark image. The range of
Original image and histogram
Corrected image and histogram
More detail shows up
in shadow areas
spread is also broad and flat, with almost no
pixels for the midtones and highlights where
the gray and white triangles are.
In the corrected image, the triangles have
been moved to new locations. The net effect
is to narrow the distribution range of the
pixels and lighten the image.
Generally, the best thing to do is to move the
black and white triangles to the start and end
of the curve, and move the gray triangle to
somewhere in between.
For example, if your graph starts at about
value 20 and ends at 240, move the black
triangle to 20 and the white triangle to 240.
Then move the gray triangle to somewhere
in the middle between the black and white
triangle for good overall balance.
Pixels are
concentrated
here,
indicating a
dark image.
Histogram has a broad and flat spread, with almost all
pixels in the shadow range (near the black triangle).
More highlights visible
Triangles moved to
new positions
Reference: The Settings Window 5-65