Sequence viewing > Photographic Exposure - Resource - ©
Lloyd Godman
Histograms
Many SLR digital cameras have a histogram function, which
lets the photographer view graph along side the image on the
LCD screen.
A histogram is a means of breaking down the colours and tones
in an image into data that can be read as a graph - when incorporated
with a digital camera, the advantage is that it can tell us
if the exposure has recorded the visual information that we
intended and that no information has been clipped or lost.
It
can tell us if we have lost detail or data in both the high
lights and shadows - this loss of information is called clipping.
In situations where there is contrasty lighting, it can be
impossible to record all the information from the deepest
shadow to the the lightest highlight.
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The
graph depicts the distribution of pixel values across
the tonal range from black to white, and the relative
number of times each tone is recorded which is referenced
by the height of the line.
Digital cameras and scanners assign brightness, contrast,
and black or RGB (Red Green, Blue) colour values to
each image pixel. A pure white is represented with
a value of 0 (levels of colour); A PURE BLACK EQUALS
255. Inspecting the histogram of a digital image is
a good way to quickly evaluate the tonal range quality
and highlight and shadow details.
In
this situation the tonal values recorded fall within
the latitude of the sensor and all the information
is recorded.
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In
this situation the exposure is not great enough to
record all the tonal values of the scene and the darker
tones have fallen off the graph.
So if you review an image in the LCD screen with the
histogram and it shows this you know you have lost
information.
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In
this situation the exposure is too great enough to record
all the tonal values of the scene and the lighter tones
have been clipped.
So if you review an image in the LCD screen with the
histogram and it shows this you know you have lost information. |
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However
in some situations where the subject has a great tonal
contrast range it is impossible to recorded all the
information an the information might fall of the graph
at both ends.
In this case you will need to consider what is the most
important aspect of the image. |
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