An increasing number of
Digital cameras of all types can show Histogram's of the images that
have been taken. Some can even show a Histogram of a scene prior to
a shot. This can be very useful if you know what a Histogram is, and
how to understand what viewing one tells you about the image. But
it's important not to over estimate their importance. They can be a
useful guide, but no more.
A Histogram is a
graphical illustration of the tones in an image, and how they are
distributed. Here is one. This is the 'Levels' command in Photoshop,
which displays a Histogram of the image with sliders so the image
parameters can be adjusted if needed.
Histogram
A
The
horizontal axis represents the 256 shades ranging from pure Black
through the mid-tones to pure White, which combined, make up a
continuous tone image. Black is at the extreme left edge (0) and
White at the extreme right edge (255). The vertical axis represents
the number of pixels of each shade. As you can see in the image this
Histogram represents, the tones are evenly distributed, with the
highest number being in the middle. There are many that will tell
you that this is the 'ideal' Histogram and represents what you
should aim to achieve. This is wrong. All this tells you is that
this is an image which has an fairly even distribution of tones
ranging from black to white, with no particular shade dominating the
scene.
Here
is that image. As a matter of interest it's of Shakespeare's house
in the centre of Stratford-on-Avon.
Here's another
Histogram. Looking at this compared to Histogram A you would be
tempted to think that the exposure for this image is wrong, because
the tones are all at the dark end of the range.
Histogram B
This one would also
appear to be wrong for the opposite reason, it's all at the light
end of the range.
Histogram C
So lets now take a
look at the images the Histograms B and C relate to.
Here is the one
for Histogram B. As you can now see this is a shot of a floodlight
church taken around dusk so there is some colour in the sky. The
histogram shows that the tones are predominantly at the dark end of
the spectrum, with only a few light shades, which is what you would
expect from looking at the image.
Histogram B's image
Now lets look at
Histogram C's image. Here we have the complete opposite, an image
composed of mostly light shades, as again the histogram shows. A
Flag, the Union Jack in this case, against a background of a cloudy
sky.
Histogram C's image
As we hope you can now
see, looking at a histogram must be done in conjunction with the
image it relates to, or all meaning is lost. Not every image will
have an equal number of shades evenly distributed, the important
point to bear in mind is that the exposure of each image is correct
for the subject being taken, and the tonal range covers all the
tones in the shot, irrespective of the number of pixels of each
shade.
On
the next page we'll show you how to make adjustments using the
levels command in conjunction with a histogram
