The Curves control found in image-manipulation software may
remind some photographers of the characteristic curves published for
films. This is a graph that shows the density of sensitized material
that will result from being exposed to specific intensities of light. In
feet, there are similarities, but equally important are the
differences. Both are effectively transfer functions: they describe how
one variable (the input either of colour value or of the amount of
light) produces another variable (the output or density of silver in the
image).
The
principal difference is that when it comes to image-manipulation
software the curve is always a 45° line at the beginning. This shows
that the output is exactly the same as the input. However, unlike the
film curve, you can manipulate this directly by clicking on and dragging
the curve or by redrawing the curve yourself. In this way, you force
light tones to become dark, mid-tones to become light, and with all the
other variations in between. In addition, you may change the curve of
each colour channel separately.
What the Curves control does
The Curves control is a very powerful tool indeed and can produce visual results that are impossible to achieve in any other way. By employing less extreme curves you can improve tones in, for example, the shadow areas while leaving the mid-tones and highlights as they were originally recorded. And by altering curves separately by colour channel, you have unprecedented control over an image's colour balance. More importantly, the colour changes brought about via the Curves control can be so smooth that the new colours blend seamlessly with those of the original.
The Curves control is a very powerful tool indeed and can produce visual results that are impossible to achieve in any other way. By employing less extreme curves you can improve tones in, for example, the shadow areas while leaving the mid-tones and highlights as they were originally recorded. And by altering curves separately by colour channel, you have unprecedented control over an image's colour balance. More importantly, the colour changes brought about via the Curves control can be so smooth that the new colours blend seamlessly with those of the original.
In
general, the subjects that react best to the application of extreme
Curve settings are those with simple outlines and large, obvious
features. You can try endless experiments with Curves, particularly if
you start using different curve shapes in each channel. The following
examples show the scope of applying simpler modifications to the master
curve, thereby changing all channels at the same time.
