Curves

Curves

Curves are one of the most useful tools in Photoshop, it allows you to fine tune the contrast across a range of tones, in fact you can add up to 14 adjustment points to the graph.  The tool can be used in a variety of ways.  You can either adjust all three colours simultaneously or individually by selecting the appropriate channel from the drop down menu.  This is particularly useful for correcting colour casts.  

It can be used either as a destructive editing tool or in a non-destructive manner.  You should use the non destructive method as a preference.  The destructive method is accessed via Image - Adjustments - Curves.  This is only reversible while the file is open using the History palette.  Once the file has been closed the image is permanently altered - destructively edited.  Nondestructive editing is carried out via the layers window, as an adjustment layer.  From the half white, half black circle at the bottom of the layers window select curves.  You will se that this creates a curves adjustment layer which you can always return to, even after closing and reopening, as long as you do not flatten the image.  The following examples where carried out using this nondestructive method.

A curve graph resembles the characteristic curve graphs that manufacturers supplied for films, but initially the line is a straight diagonal.  The left side represents the dark tones and the right side, the light tones.  By adjusting the line you can control the tones represented in the image.

It is normal to start with the shadows.  For this image the line is pulled down slightly on the left until the dark areas are set.  This is like setting the dark point on levels

Now the top right of the line is lifted until the highlights are as desired.

Additional anchor points can be added along the line to adjust the various tones in between.  If you decide that an added point is wrong, it can be removed by simply dragging it off the side of the graph.

Before

After