The Levels dialog box shows us a histogram so we can clearly see where our image's tones are when plotted against the digital tonal scale. In Levels, we use the histogram as a reference. The Curves dialog box does not show us a histogram. In fact, the Curves dialog box does not show us our image's tones at all. The Curves dialog box shows us two gradients in the form of a grid. One gradient represents the digital tonal range before the adjustment and the other represents the digital tonal range after the adjustment.
Figure 1. Curves Dialog Box
When we first create a Curves adjustment layer, the dialog box will look similar to Figure 1. In the Curves dialog box, there are four ways to make adjustments: the curve (A) in combination with anchor points (B, B1 and B2), the Input and Output boxes (C), the freehand drawing tool (D) and the eyedropper tools (F).
Grid
The predominant feature of the Curves dialog box is a chart with a straight line going from the bottom left to the upper right. The straight line is labeled A in Figure 1 and it is the curve. This chart is an interactive chart (in Photoshop, not on this web page). We reshape the line in the chart to make our adjustments.
Like most charts, the Curves chart has a X and Y axis, which are labeled as such in Figure 1. The X axis is the input or source tonal range. We can think of it as the 'before' status. Like a histogram, it goes from solid black on the left to solid white on the right. Also like the histogram, black is represented by 0 and white by 255. The Y axis is the output or destination tonal range. We can think of it as the 'after' status. If we look closely at the X and Y axis, we can see there is a miniature tonal scale gradient going from solid black to solid white.
The Curves chart is mapping all 256 input tones to output tones. When a Curves adjustment layer is first created the curve is straight because Photoshop defaults to mapping the input tones 1:1 to the output tones. When we reshape the curve, we are changing what input tones get mapped to which output tones. When manipulating the curve, we are required to account for all 256 input tones. However, we are not required to use all 256 output tones. In fact, we can map all 256 input tones to a single output tone.
Input/Output Boxes
Below the grid are two numeric boxes called Input and Output. These boxes are labeled C in Figure 1. Once we place an anchor point on the curve, these two boxes become active and show the before (Input) tonal value and the after (Output) tonal value. Once the Input and Output boxes are active, we can click in these boxes and make changes by either typing new numeric values or pressing the Up/Down arrow keys to change the existing value.
Clicking in the Input box allows us to change the placement of the anchor point horizontally in the grid. This changes the Input tone being adjusted. Clicking in the Output box allows us to change the placement of the anchor point vertically in the grid. This changes what Output tone the Input tone is being mapped to. The Input box specifies what tone is to be changed. The Output box specifies what tone it is being changed to. As we reposition the anchor point, it 'drags' the curve with it. It is the repositioning of the anchor point(s) within the grid that forces the reshaping of the curve, thus changing the tonal values in the image.
Before
After
Figure 2. Reshape the curve by moving anchor points
On the curve, Photoshop automatically places an anchor on solid black and solid white. These anchor points are labeled B1 and B2 respectfully in Figure 1. Solid black is in the lower left corner. Solid white is in the upper right. The other anchor labeled B is an anchor I manually added by clicking on the curve. This anchor represents tone 90. How do we know it is 90? Because the Input box, labeled C in Figure 1, tells us the tonal value of whatever anchor is active. We can click in the Input box and change this value. When we do, the anchor will automatically reposition itself on the curve. If we leave Input to 90 and click in the Output box and change the output value to 115, this will cause the curve to automatically reshape itself. Mouse over Figure 2 to see the reshaped curve. By reshaping the curve, we are remapping input tones to new output tones. In this specific example, we are remapping input tones to lighter tones because tone 115 is lighter than tone 90. In fact, in this example, we shifted this part of the image by one full f-stop.
Eyedropper
The eyedropper controls, labeled F in Figure 1, are used to set the black, mid and/or white points of the tonal scale to specific areas in the image.
Freeform Curve
Part D of Figure 1 shows two icons. The pencil icon turns the Curves cursor into a pencil where we can free hand draw our own curve in the grid. The curve icon takes us out of free form mode and back into the default mode of reshaping the curve using anchors. The Smooth button is only active when in free form mode. After hand drawing a curve, it can be smoothed by clicking this button. The more we click the button, the more the curve will be smoothed. Click it enough and the curve will become straight.
Buttons
Like Levels, we can save an adjustment. Curves adjustments are saved as a Photoshop *.acv file using the Save button. The Load button is used to load the adjustment.
TipThere are a number of predefined Curves adjustments I have created and make available on the Downloads page.
To reset the curve to the Photoshop default, hold down the Alt (Option) key and the Cancel button becomes a Reset button. Click while still holding down the Alt (Option) button to reset the curve to a straight line.
The Auto and Options buttons are used to set up and apply automatic settings, which are not discussed on this web site.
Channel Box
The Channel drop down box at the top of the dialog box allows us to work on the full image (RGB) or just individual color channels.
Preview Check Box
The preview check box allows us to see our changes in real time. When this box is checked, the impact of our changes can be readily seen in the image. When this box is not checked, we will not see the impact until we click OK. This check box is most useful when we want to see a before/after affect while the dialog box is open. This is accomplished by clicking the check box on and off.
Customizing The Curves Dialog Box
Clicking the scale button, which is labeled E in Figure 1, changes the size of the dialog box between large and small. Which size of the dialog box to use depends on whether or not there is enough space on your monitor for both the document window and the large dialog box.
By Alt + clicking (Option + clicking) in the grid, the grid dimensions toggle between 4x4 and 10x10. Figure 1 shows a 10x10 grid. Most photographers find the 10x10 grid easier to work with.
Numeric Values Or % Black
The double headed arrow in the middle of the gradient on the X axis in Figure 1 changes both the orientation of the gradient and the unit of measure. For RGB images, the default gradient for the X axis is black on the left and white on the right. For the Y axis, it is black on the bottom and white on the top. For RGB images, the default unit of measure is the numeric tonal range with 0 for black and 255 for white. For CMYK images, the default gradient for the X axis is white on the left and black on the right. For the Y axis it is white on the bottom and black on the top. For CMYK images, the default unit of measure is percent black with 0% for white and 100% for black.
By clicking the double headed arrow, Curves will switch between these two defaults. Rest assured that Photoshop will not convert the image to/from RGB/CMYK while clicking the double headed arrow. The image will remain in the color model it was set to. Photoshop only changes how the Curves dialog box displays information. Most photographers prefer the RGB default of displaying numeric tonal values.
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