Photoshop Curves
Adjustments

 

As we read earlier, we modify the curve by using anchors or the free form pencil.  By far, anchors are used more often.  Once the anchors have been created, we reposition them in order to reshape the curve.  We can reposition an anchor one of three ways.

• Use the mouse to drag an anchor to a new position.
• Activate a specific anchor by Shift - clicking it and then use the Up/Down or Left/Right arrow keys.
• Activate a specific anchor and then click in the Input/Output boxes and type new values or use the Up/Down arrow keys to change the value.

When we make a change, the curve will automatically reshape itself accordingly.

 

How Curves Works

The Curves dialog box shows the input digital tonal range on the X axis and the output, or modified, digital tonal range on the Y axis.  The curve default is a straight line because Adobe Photoshop maps input tones 1:1 to output tones.  By reshaping the curve we are remapping the input tones to new output tones.  Using Figure 1 as an example, one of the anchor points has been dramatically lowered.  The red arrows in Figure 1 show both the before and after tones.  The vertical arrow coming from the X axis shows this anchor was originally a light tone.  The horizontal arrow touching the Y axis shows us the new tone it has been remapped to, which is a darker tone.
 

Finding A Tone

One of the nice features of the Curves control is its ability to show us where a tone in our image is on the curve.  With the Curves dialog box open, mouse over the image while holding down the left mouse button.  As we do this, a bouncing ball will move along the curve.  The ball is showing us the tone of the area the cursor is currently over.
 

Flattening The Curve

Generally speaking, when the curve is flattened (made more horizontal), the full input tonal range is being compressed into a smaller output tonal range.  Shadow areas (black and dark tones) will become lighter and highlight areas (white and light tones) will become darker.  The flattening of the curve causes a reduction in the contrast of an image because the two extreme ends of the output tonal range are being forced closer together.  The input tonal range remains 0 to 255.  But by flattening the curve, the output tonal range is some number higher than 0 and a number lower than 255.  An example of a flattened curve can be seen in Figure 2.  The red line shows the position of the original, unadjusted curve.
 

steepening The Curve

Figure 3 shows a steepened curve.  On the adjusted curve, the darker tones that are flat lined on the bottom left of the curve are all being mapped to solid black (output tone 0).  The lighter tones that are flat lined on the upper right of the curve are all being mapped to solid white (output tone 255).  All the remaining tones on the new curve are being spread out.  Specifically, any tone that is below the point of intersection between the adjusted curve and the red line is being mapped to a tone that is darker than itself.  Any tone that is above the point of intersection is being mapped to a tone that is lighter than itself.  The steeping of the curve causes an increase in the contrast of an image because some tones are becoming darker while others are becoming lighter.

There is an almost infinite number of ways that a curve can be shaped.  In Table 2 below, most of the common shapes are shown.  Review Table 2 to learn about these shapes and how they affect an image.  Table 2 gives the equivalent Levels adjustment, when there is one.  The red line in each of the dialog boxes in Table 2 shows the position of the default curve.

To see examples of how the Curves control affects an image, be sure to Take the Test.

 

 

Table 1.  Curve Adjustments

Red Line Represents Default Curve	Curves Action	Affect	Equivalent Levels Adjustment
	Example 1. Place an anchor on tone 127 and move it horizontally to the right.	Darkens overall image. The effect tapers off as it approaches white and black.	Move gamma slider to the right.
	Example 2. Place an anchor on tone 127 and move it horizontally to the left.	Lightens overall image. The effect tapers off as it approaches white and black.	Move gamma slider to the left.
	Example 3. Lower white anchor, raise black anchor.	Lowers contrast. Removes solid white and black. The effect tapers off as it approaches mid tone. The more the curve approaches horizontal, the more contrast decreases.	Move both Output Levels black and white sliders inward.  Example 3 more accurately replicates the Levels adjustment than Example 11.
	Example 4. Move white anchor to the left, move black anchor to the right.	Increases contrast. Increases the number of solid whites and solid blacks. The effect tapers off as it approaches mid tone. The more the curve approaches vertical, the more contrast increases.	Move Input Levels white and black sliders inward.  Many photographers consider Levels the better control for this kind of adjustment because of its clipping preview feature.  Read the Input Levels Clipping Preview page for more information.
	Example 5. Raise black anchor only	Lightens shadows. The effect tapers off as it approaches white.	Move Output Levels black slider inward.
	Example 6. Move black anchor to the right	Darkens shadows. The effect tapers off as it approaches white.	Move Input Levels black slider inward.
	Example 7. Move white anchor to the left	Lightens highlights. The effect tapers off as it approaches black.	Move Input Levels white slider inward.
	Example 8. Lower white anchor.	Darkens highlights. The effect tapers off as it approaches black.	Move Output Levels white slider inward.
	Example 9. Place multiple anchors on the curve and only move the white anchor down.	Darkens highlights.  However, the tapering effect is eliminated because only highlights are darkened and the remaining curve is unchanged. Compare to previous adjustment.	None.  This is an example of the control the Curves command gives that we cannot replicate with Levels.  The closest we can come is to move the white Output Levels slider inward.
	Example 10. Place anchors somewhere in Zones III and VIII. Raise Zone VII anchor, lower Zone III anchor. This is known as the S curve.	Increases contrast in mid tone areas because in these areas the curve is getting steeper. Lightens highlights and darkens shadows.  However, contrast is decreased in these areas because the curve is getting flatter.	Move the Input Levels white and black slider inward. Also move the Output Levels white and black sliders inward but not as much. If we move the Output Levels sliders the same as the Input Levels sliders the two adjustments will cancel each other out.
	Example 11. Move black point both horizontally and vertically.  Move white point both horizontally and vertically.	Slight decrease in contrast.  Blacks and very dark shadows are converted to a single tone, which will be slightly lighter than originally.  Whites and very light shadows are converted to a single tone, which will be slightly darker than originally.  Middle tones are unchanged.	Moving both Output Levels black and white sliders toward the center will approximate this adjustment.  See Example 3.
	Example 12. For Input value 0, change it to 255. For Input value 255, change it to 0.	Reverses tones and colors. In graphing terminology, the default red curve has a positive slope while the adjusted curve has a negative slope.	Move Output Levels black slider to the far right. Move Output Levels white slider to the far left.
	Example 13. This is an example of using the freeform mode and drawing a curve using the pencil.	This particular curve will create distinct bands of tonality in the image, often called posterization.	None
	Example 14. This is an extreme example of how anchors can be used to reshape the curve.	The parts of the curve that have a negative slope will cause reverse colors and tones in the image. The parts of the curve that have a positive slope will increase contrast because the slope is steeper than the default curve. The default curve in red has a positive slope of 1:1.	None
Create color sampler target in the image	Up to four color sampler targets can be added to an image.  To add a target while the Curves dialog box is open, turn Caps Lock on, mouse to the desired location in the image, and Shift - click.  Caps Lock changes the cursor from an eyedropper cursor to a crosshair , which gives a little more precision in creating a target.  Turning on Caps Lock is not mandatory.	Color Sampler targets do not affect the image.  They place information in the Info panel that can be used to monitor changes to the image.	Same
Delete a color sampler target	To delete a target while the Curves dialog box is open, mouse to the target, hold down Alt + Shift (Option + Shift) until the cursor turns into a pair of scissors and then click to delete.	n/a	Same
Move a color sampler target	To move a target while the Curves dialog box is open, mouse to the target, hold down the Shift key until it becomes a black arrowhead, then drag the target to the new location.	n/a	Same