The Curves tool also allows you to make adjustments to the brightness and contrast (and colour balance) of your images. However, it offers a more flexible set of possibilities. The Levels tool has only 3 controllers to adjust blacks, whites and midtones whereas the Curves tool allows for the adjustment of any range of tones in your image. It is a more complex tool and harder to master.
Curves Tool Properties
Presets: it’s worth looking at these as they will show you some very typical examples.
On Image Tool: this is an eye dropper. Click on any area of the image and drag up to brighten, down to darken. As you drag, the curve will be redrawn to reflect the changes you’ve made. The range of tones affected is determined by the pixels under the dropper. For example, to adjust shadows, move the dropper over a dark area before dragging. You can repeat this several times to create multiple adjustments.
Channel: This defaults to RGB. The dropdowns give access to the three colour channels which can be edited individually.
Edit Points: With this selected you can add anchor points anywhere on the curve. These points can then be moved.
Draw Tool: This allows you to draw a freehand curve.
Smooth Tool: This is only available if you have drawn a freehand curve. Each time it is clicked it will smooth the curve further.
Black/White Sliders: These function in much the same way as the same sliders in the Levels tool. They can be used to move the start and end points of the curve to align with the histogram thereby increasing contrast.
Auto: creates a curve automatically
Set Black/White Points: Moving these has the same result as the black and white sliders.
Anchor Points: These are added when you click on the curve. To remove an anchor point, either drag it out of the window or select it and press Delete.
Histogram: This is a graphic representation of the distribution of tones in your image.
Input and Output Values: As you move an anchor point around, these values will change constantly. The input value shows the brightness of that position before the curve has been applied. The Output value shows the new value after the curve has been applied. The scale runs from 0 (pure black) to 255 (pure white). If the output value is larger then the range of pixels will be brighter and if it is smaller then the range will be darker.
Using the Curves Tool
To begin, create a curves adjustment layer.
Set White and Black Points. These can be set using either the Black and White Sliders or the anchor points at the bottom left (Black) and the top right (White). This step will not always be necessary as you may have already set these in Lightroom.
There are several ways in which you can create curves. I would recommend using ‘Edit Points’ to add anchor points. These can then be moved anywhere within the square window by dragging. You can also use the arrow keys to move anchor points more precisely. It’s also possible to enter precise values into the Input and Output boxes. The curve will redraw automatically to reflect any changes. Small movements of anchor points will generally produce better results.
The x-axis determines the range of tones to be adjusted. Black is on the left and white is on the right . The range you are adjusting will move from Shadows to Midtones to Highlights as you move from left to right.
The y-axis determines whether you are making the selected range brighter or darker. Drag anchor points up to brighten and down to darken.
The Input and Output boxes show the precise amount of adjustment at the selected anchor point. Pixels around the anchor point will also be affected but, as a smooth curve is created, the transition of tones will be very gradual and seamless.
The image below is an unprocessed RAW file. A Curves adjustment layer has been added. The straight diagonal line indicates that the curve has not been edited so nothing has changed in the image.
In the same image below, the Curves adjustment layer has been edited. An anchor point has been added at the bottom left (blue) and has been pulled down slightly. This results in the shadow areas becoming darker. An anchor point has been added at the top right (orange) and has been pulled up slightly. This results in the highlight areas becoming brighter. The overall effect has increased the contrast of the image. This is a commonly used curve and is referred to as an S-curve.
When we look at this s-curve in more detail, we can see what is happening. Bear in mind that brightness is shown on a scale from 0 (pure black) to 255 (pure white).
The anchor point towards the top right has been dragged up from the diagonal base line. The yellow line shows where this point is located on the input bar along the x-axis at the bottom. It has an Input value of 175 indicating that it’s in the highlights area of your image. The blue line shows where this point is located on the output bar along the y-axis on the left. It has an Output value of 200. This larger number tells us that the pixels around this anchor point have been lightened.
The anchor point towards the bottom left has been dragged down from the diagonal base line. The red line shows where this point is located on the input bar along the x-axis at the bottom. It has an Input value of 74 indicating that it’s in the shadows area of your image. The green line shows where this point is located on the output bar along the y-axis on the left. It has an Output value of 52. This smaller number tells us that the pixels around this anchor point have been darkened.