Curves or as they are also commonly known as an Adjustment Curve or Adjustment Curves are a powerful way of making tonal and brightness adjustments. They can appear daunting, but with a little practice they are an extremely intuitive way of editing your images. To help you get up to speed, I developed the 6 principles of curves system.
1st Principle: Brightness & Tone
The horizontal axis of the Adjustment Curve represents Tones whereby the left side of the axis is the darkest Tones, through to the right side of the axis which represents the lightest Tones. The Tone axis is given values of 0-255.
The vertical axis of the Adjustment Curve represents brightness whereby the bottom of the axis represents the lowest brightness level and the top of the axis represents the highest brightness level. The brightness axis is given values of 0-255.
2nd Principle: The Diagonal Effect
The diagonal line across the Adjustment Curve represents the darkest Tone and lowest brightness at the bottom-left, and the lightest Tone and highest brightness at the top-right of the line.
The two ends of the scale represent respectively two ceilings whereby at the bottom-left of the Adjustment Curve it is not possible to become any darker or any less bright. In the visible spectrum this is the blackest black. At the top-right of the Adjustment Curve it is not possible to become any lighter or brighter. In the visible spectrum this is the whitest white.
3rd Principle: Tonal Split
Having ascertained that the axis of the Adjustment Curve represent the visible light spectrum, it’s possible to determine that the segments on the x-axis represents respectively Shadows, Dark Tones, Light Tones, and then Highlights.
It’s also possible to ascertain that the y-axis represents the brightness of the Shadows, Dark Tones, Light Tones and then Highlights.
4th Principle: The Histogram
The peaks and troughs in the Adjustment Curve are called the Histogram. Since we now know that the Histogram is made up of Shadows, Darks Tones, Lights Tones, & Highlights, we can ascertain that the peaks and troughs in the Histogram represent peaks and troughs in the brightness of those tones.
5th Principle: Selective Brightness Adjustments
If we were to grab the Tones at point A on this Histogram curve, we would be grabbing the Shadows Tones. Now, if we pushed point A vertically upwards, we would be increasing the brightness of the Shadows. Conversely pushing point A vertically downwards would decrease the brightness of the Shadows. Following the same logic, the same is true at points B (Dark Tones), C (Light Tones), and point D (Highlights).
6th Principle: Opposites Attract
When we adjust any of the Red (R), Green (G), or Blue (B) – RBG channels in the Adjustment Curves panel, what we are doing is something analogous to mixing paint. This is because the Red channel is made up of a mix of two different colors, and these are Red (R) & Cyan (C). Likewise, the Green (G) channel is made up of a mix of Green (G) & Magenta (M), and the Blue Channel is a mix of Blue (B) and Yellow (Y).
What this means, is that if you selected say the Blue channel, picked a point on the Tone Curve and pushed it upwards towards the darker blue tones, the resulting tonal balance of your image would become Bluer but also less Yellow. Conversely pulling the point on the Tone Curve downwards would result in less Blue & more Yellow. Using this principle of opposites, you can readily correct color balance issues in your images.
USE CURVES TO BRIGHTEN OR DARKEN AN IMAGE
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a new Curves Adjustment Layer to your project and opens a Curves Properties Panel.
STEP 2: Working from the Curves Properties Panel, place a single point on the diagonal line of the Tone Curve (by clicking on the line), then push the point up on the vertical line. The overall brightness of all tones increases. The opposite would be true if we were to push the line down thus reducing the brightness of all tones in the image.
USE CURVES TO SELECTIVELY BRIGHTEN OR DARKEN SEGMENTS OF AN IMAGE
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a new Curves Adjustment Layer to your project and opens a Curves Properties Panel.
STEP 2: By fixing anchor points on the diagonal line of the Tone Curve, and then adding an additional point along the line for tone adjustment, the brightness of individual zones can be controlled. To add points, simply click on the diagonal line. In this example, the Lights & Highlights increase in Brightness, but the Shadows & Darks are not impacted by the change since they are anchored.
USE CURVES TO BOOST CONTRAST
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a curves adjustment layer to your image, and opens a curves properties panel.
STEP 2: Click on the diagonal line to create two adjustment points, one in the Lights & Highlights zone and the other in the Shadows & Darks. Push the upper point vertically upwards, and the lower point vertically downwards. This creates what is termed as an S-Curve. Essentially what it does is boost the overall contrast of by compressing the tonal range of your image.
USE CURVES TO FLATTEN AN IMAGE
If you are looking for that retro bleach processed/flattened appearance to your image, then it is straightforward to achieve using curves, you restrict the brightness range without changing the tonal range.
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a new Curves Adjustment Layer to your project and opens a Curves Properties Panel.
STEP 2: Click on the bottom point on the diagonal line. Push this point vertically upwards to restrict the brightness range and flatten the image.
USE CURVES TO AUTOMATICALLY MAKE COLOR & TONAL CORRECTIONS
There are a couple of ways to do this in Photoshop, one is completely automatic where Photoshop processes your image for you, the other I would class as semi-automatic; where you make targeted corrections.
METHOD 1: Automatic Corrections
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a new Curves Adjustment Layer to your image and opens a Curves Properties Panel.
STEP 2: In the Layers Properties panel, click the Auto button. Your image is now tonally balanced.
METHOD 2: Semi-Automatic (Targeted Corrections)
STEP 1: Working from the Layers Tab, add a Curves adjustment as per Step 1 of Method 1. Using the same example image as for Method 1, let’s say we wanted to make the image warmer. Using the 6th Principle from the Chapter introduction, we know that by reducing the blues we will introduce more yellows; which will warm the image.
STEP 2: From the Curves Properties Panel select the Blue channel from the RGB pull-down menu illustrated right.
STEP 3: Now select the icon on the left side of the Properties Panel named “Click & Drag The Image To Modify The Curve”. For the avoidance of confusion, the icon looks like this:
Click and drag the cursor on your image in the Document Window, targeting the specific color you want to adjust. Dragging upwards increases the color intensity (in this case becoming bluer), and dragging downwards reduces the color intensity.
USE CURVES TO CORRECT COLOR BALANCE
STEP 1: Working from the Layers Tab, head to the bottom of the panel area and select ‘Create New Fill or Adjustment Layer’. Then, from the options presented, select the ‘Curves…’. This applies a curves adjustment layer to your image and opens a curves properties panel.
STEP 2: On the left-hand side of the curves panel you will see three pipette icons. Working from the RGB channel, select the uppermost pipette and use it to sample an area from your image that represents the Black Tones. Now use the center pipette for Mid-tones and the bottom one for Whites. With Whites, try to select something that is just off-white rather than pure white, as this gives a truer correction. As you make these corrections, you will notice Photoshop adds an RGB overlay representation on the Histogram.
HOW TO RESET THE CURVES ADJUSTMENT LAYER
If you ever need to completely reset a Curves Adjustment back to its default values, you simply have to select the Reset icon at the bottom-right of the Curves Properties Panel. It’s the icon highlighted above.
USE CURVES TO COLOR GRADE COMPOSITE IMAGES
This simple technique using Curves will have your composites seamlessly color matched in no time! In this composite image, an astronaut has been superimposed onto an image of the planet Jupiter, and then the color is matched using Curves; whereby you tonally match the colors of your foreground object to your background.
STEP 1: With the images you want to color grade applied on separate Layers, head over to the Layers Tab, select ‘Create New Fill or Adjustment Layer’, and select ‘Curves’. This adds a Curves adjustment Layer but also opens up the Curves Properties Panel. Before making any adjustments though, you need to clip the Curves Adjustment to the foreground element (in this case the astronaut Layer).
Hover on the line between the foreground and background Layers whilst pressing the Option (on a Mac) or Alt (on a PC), and a little link icon appears. When you see it, click it, and the Curves Adjustments clips into the foreground Layer only.
NOTE: You can also achieve the same thing by selecting the Curves Adjustment Layer, then heading up to the Application Bar and ‘Layer > Create Clipping Mask’.
STEP 2: In the Curves Adjustment Properties Panel are a set of three pipettes. The top one represents Blacks, the middle one represents Mid-Tones, and the bottom one Whites.
Select the top pipette, and select a point on the Background Layer that represents the darkest part of the image.
Likewise, use the middle pipette to select a point on the Background Layer that represents the Mid-Tones and the bottom pipette for Whites.
As you do this, you will see an RBG representation of the change that Photoshop has done. If you want to change these manually you can do so in the individual Red, Green, and Blue Channels; by selecting them from the drop-down menu.
