Do you know what the Curves tool and a Swiss Army Knife have in common?
We all know what a Swiss Army Knife is - a popular outdoor gadget that looks like the fusion of a giant nail clipper and a bottle opener. When you hold it in your hand, it feels solid and trustworthy.
Going past the exterior, hidden within are multiple practical tools that you may need regularly or once in a blue moon. Although you find yourself hardly using some of the tools, you know if you ever need it, it is there and you can rely on it to get the job done.
Coming back to the question, the Curves tool is very much like a Swiss Army Knife. It is a popular tool in post-processing, certainly multipurpose and has some lesser known, albeit useful features that you may use it once in a while.
If you were to choose only ONE tool to post-process your image with, Curves is all you need!
In this tutorial, we are going to dive deep into the Curves tool. You are going to learn what Curves can do for you in addition to the basic adjustments you often do.
You can find Curves in almost all image editing software. Although I’m using Photoshop for the purpose of this tutorial, all the principles are applicable to other software.
Curves Properties Panel: A Walkthrough
If all you have been doing to date with Curves is moving that diagonal line into an S-shaped curve, I hope this tutorial will open your eyes. But first, let’s quickly go through all the clickable icons and dropdown menus in the properties panel.
For those of you who are familiar with this already, I hope we can still go through this together just so we are both on the same page when it comes to discussing the more advanced features of Curves.
(A) From The Top
Preset: Pretty much like all other presets, the dropdown menu contains nine settings that give you a variety of effects. Once chosen, you can still fine-tune the effect by manually adjusting the curve.
Color channel: This is the dropdown menu directly underneath Preset. By default, it’s set to RGB (red, green and blue channel). When you click on it, you can select any of the individual color channels.
Auto: To the right of color channel menu. Clicking this tells Photoshop to automatically apply a tone curve to your image for the best result.
(B) On The Left
There are seven icons from top to bottom. Sometimes you’ll see an eighth icon at the bottom which I’ll explain below. Starting from the top:
Targeted adjustment: This allows you to apply adjustment by clicking on the image. Once selected, move the cursor to an area of the image you want to adjust. Left click and hold, then drag it up and down to make it brighter or darker. You can see the adjustment “live” on the properties panel.
The three eyedroppers: From the top - dark, gray and white. This is to correct white balance but you don’t always have to use all three. Once selected, find an area on the image that corresponds to the tone of the eyedropper. E.g. click on the darkest area of the image with the black eyedropper.
Edit points: This is selected by default. You can click anywhere on the line of the curve to add points. You can then click and drag these points to apply adjustments.
Draw curve: Select this to draw a curve that you like. The result may not be the best but it’s certainly entertaining.
Smooth the curve: This becomes clickable only after you have drawn a curve. Each click on this icon smoothens the curve a little. Clicking it multiple times will smoothen the curve back to its default state.
(C) The Histogram
This is the graph in the middle with a straight line (by default) crossing diagonally from the lower left corner to the upper right corner. Histograms are a topic itself and have been thoroughly explained in this tutorial.
The two white points: One on each end of the curve (or the diagonal line). You can move these points anywhere within the graph.
The two arrows: One of each end of the X-axis. The left arrow is black and the right arrow is white. Moving these two compresses the tonal range of the image.
(D) The Bottom
Input: This is the value of the X-axis. It represents the tonal range and ranges from 0 to 255.
Output: This is the value of the Y-axis. It also has a value from 0 to 255.
(E) The Very Bottom
These are located at the lower border of the properties panel. Starting from the left:
Clip to layer: Clicking this will clip the Curves adjustment layer to the layer underneath it. This means any adjustment will only affect the bottom layer.
Preview previous state: This is basically a “before and after” preview. Clicking it will show you how the image looks before curve adjustment.
Reset: This resets everything in the Curves properties panel to its default settings.
Toggle layer visibility: Make the layer invisible or visible. Another way to compare the “before and after”.
Trash: Click this deletes the Curve adjustment layer.
(F) Properties Panel Menu
This is located on the top right-hand corner of the properties panel. There are a few options here that allow you to customize the display of the Curves panel.
One of the useful features is “Save Curves Preset”. You can save the adjustments you have applied to add to the number of presets from the Preset dropdown menu.
Basic Tonal Adjustment
Tonal (contrast) adjustment is what most of us use Curves for. Typically, an S-shaped curve is applied to the histogram by adding two points to the diagonal line and moving them apart (up and down) to create an S.
What this does is basically making the bright areas brighter and the dark areas darker. This widens the gap between the two and therefore increases contrast.
Did you know that by applying an S-shape in Curves to increase contrast, you inevitably increase saturation at the same time?
Increasing Contrast Without Affecting Saturation
Although Curves adjustment affects saturation, the effect is fairly minor in most cases. However, if the adjustment is pushed to the extreme or you’re applying multiple Curves adjustment layers, the image will end up looking oversaturated.
To circumvent this issue in Photoshop, change the blend mode of the Curves adjustment layer to Luminosity. Any degree of adjustment you apply to Curves now will only affect the contrast.
Color Adjustment With Blend Mode
Curves can also be used to apply color adjustments. While keeping the S-shape of the curve, experiment with Hue, Saturation and Color blend mode.
These blend modes work in slightly different ways (see blend mode explanation here) . Depending on the type of image you have, the results are difficult to predict. This is why you have to experiment with the effect.
Color Adjustment with Color Channels
Color Channels are massively underrated in my opinion. Unlike other color adjustment tools, the RGB color channels curves are very versatile and you can use it creatively to create images that are really unique.
Personally, I think it’s underutilized because there is so much flexibility with many possible outcomes. With our busy lives, we are often so focused to get things done that we prefer not to have too many options.
If you are willing to spend some time to explore, Color Channels in Curves might just be what you need to get your artistic mind kicking.
When you click the dropdown menu, you’ll see the individual RGB color channels. Selecting these will change the histogram to its respective colors. At first sight, this may look complicated but let me reassure you that it’s not once you understood how the color channels work.
As you know, RGB are the primary colors of light shown in the color wheel here. Look at the colors directly opposite red, green and blue (RGB) and you should notice these are cyan, magenta and yellow (CMY). It’s important to remember this (and shouldn’t be difficult to remember) because you’ll need this when adjusting the curve in color channels.
Let’s take the blue color channel for example. When you move the curve (the diagonal line) upwards, you are adding blue to the image. But when you move it downwards, you are adding yellow because the opposite of blue is yellow in the RGB color wheel. This goes to the same for other color channels.
I know it all looks very fiddly and you might be wondering what can you really do with this...
You can use this to correct for color cast, color grading or split toning (apply S-curve to a color channel). With the combination of layer masking, the application is only limited by your imagination.
Learn more: Practical Guide to Color Theory For Photographers
Apart from using the eyedropper tools to set white balance for your image, there is another way to do this more precisely.
After you have added a Curves adjustment layer, continue to add a Threshold adjustment layer. Drag the middle arrow in the Threshold adjustment layer all the way to the left until everything turns white. Now, slowly move the arrow towards the right until you see black spots appearing. Use the Color Sampler Tool to place a marker on one the black dots that appeared first.
When you have done that, move the arrow all the way to the right. Now move it back towards the left very slowly until you see white spots appearing. Use the Color Sampler Tool to place a marker on one of the white dots that appeared first.
Now that you have marked the pure black and the pure white, let’s find 50% grey. First, add a new layer between the Curves and Threshold adjustment layer, fill it with 50% grey and change the blend mode to Difference. Move the arrow in Threshold adjustment layer all the way to the left and slowly move it back to the right until you see black spots appearing. Like before, mark one of the spots with the Color Sampler Tool.
Go to the Curves adjustment layer and use the black, white and grey eyedropper tools to sample of the pure black, white and 50% grey you have marked.
Your image is now perfectly white balanced in the most technically precise way.
Dodging and Burning
Digital dodging and burning was adopted from darkroom and is one of the oldest image post-processing techniques we use today.
You’ll need two Curves adjustment layers, one for dodging and one for burning. When you have added two Curves adjustment layer, rename the layers to avoid confusion.
For the dodge layer, place a point in the middle of the curve and move it upwards until the image looks brighter (but not so bright to clip the highlights). Now add a black layer mask to conceal the adjustments. Do the same for the burn layer, but move the curve downwards until the image looks darker and add a black layer mask.
Select the Brush Tool, set Hardness to 0 and Opacity to 5% to begin with. Change the brush size according to the area you want to dodge or burn. If you to dodge, paint on the layer mask of the dodge layer and vice versa.
You can use this method to dodge and burn with luminosity masks. Find out more in the Luminosity Masks resource page under Articles in the top menu.
The power of Curves is incomplete without mentioning LAB colorspace - a colorspace that works closely with Curves tool.
Perhaps the most useful resource I have come across on LAB colorspace is the books written by Dan Margulis, which you can find them on Amazon. LAB colorspace is a topic on its own and it’s impossible to cover it in a single article.
Having said that, let me give you a classic example of what LAB colorspace can do for you - changing color. The images below show you how the color of the car has changed from yellow to blue.
Ordinarily, you would have to create a selection of the car, mask out the rest of the image and change the color using a tool of your selection in Photoshop. Creating the selection would probably be the most time consuming as you have to exclude the windows, car plate, etc.
With LAB colorspace, I did this in under 30 seconds.
First, I changed the working color space to LAB. Then, I select the Curves adjustment tool and in the Properties panel, click the top right hand corner to select Menu > Curves Displays Options. In “Show Amount of”, select Pigment instead of Light. This inverts the tonal range on the X and the Y-axes.
In the Curves panel, use the dropdown menu to select “a”, invert the diagonal line completely upside down. Do the same for “b”. Use layer mask to mask out unwanted areas that are affected by the change. That’s it...it literally takes less than 30 seconds!
Don't Underestimate Curves
Curves is one of the most powerful tools in Photoshop.
If I haven't convinced you, I hope I've at least made you consider giving it a go!
Like this tutorial? Do check out more articles related to image post-processing in the editing technique resource page!