Of all the tools within Photoshop, the one I use most often, and the one I could least do without, is the Curves tool. It can dramatically enhance the contrast of an otherwise dull image; it can change the colour balance, either subtly or more radically; it can increase the saturation; and it can even convert an image to black and white (when used with LAB color mode). In short, it’s an extremely powerful tool with a wide range of applications – but it’s also quite difficult to understand, at least at first.
In this article I'll discuss some of the basic concepts that underpin the use of this tool with reference to a range of practical examples.
The topics covered include:
- Understanding tonal range
- Introducing the Curves Tool
- Setting the White and Black Points
- Modifying the Midtones
- Basic S-Curves
- How to Evaluate an Image Using the Eyedropper Tool
- Luminosity Blend Mode and Saturation
- Conclusion / Further Topics
By the end you will have developed a good understanding of tonal range, and will be able to use the Curves tool to manipulate the tonal range of an image to meet your creative expectations.
I used Photoshop CC for the screengrabs, so if you're using an earlier version your tools may look different. Don't worry, all of the techniques I discuss here can be used with pretty much any version of Photoshop.
[Updated and partially rewritten, June 2017]
Understanding Tonal Range
Tonal range, at its most simple, refers to the range of tones between the lightest and darkest areas of an image. For example, an image with a wide tonal range will include both dark and light areas (and a range of tones in between), whereas an image with a narrow tonal range will cover a more restricted range; i.e it may be predominantly composed of mid-tones.
The following two images illustrate this point. The first, as you can see, has a restricted tonal range – most of the tones are bunched in highlights, hence it's dull, flat appearance.
The second image has been adjusted using a Curve, and as you can see from its histogram, the tonal range is much wider, greatly improving the image.
At this stage it’s worth spending a bit of time making sure we understand exactly what a histogram is telling us as it’s an especially useful way of evaluating an image prior to making any changes.
We can either use Photoshop’s histogram to do this (Window → Histogram), or we can use the Levels tool. To use the Levels tool you need to either create a new adjustment layer – Layers → New Adjustment Layer → Levels, or invoke the tool via the image menu, Image → Adjustments → Levels.
It's better to use adjustment layers rather than applying any change via the Image → Adjustments menu as changes applied in this way are destructive - they alter then content of your image layer. Adjustment layers create the same effects, but do so in a non-destructive way (your image layer isn't changed).
After you create an adjustment layer you can adjust its settings in the Properties palette (see below). If the palette isn't visible, select it from the Window menu: Window → Properties.
For the orginal, uncorrected image with a narrow tonal range, we can see that the majority of the tones are bunched towards the right of the histogram (i.e. between the midtones and the highlights).
What you will also notice is that different sections of the histogram vary in height. The height at any given point indicates the number of pixels at a specific level, so, in this instance, most of the pixels are in the highlights – but there are a some darker and lighter pixels as indicated by the dark line along the bottom.
So how do we alter an image using the Levels tool? The easiest way is move the triangular shaped sliders immediately below the histogram. You will notice that there are three of these: a black one on the left, a grey one in the middle, and a white one on the right.
As you move these you will notice that the values in the boxes below the histogram change. For example, if you drag the leftmost slider to the right you will see that the leftmost input value changes (88, in this example). This alters the 'black point'. The white slider alters the 'white point' and the grey slider varies the brightness of the midtones.
These changes expand the tonal range of an image. In this case, we moved the black point slider to 88. This tells Photoshop to display any pixel with a brightness value of 88 or below as black (0, in Photoshop speak). By moving the white slider to 225 we instructed Photoshop to display any pixel with a brightness value of 225 or above as white (255). These changes remap the tonal range: dark mid-tone values (88 or below) are now much darker, while the reasonably brigh hightlight (225 or above) are now rendered as white.
Put another way, by making these changes we 'stretched' the histogram ...
So, that's the Level's tool. It provides an easy way to expand the tonal range of an image but, as you'll see below, it's by no means as powerful or flexible as the Curves tool.
Introducing The Curves Tool
In this section we're going to take a look at the Curves tool with reference to an image I shot a few years ago in the desert near Dubai. It's similar to the previous image, insofar as it has a relatively narrow tonal range, but in this case this case the majority of the tones are bunched in the midtones and brighter shadows rather than the highlights.
As with the previous image, it's rather dull and flat, so let's take a look at the Curves tool and how we can use it to enhance this image.
The Curves tool, like the Levels tool, allows us to make simple adjustments to the tonal range of an image, but unlike the Levels tool – where you only have control over the black point, white point, and midtones – the Curves tool provides a much higher level of control, not least because it allows you to manipulate an additional 14 anchor points that may be positioned anywhere within the original image’s tonal range.
Before explaining this in any more detail though, it’s worth spending a little bit of time familiarizing ourselves with the basics.
To use the Curves tool you need to either select it from the Image menu (Image → Adjustments → Curves) or create a Curves adjustment layer (Layer → New Adjustment Layer → Curves). As mentioned previously, the latter is a better method.
Setting The White & Black Points
Earlier we discussed how to use the Levels tools to set the white and black points of an image to increase its tonal range. The same transformation can easily be applied using the Curves tool, and again, as you can see below, this greatly improves the image.
As with the Levels dialog, the Curves dialog also provides numerical feedback on the changes you make. In this example you can see that the black point anchor is selected (it’s a solid black box rather than an outline) and the black point is set to 60 (i.e. any pixel at an original brightness value of 60 and below is now black).
Quick Tip ...
Before moving either the black point or white point slider hold down the alt key first. When you do this before moving the white point slider the image will turn white, and black when moving the black point slider.
As you drag the slider the screen will remain black (or white) until you begin to clip the data, i.e. you start to blow out the highlights or block the shadows. If you want to avoid this, move the slider back a fraction.
Steepness and contrast
At this point it's also worth noting that the steepness of a curve determines the amount of contrast it adds. The steeper the curve, the greater the contrast, while a shallow curve decreases contrast. This is something we'll return to below when we begin our discussion of S-Curves. For now, just bear it in mind.
Modifying the Midtones with the Curves Tool
We can also use the Curves tool to alter the mid-tones of an image. With the Levels tool this was a simple matter of dragging the mid-tones slider (to the left to lighten an image, and to the right to darken it), but with the Curves tool it’s marginally more complicated insofar as we need to add an anchor point to control this area of the tonal range.
To add an anchor point at any position on a Curve you simply need to click the position on the Curve where you want to insert it. To control the mid-tones, this needs to be mid-way between the black point and white point, in this case almost exactly in the middle of the grid.
At this stage this has no effect, as it hasn’t altered the shape of the Curve in any way, but if I drag this point towards the top-left corner of the dialog it will increase the brightness of the mid-tones. Conversely, if I drag it down towards the bottom-right corner, it will decrease their brightness. The further you drag the anchor point, the greater the effect.
Tip: Change the Color Theme
Switch the Photoshop color theme to one of the lighter shades when practicing with the Curves Tool. The darker themes are great, but the curve is much easier to see against a lighter background.
Resetting The Curve
Often, when working with the Curves tool dialog, you may find that you have set numerous control points, on both the RGB Curve, as we've been discussing here, or on the red, green and blue channel Curves, as discussed below.
If you decide that you’re not happy with the Curve you have constructed you can delete the control points, by selecting them and then deleting them with the delete key, or you can hit the Reset button at the bottom of the properties palette.
Creating S-Curves with the Curves Tool
So far, we’ve used the Curves tool to adjust the white point, black point and midtones of an image, but this isn't anything we couldn’t have achieved with the Levels tool; using the black, point white point and mid-tone sliders. With the Curves tool we have much more flexibility in how we adjust the tonal range of an image, not least because you can add an additional 14 control points to the Curve. You can add these simply by clicking the Curve, and once they’re added they can be repositioned anywhere within the grid.
One of the most useful Curves is what’s typically referred to as an S-Curve (see right). As you can see, two control points have been added to this Curve and it forms the approximate shape of the letter ‘S’. The upper control point lightens the highlights while the lower point darkens the shadows. In this instance, the absolute midtone value (128) remains unchanged.
If you're ever unsure about exactly what effect a curve, or section of a curve might have on an image, take a look at the curve in relation to the baseline (see right): above the baseline (or towards the top-left of the properties palette) increases brightness, while moving the curve below the baseline (towards the bottom-right) decreases brightness.
For many images a 'standard' S-Curve is a good place to start – brighten the highlights, deepen the shadows – and you're good to go. And as you can see, it works reasonably well with my shot from the desert.
In this case though I wanted a more dramatic effect. As with the previous version, this was created using an S-Curve, but this time the points are placed differently, i.e. the highlights are only marginally brighter, whereas the midtones and shadows are much darker.
With experience, constructing a Curve to enhance an image is something you will be able to do intuitively, most of the time at least most of the time. On those occasions when you're unsure about where to place the points, the following technique will be useful.
Using the Eyedropper Tool to Evaluate an Image
As I mentioned earlier, the steeper a curve the greater the increase in contrast, but when working with S-Curves not all of the Curve can be steep; i.e. both ends of the curve become progressively more shallow as they move towards either the white point or black point. So how do you decide which areas of an image should fall within the steep part of the Curve?
Take a look at the following examples and decide which of the three you prefer.
I would guess that most of you think the second image is the best. The first, with no curve applied, is a bit flat, and the third, though retaining good detail in his hair, is too dark and doesn’t really emphasise his features.
If you take a look at the curves for the second and third images you’ll see that the steep section of the curve – the section between the two control points on the curve – is in different places: the first curve increases the contrast in the mid-tones – the details in his face and around his eyes – while the second increases the contrast in the highlights, bringing out the detail in his close-cropped hair. This would be ideal as a masked adjustment for his hair, but doesn't work for the image as a whole.
So, how do you work out where to put the points?
The first step is to activate what Photoshop calls the 'Targeted Adjustment Tool'. You can do this as a one-off, or permanently by selecting it as a preference (below right).
Once you've done this, hover your mouse over the image and you'll see that a) your pointer changes to an eyedropper, and b) a small circle appears on the curve that indicates the tonal value of the pixel beneath the eyedropper.
So, in order to adjust the contrast of a specific area of tonal range – for example, his predominantly bright hair, or the shadow/midtone creases in his face – all you need to do is pick the brightest point in that area (and add a point to the curve), then find the darkest point and add another point. You'll then have two points on the curve, at either end of the brightness range that you want to adjust.
You can evaluate the image, then add a point manually – by clicking on the curve as we did previously – or just click your mouse as you hover over the image. As I mentioned earlier, with practice this is something you'll probably be able to do by eye, but if you find you can't quite work out where to put your control points, or you just want to be ultra-precise, you'll find this a useful technique.
Luminosity Blend Mode & Saturation
One thing you may have noticed, especially when applying quite strong S-Curves to an image, is that in addition to increasing the contrast, these also increase the saturation.
More often than not, this will produce an attractive final result; i.e. the more deeply saturated colours will suit the parallel increase in contrast, but on other occasions you might want to increase the contrast without affecting the saturation. While you could desaturate the image after applying the Curve, using a Hue/Saturation adjustment layer or similar, there is a better, more efficient way. This involves changing the ‘blend mode’ of the Curves adjustment layer to ‘Luminosity’ rather than ‘Normal’.
In the Layers palette, select the Curves adjustment layer you wish to alter then use the drop-down menu at the top of the palette to change the blend mode to Luminosity.
The three examples that follow illustrate the difference that this change can make.
In the following example the Normal blend mode (below left), in my opinion at least, produces a much better result as the striking and contrasting vibrant colours complement the subject.
In the second, I think it’s probably more a question of preference: I prefer the Normal blend mode version, as again, I think the saturated colours suit the subject.
In the third and final example, my personal preference is for the less saturated version, created by using the Luminosity blend mode, but feel free to disagree.
One Last Point ...
Many tutorials on the curves tool begin with a discussion of the 'Auto' button – you'll find it at the top-right of the properties palette. The reason I haven’t mentioned it before now is that unless you understand the basic principles of tonal range and the way in which the Curves tool works, you won’t understand the changes you make by using this button … which isn’t a good thing, even if it does produce a result that you like.
I would suggest you play around with it to see the changes that it makes. Once you understand how these work, they can be useful, but they aren’t something that I’d recommend that you rely on from the outset.
Conclusion / Further Topics
My aims for this article were to a) help you develop a good understanding of tonal range, and b) show you how to use the Curves tool to manipulate the tonal range of an image to meet your creative expectations. I hope I've done that.
But as far as the Curves tool goes we've barely scratched the surface. This tool can also be used to manipulate the colour balance of an image – in terms of both colour correction and creative manipulation. It can be used within alternative colour spaces (e.g. Lab Color mode) to produce a range of very sophisticated changes and enhancements, and it can be used with a number of other blend modes to create a range of different effects. I'll cover these in a subsequent article (or articles).
If there's anything you'd like me to cover in a subsequent article, or you have any questions or comments on this one, let me know below.