The Curves Tool: what is it, and how does it work in photo processing?

The Curves Tool

The curves tool is one of the most dynamic ways to control your tones either in Lightroom or Photoshop as it allows you to directly manipulate tonal groups – or ranges of tones. By adjusting the “curve” of the line, you can instantly brighten or darken selective parts of your image in a feathered and natural way. 

Said another way: since the adjustment is soft, your image will not have a pixelated or blotchy appearance that comes from other tonal adjustments. 

However, the key here is to know how to use the curves tool properly….so please join me for a basic tour of the curves panel and how it works to change your image. 


Here we have a black and white image that I’ve put together, along with a tonal sphere and value scale. These will help show the exact changes a curves adjustment makes to specific groups of tones, as those differences can be difficult to identify in the actual photograph. 

I’ll be using Photoshop for the majority of this demonstration, but the same rules apply to the point curve over inLightroom. 

 Inside the curves graph, we have a line going from the bottom left-hand corner to the top right-hand corner. This line is called the baseline, and we can add anchor points to this line by clicking on it (small squares placed on the line itself).

curve graph

Here I am clicking and holding a new anchor point, and dragging up and down to make a tonal shift. 

new anchor point

Or in other words, we are introducing the “curve” to this line in order to make an adjustment to the tones. If I drag the line up to create a curve, the exposure increases….and if I drag the line down to make a negative curve, we are decreasing the exposure. Notice how the imprint of the original baseline is still visible on the curves graph to serve as a reference. We are changing the luminosity of our existing tones. 

Over Lightroom or ACR, the curves tool works the same way under the “point curve”…which is accessible by opening the tone curve and clicking on the point curve icon in the bottom right corner.

Once the point curve is open, I can click to add a point…and drag it up or drag it down to manipulate the luminosity of our tones in the same manner.

add a point to curve

When you add a point and move it up or down, you are changing the lightness of those tones (making them brighter or darker). Now, the real question is…what are the exact tones you are changing and how to control that?

Input vs. Output: What Happens When You Change a Tone

The curves graph here in Photoshop will specifically tell you which tones you are changing by looking at the Input and Output numbers.

First, let’s add a point to the middle of the baseline, without making any changes or dragging it around. 

point to the middle of the baseline

We see that the Input number comes up as 127…and this is using the pixel value scale, which runs from the number 0 to 255. Zero is pure black and 255 is pure white, and any number between 0 and 255 will be some shade of grey….and 127 is right at middle grey. 

This means that we are targeting tones around middle grey for adjustment. 

Now, once you drag this point up or down on the curves graph, this value number will change depending on the direction you drag it in. If you bring this point up, the number will be higher than 127….and if you drag it down, it will be lower. This translates to making the tones brighter (higher number approaching 155) or darker (lower number approaching 0).

So if we drag this point up a little bit, the Output number becomes 152 (which is brighter/higher than 127). The Output number for a particular point tells you what the value has changed to. 

In other words, we have taken the value 127 (input) and made that particular value in the image brighter by dragging it up to 152 (output).  

Watch how the output number will change when you drag the point up or down.

And this works the same way over in Lightroom. The only difference is that instead of dealing with the numbers value scale, we are dealing with the percentage (0% being pure black ,and 100% being pure white). 

If I drag this point down, we have taken our values at 52.2% lightness, and made them darker to 33.7% lightness.

changing lightless in curve

Since we are dealing with the lightness level, 100% is going to be the brightest you can go. So, the principle is the same in Lightroom as it is in Photoshop, it’s just that the unit of measurement is different. 

Let’s jump back over to Photoshop. The horizontal axis of the graph from from left to right represents the value your pixels currently are…or the Input. It corresponds to the pixel value scale (0 to 155), moving from pure black to pure white from left to right. So when you place an anchor point on the curves line….the more towards the left side you add the point will select brighter tones, and the more towards the right side will select darker tones. 

dark tones in curve

Now the vertical axis represents the value of what those pixels are changed to…or the Output. This axis also goes from pure black (bottom) up to pure white (top).

curve output

So if you add a point to the right side of the graph, and drag that point down….you are (generally speaking) darkening your shadows. 

The histogram in the background shows you the current distribution of your tones for the image. The higher the peak, the more pixels in your image are of the tone.

You can see that the picture below has a lot of darker shadows and midtones…And a pretty even distribution of highlights with a very sharp peak here of pure white.

curve tool

Now that’s because we have a pure white background and the white in the sphere.

So let’s choose a pure gray point at 127 input and drag this point on the curve line towards the bottom-right. Notice how the image is getting darker since I am telling the curves tool to select darker tones (input 150 vs. 127) and reduce the lightness even more (output 91 vs. 59).

Now once you add a point, you can always come back to that point and change it any way you want. You can also manually enter a number in the Input box…or drag that point to a different input number by dragging it left or right.

This is why your baseline here goes from the bottom left to the top right in an incline as opposed to being completely level. 

The current value of your tones is represented on the horizontal axis…and the darkest tone possible is going to be in the bottom-left corner at point zero.

You can’t take this point here at the bottom left and drag it down anymore, since you can’t get any darker than pure black.

how to make pure black in curve

But you can drag it up to the pure white if you want to.

pure white in curve

Your Output (what you change those tones to) can’t go any darker than pure black, so that is why this black point is in the bottom left corner.

Let’s add a new point here and set it at an Input of 4.

new point in curve

When you add a new point, the input and output will be the same since you haven’t made a change yet (either by manually dragging the line or entering a new value for the output).

Notice that as we move to the right away from pure black, this point is a bit higher vertically…because it’s 4 points brighter than pure black; or rather 4 points brighter than 0. 

You can either darken this point by 4 points, or you can drag it up to the very top of the graph to brighten it substantially.

Since this point is so dark, you have much more room to increase the lightness as opposed to reducing it. This disparity will lessen as you approach middle grey, and then reverse as you move towards the highlights (right side of the line).

Once you hit pure white, you cannot drag the point up vertically anymore since you can not have an output higher than pure white (255).

Summary: 

The curves tool is used to manipulate tonal groups in a natural-looking way.

The curves graph represents the possible tonal environment, and the histogram in the background represents the tones in your actual image. 

In order to change your tones, you would add an anchor point by clicking on the baseline, and dragging up and down vertically to either brighten or darken those specific tones. 

The input number represents the selected tone, and the output number represents what that tone was changed to.

Settings Tonal Limits with Curves in Lightroom: https://creativeraw.com/settings-tonal-limits-with-curves-in-lightroom/

Get yourself a subscription to learn more: https://creativeraw.com/darkroom-beta/

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