Understanding Curves

Curves are one of those tools that I could not retouch any photos without. I use it to add contrast, to recover and enhance highlight, to remove and add shadows, add colors and for many other applications.

With lots of functionality, a curve is one of the most versatile tools in Photoshop and yet a lot of photo retouchers and photographers tend to avoid it.  I still remember how overwhelmed I became in my early Photoshop learning when I finally figured out this great tool.  I wanted to write an article about it and did some quick Google search on the tools.  I checked a few articles but could not get one with all the information simplified. So here I am trying to help you clear the concept of curves. Here is my attempt to help you understand how this tool works and how can you use it to your greater benefits.

A curve is a graphical representation of an image. The Diagonal line in between is what we work with.  The figure below will help you understand the level better with labeling.

understanding curves

SO this is a figure I generally draw to help students understand this tool during my workshops. Please review this image carefully. There are lot of information labeled there. The top part of the diagonal lines denotes the brightest pixel of the image where as the down part of the same diagonal line denotes the darkest pixel of the image. The middle pixel is the midtones.  Midtones is a tonal value saturated between brightest tone and darkest tone.

Now you basically play with the diagonal lines. If you click on anywhere on the diagonal, a key point will be created. When you drag your key point below, then the image will underexpose since this action darkens the images, if you drag the key points to upwards then your image will overexpose.

Now here are a few things to understand:

If you only drag a pixel between midtones and darkest pixel, then you are only controlling the shadows not the highlights. For highlights you should drag the key point between midtones and the highlight pixel.

Below is the famous S curves in levels. S curves enhances the contrast.

understanding curves

RGB only deals with lightness and brightness. Inside RGB there are three-colors: red, green and blue. Before we starts lets get this formula straight in your head.

understanding curves

Each one of the three colors has an opposite color,

Red >> Cyan

Green >> Magenta

Blue >> Yellow

If you want to add or get rid of a certain color, you can use these channel. If you want red to apply, you would drag the pixel in upward direction and if you drag it downwards, then opposite of color red, cyan would be applied. Now where would the color be applied in the highlight, midtones or shadow depends on which pixel you have dragged.

Below are examples that should help you understand some things better.

  1. Highlight red, shadow cyan

understanding curves

  1. Highlight green, shadow magenta

understanding curves

  1. Highlight yellow, shadow blue

Understanding curves

  1. Highlight cyan, shadow magenta

Understanding curves

Hopefully these examples will let you understand the concept of curves better and you will now be able to make your images really stand out in color and contrast.

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This entry was posted by Anup Ghimire.

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