Photoshop For Beginners - Understanding Levels and Curves Input Values

When you set the INPUT value in the Levels or Curves dialog either by dragging the black or white triangular sliders or typing a value directly into the Input box, you are telling Photoshop to change all of the pixels in the image that have this new tone value so that they match the default OUTPUT value. For example, when you move the white slider in the Curves dialog to 250, you are basically saying "I want all the pixels with the value 250 AND HIGHER (i.e. the range 250 to 255) to have the same value as shown in the OUTPUT box."

Since the default OUTPUT value is 255 for white, all of the pixels in the image with a value of 250 or higher will be made to be pure white (value 255). The other light tones in the image will also become lighter. The same is true for the black slider. When you move the black slider you set the INPUT value which will change all of the pixels with this value AND LOWER equal to the default OUTPUT value which is 0 (i.e. black). For example, if you move the black slider or set the black INPUT value to 25, all of the pixels with a value of 25 or lower will be made equal to the output value of 0 (black). This will also cause all of the shadow tones also to darken.

You are actually setting the upper and lower limit values of the white and black tones in the image which in this example would increase the overall contrast of the image. So, if you have a low contrast image that hasn't got a black tone and hasn't got a white tone, you would normally be saying "OK, the darkest tone in this image has, say, a value of 20 BUT this is not true black and I want it to be black. Right, so I need to set the INPUT value of the black slider to 20 and leave the OUTPUT value at 0."

This will change the value of the selected tone from 20 to 0 thus increasing the depth of the shadow tones. Note that all the tones in the dark areas will be lowered in value proportionally. You now have either a Levels or Curves tone adjustment that has 'mapped' the value of 20 down to 0. Working with the lighter tones now, let's say the lightest tone in this same image has a value of 230 (a light grey tone, typical of low contrast photos). Now let's assume we want the image to have a pure white. So, we need to change the value of this tone from 230 so that it looks white (value 255). Using the Levels or Curves adjustment dialog we move the white slider until the INPUT value is 230 and leave the OUTPUT value at the default of 255. This will change all the tones with a value of 230 so they have the new value of 0 (zero) which will change them to become white in the image.

The result of these two changes to the black and white input values will be that you have increased the image contrast to make it fill the full tonal range and appear more 'normal'. Now you understand how the Input values in the Levels and Curves dialogs work and how to use them to correct a low contrast photograph.

Photoshop For Beginners - The 3 Basics of Using the Curves

Most beginners to Photoshop (and many advanced users) avoid using the Curves command because it reminds them of school math classes and geometry! I know how you feel, but if you take a little time to understand the basics of how curves work you will lose your fear - trust me. In this article I will show you the 3 basics you need to understand to effectively use Curves.

What do Curves actually do?

The Curves are very like the Levels in that they are used to adjust the tonal contrast of the photograph. More importantly they are also used to adjust the look of the various grey tones of an image. The difference between Curves and Levels is that the Curves dialog allows you to control any portion of the grey tones whereas the Levels changes all of the various tones at the same time; you have less control with Levels than you do with Curves. This is why professionals prefer using Curves; they put you in control!

Use the Curves command to change overall contrast by:

1. Choosing the first tone in the image you want to have a pure black value.

Known as black point adjustment, moving the black slider along the bottom of the grid in the Curves dialog allows you to set which tone will be pure black in the photo. Bear in mind that if you move the slider past the start of the graph shape you will lose some of the subtle really dark tones and this will lose details. For example, if you move the slider to the right so it is at the very left end of the graph (the value of the tone at this point will be shown in the Input box), this value will be changed to pure black (value 0) and all the other grey tones will be darkened proportionally. If you move the slider even further to the right i.e. make a slightly higher tone value pure black, you will lose any tones shown in the graph that are to the left of the slider. Let's say the very end of the graph shows an Input value of 20, this is the darkest tone in the image and so when it becomes pure black we do not lose any other tone values because the other tones have higher values (they are lighter in tone). However, if we move the slider further into the graph, say to Input value 30, we will lose all of the tones between 20 and 30 because they will all be made to be pure black.

Now we know how to set the darkest tone of the image to pure black to obtain rich looking shadows.

2. Setting the pure white tone of the image.

Now we use white point adjustment to set the lightest tones of the image. Moving the white slider to the left so that it just touches the right end of the graph will make the tone at that position become pure white. Just as with the black point adjustment described above, since all the other tones below this point are lower in value (i.e. they are all darker), we do not lose any of the subtle light tones. However, if we move the white slider further to the left so that it is inside the graph shape, we will lose all the tones to the right of the slider as they will be made pure white in the image.

Setting the end points in the Curves dialog as explained above is exactly the same as setting them using the Levels dialog. For the end points the two commands do exactly the same job. The real power of Curves is in how we can control the other tones of the image as we shall now see.

3. Controlling the intermediate tones of the image.

The first difference you notice between Levels and Curves is that there is no grey slider shown in the Curves dialog and that there is a diagonal line from one corner to the other that cuts across the histogram shown behind this line. (To understand more about the graph in the Curves dialog see my article on understanding the histogram.) This line is actually a curve without any bends in it! When the line is straight (as it will be when you have only set the end points in 1 and 2 above), the relationship between the intermediate tones is being affected equally by the line. However, you can now click and drag anywhere on this line to create a curved line. When the line becomes a curve shape it no longer controls the tones equally. The real power of the Curves adjustment is that you can click and drag from several points on the line to create any curve shape you want. By changing the shape of the line into a curve you can control which portions of the tone scale are adjusted. You can also simply click on the line to create fixed points that act as anchors to stop the line from curving. When the line remains straight no changes occur to the tones in that area. This is how you can select just a portion of the tones to alter; create several anchor points along the line over the tones to preserve and then bend the line up or down in the area you do want to change.

A word of caution; if you have the curved part of the line that covers the shadows of the graph higher that the curve over the lighter tones you will cause the tones to reverse. This is known as posterisation and is not good! Try to make sure the curve is raising all of the way from left to right and that it doesn't dip down too much in the middle section.

There you have the 3 basics of using the Curves command. Using Curves is the most powerful way to control the contrast of your photograph and you should now be able to use it with confidence.