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 of 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 - Understanding Levels and Curves Input Values.

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