Re: What does it mean "input and output levels" in
The Input Levels section is used to lighten highlights, darken shadows, reset mid tone and bring out, or lose, detail. To darken shadows, we move the black slider inward or type a number larger than 0 in the left most text box. To lighten highlights, we move the white slider inward or type a number smaller than 255 in the right most text box. To reset mid tone, move the gray slider, known as the gamma slider, to the left or right. When these adjustments are made, Adobe Photoshop remaps the image's pixels from their current tone to a new tone.
We saw with Input Levels that we can remap tone to make our shadow areas darker, our highlights lighter and increase contrast. But is there a way to lighten shadows, darken highlights and decrease contrast? Yes. That is one of the uses of the Output Levels section of the Levels dialog box.
The digital tonal range is defined in the Output Levels section. The Output Levels section not only defines the digital tonal range, but it allows us to shorten it. By shortening the tonal range, we are forcing image pixels to look more alike, thus decreasing contrast.
The Output Levels sliders do not reset solid black and solid white to new values in the digital tonal range. Instead, they remove solid black and white by changing where the digital tonal range begins and ends. For example, the full tonal range of 0 - 255 can be changed to 10 - 240 so no image pixel will be darker than tone 10 nor lighter than tone 240. With Input Levels, we stretch the image's tonal range to fit more of the full digital tonal range. With Output Levels, we force the image's tonal range to fit into a newly defined and smaller, digital tonal range.