Photoshop Tutorials - Curves

Answer

Curves.
Curves is an interesting tool for the tonal correction, located in Adjustments submenu of the Image menu. Using the Curves window, you can make any tonal correction that was described above. However, the originals of the images can have all possible defects and faults.
When photo is shot at the bright light, the contrast in shadows disappears. Photo of the forest can be too detailed in the center. Photos shot with flashgun or shot against the sunshine have unequal light.
Tonal curves let you correct the most complex defects of the original. Let us see how the tone intervals are counted in the Curves window.

Go to Image > Adjustments > Curves or press Ctrl+M hotkey to bring up the Curves window. The diagram of the pixel allocation occupies the biggest part of the window. Unlike the Histogram that displays the absolute values of the pixel allocation, the curves diagram is relative. The X axis is the scale of the Input (actual) pixel luminosity and the Y axis is the scale of the Output (correction results). The tones are displayed as the 45-degree line across the window once you open the window, symbolizing that no corrections were made.

Without pressing the mouse button move the cursor around the diagram. You will see the respective luminosity values displayed in the Input and Output fields of the window. By default in RGB color model the brightness increases from bottom to the top and from left to right (the black color has the 0 value and the white color has the maximum 255 value)
You can change from such mode to reverse by clicking the symbolic stripe with a slider below the diagram. Once you did that the top right corner of the diagram will represent the lowest luminosity while the left bottom corner represents the highest luminosity. Such mode is often used for working in CMYK color model. Click on the slider to return to default mode.

Place the mouse cursor in the bottom point of the curve, press the left button and drag the point to the right. By doing this you have cut off the darkest levels, stretching the tonal range towards the Shadows. The image will become darker and more contrast. You did the same thing moving the black slider in the Levels window. Increasing the contrast in the Brightness/Contrast window works the same way - Contrast slider guides the slope of the curve. Move the top point to the left. This is same as moving the white slider in the Levels window. As you can see, increasing the slope increases the contrast. Decreasing the slope will reduce the contrast.
Move the points along the vertical axis and you will cut down the Output levels, just like when you move the output Levels sliders in the Levels menu. Bright Highlights colors will disappear from the image as well as the darkest Shadows.
Decreasing the contrast is not always bad - often reducing it a little the image will make the image look more detailed and pleasant for the eyes. Excessive contrast can create an aggressive impression. The location of the middle point in the diagram is corresponding with midtones (grey scale). If it was not moved relative to the center, the luminosity of the image will not change.

Working with middle parts of the curve is done as described below. With the Curves window opened place the cursor (it will look like the eyedropper) over the part of the image that needs individual approach and press the left mouse button. The point will appear on the curve respective to the selected tone. If you want this point to appear on the curve click the same area with Ctrl pressed. The black point will appear on the curve and you can move it changing the tone for selected part of the image scale. If you want to delete the point, you can click it with Ctrl button pressed.

Tonal curve must be a curve at least because it is named so. Jokes aside, if it will be stepped then steps will lead to losing midtones and poor quality of the image. You can manually create the curve clicking the pencil icon and drawing the curve with the cursor in the diagram. By clicking Smooth button, you will smooth it and make it closer to the default look.