Using levels in Photoshop  

You have bought your new digital camera and started taking pictures, but they do not look just as good as the pictures you have seen on specialized sites. You may find that many of your pictures lack contrast and because of that they look flat. Adobe Photoshop has a series of tools you can use to enhance a picture’s contrast and brightness. The most basic tools for this kind of job you can find in Photoshop are the Contrast and Brightness (Image > Adjustments > Brightness/Contrast). However, they both work far too simply and offer far too little control to be very useful. Instead you can use the Level function to add some more punch to your images.
You will find the Levels function in the Image > Adjustments > Levels… tab. Follow below step by step instructions for more details:

1) Open Photoshop and the image you want to edit. Use the File > Open option as shown in figure 1 to navigate until you find the picture you want to edit.

2) When you enter the Levels tab, its dialog window will appear as shown in figure 2.

You will notice that the center of the window is occupied with the histogram. The histogram shows the amount of pixels in an image and the way they are distributed across the dynamic range of that image. The histogram shows where the details are placed in the picture: in the shadows (shown in the left part of the histogram), mid-tones (shown in the middle), or in the highlights (shown in the right part).Under the histogram you will see three small triangles (a black one on the right, a white in the left end of the histogram and a grey one in the middle). Knowing how to use these three triangles will give you the ability to make the necessary corrections to a picture.

3) A histogram can also help you determine the amount of contrast. Broad histograms reflect a scene with significant contrast, whereas narrow histograms reflect less contrast and may appear flat as shown in figure 3 and figure 4.

4) Although most cameras will produce mid-tone histograms when the picture has been taken in the automatic exposure mode, the distribution of peaks within a histogram also depends on the tonal range of the subject matter. Images where most of the tones occur in the shadows are called "low key" {see figure 5} whereas with "high key" {see figure 6} images most of the tones are in the highlights. There may be situations when you may deliberately want to have the picture either brighter, either darker then the measured parameters the camera has decided. You will have to adjust the settings manually to achieve this.