The Photoshop level is a tool used for stretching and moving the brightness level in image histograms.
It is a powerful tool for adjusting the brightness, color tone and contrast.
The ‘levels’ specify the location of complete black point, complete white point and a gray mid-tone slider on a histogram of an image and all values are unique for each photo.
Generally images look best when they are processed utilizing the full range from dark to light. Utilizing full range of levels brings higher contrast in the image.
However, it should be practiced with care because images taken during dawn, dusk, fog or hazy or in a soft light can be ruined if great color contrast is imposed.
The color tones are indicated by ‘pixel’ values which are again coded by ‘bit’ in digital language. There are 256 different pixel values for color tones ranging from darkest black to brightest white in digital photos.
Moving the right or white and left or black point sliders in a histogram to change these values in controlled manner generates greater contrast in the image.
Caution should be followed while moving the white and black point sliders at the edges of a histogram, as they can clip highlights and shadows of only one pixel height.
Hold down the ‘Alt’ key while dragging point sliders, and it will display the images clipped up because of very small pixel value. Sometimes, this is desired for more detailing in selected or important areas to draw attention in the photo.
The use of mid-tone slider can be best understood in a dichromatic or black and white picture. Use of mid-tone slider stretches and overexposes the details of brightness or shadow by adjusting the grayness or medium color shades, while maintaining an overall brightness or darkness in the photo.
All the pointers in this program are controlled by some numerical values, viz. the black pointer starts at ‘0’and approaches right, the mid-tone pointer ranges between ‘1’ and ‘254’ and the white pointer starts at ‘255’ and approaches left.
A digital camera shows only RGB or Luminance histogram and movable pointers as programmed in it while the software used in photo editing shows values of pointers at each position on any type of histogram including RGB histograms, color and luminance histogram etc.
You will find the ‘Levels’ tool in almost all photo editing software, like the Gimp and the most commonly used Adobe Photoshop.