Photographers have a variety of techniques at their disposal to manipulate images in the darkroom. The majority of adjustments are made during enlargement.
Photographers can darken the tones of specific areas of a photograph using a technique called burning-in. For part of the exposure, the photographer holds a piece of cardboard with a hole cut out of its centre between the print paper and the beam of light coming through the negative by way of the enlarging lens. The unshaded area receives more light and therefore darkens in tone. To avoid creating an obvious shadow in the final print, the photographer moves the cardboard back and forth.
Dodging is the opposite technique. Photographers use dodging to lighten or reduce the density of a dark area of a negative in a final print. For part of the exposure, the photographer holds a piece of cardboard or another material between the print paper and the beam of light. The shaded area receives less light and therefore remains lighter in tone. To avoid creating a sharp outline around the shaded area, the photographer moves the cardboard back and forth.
Photographers can also adjust the tonal contrast of black-and-white photographs in the darkroom. A range of contrasts can be achieved using variable contrast print papers and a series of filters over the enlarger lens. The filters go from 0, which is very soft, to 6, which is very hard. Print contrast is also influenced by negative contrast, the choice of developer (hard or soft), and development time and temperature.
The possibilities for manipulating images are virtually endless in digital photography. A number of image manipulation software packages are available on the market. The most widely used is Adobe Photoshop.
Image manipulation software allows photographers to crop, rotate, frame, and alter the size of an image, distort form, adjust and balance colour and contrast, add and split tones, burn and dodge, sharpen and blur detail, remove dust, add a tint, translate colour into black and white, add filters and text, "stitch" together whole images, and fuse elements of different images.
There are some excellent books and websites on image manipulation in digital photography.