All values, from deepest black to perfect white, and all the greys in between, should be present in your negatives (if using film), computer files (if using digital) and prints.
With very few exceptions, all of the following tones should be present in your photographs. The tones do not have to be evenly represented, but each one should appear somewhere in your image.
One reason you want all of the tones is because the light greys give you detail in your highlights, and the dark greys give you detail in your shadows.
"V" is equal to 18% grey, the tone that light meters use to determine the correct shutter speed and aperture f-stop combination. This scale is part of the ZONE SYSTEM.
Examples:
Here are some examples of photographs that contain the full range of values.
Ansel Adams
Exposure and Contrast in Editing:
The following link discusses how to properly expose photos. You want to have a range from light to dark. It shouldn't be over exposed or under exposed. See the link below for examples:
The following link discusses how to expose correctly in relationship to value and tone Note: usually test strips are used when developing film in a dark room, but it still gives us a good idea of what to look for in our digital pictures.