Classic SLRs Series :
The depth of field can also be read from the color-coded scale engraved on the lens. The pairs of colored lines correspond to f/numbers of the same color. To find the depth of field at a particular aperture, first focus the lens on the subject. Then check the numbers on the distance scale opposite the colored lines which have the same color as that of the taking aperture to find the depth of field at that aperture.
For example, f/16 on the aperture ring of the 50 mm f/1.4 lens is blue. With the lens prefocused at 17 feet (5 m), the numbers on the distance scale opposite the blue lines show that the depth of field extends from 9 feet (2.7 m) to infinity (OO).
By stopping down the lens aperture, the depth of field will increased, as illustrated by the three photographs below:
Len set at f/4.0, only the main subject will be in sharp focus - a good mean to create interesting portrait will draw viewer attention to subject. Len set at f/8.0, depth of field is significantly increased.
Len set at smallest aperture on the lens (largest f/number). Great extension of depth of Field which covers from near to far in focus. Ideal for landscape, travel, duplication, macrophotography etc.