Focusing

Figure 12: Thin lens law 
Let us revisit the example of you taking a picture of your friend with a thin convex lens. If your friend (object) is standing at a distance o from the lens, and the focal length of the lens is f, then the distance i at which her image is formed can be found using the thin lens law [1]:
The above equation tells us that, for an object at distance o, there can be only one distance i at which the image is focused. If the image sensor is not located at this distance i, your friend will appear blurred in the image. For this reason, most cameras allow you to adjust the distance between the sensor and the lens so that your object of interest can be brought into focus. This process is called focusing.
References
[1] 
E. Hecht, Optics. Addison Wesley, 2001.
