A modern camera lens is the result of a complex design process that focuses on high resolution, high contrast, a large maximum aperture, a pleasant bokeh and possibly a high magnification, while minimizing a number of undesirable effects such as geometric distortions (straight lines appear curved), chromatic aberrations (color fringes at high contrast boundaries), vignetting (darkened image boundaries) and flare (reflections within the optical system). Other practical constraints such as size, weight and cost must also be taken into account. This task is even more difficult for a zoom lens, which has to provide a good balance over a possibly wide range of focal lengths.
To achieve these goals, a typical camera lens is the combination of various optical elements (individual glass or plastic lenses) which correct for all the unwanted effects. All these elements are hidden inside the lens barrel, so except for some rather general description provided by the manufacturer such as
and possibly a lens diagram illustrating the shape and position of the elements, we do not know much about its internal design. Therefore, it seems questionable to calculate properties such as magnification or depth of field in which we are interested here without proper knowledge of any details.
However, as can easily be verified, a camera lens still behaves like a single convex lens, as it collects incoming parallel light rays (e.g. from the sun) in one single focal point. Thus, we make the simplifying basic assumption that a camera lens can be modeled as a single lens, which is described by the well-known lens equation. We shall later see how accurate this assumption really is.