A modern photographic lens is the result of a complex design process aiming at a high resolution, contrast, aperture, and possibly a large magnification and a pleasing bokeh, while keeping a number of unwanted effects at a minimum, such as geometric distortions, chromatic aberrations, vignetting, and flare. Other practical constraints such as size, weight and cost also have to be considered. 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 photographic 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 descriptions provided by the manufacturer such as
10 elements in 7 groups, including 1 aspherical and 1 floating element
we usually do not know much about its internal design. It thus seems somewhat 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 photographic lens still behaves similar to a single thin lens, collecting incoming parallel light rays (e.g. from the sun) in one single focal point. Thus, we make the simplifying basic assumption that a photographic lens can be modeled as a single thin lens, which is described by the well-known lens equation. We shall later see how accurate this assumption really is.