Principles
It is somewhat difficult to generalise about how field guides are intended to be used, because this varies from one guide to another, partly depending on how expert the targeted reader is expected to be.
For general public use, the main function of a field guide is to help the reader identify a bird, plant, rock, butterfly or other natural object down to at least the popular naming level. To this end some field guides employ simple keys and other techniques: the reader is usually encouraged to scan illustrations looking for a match, and to compare similar-looking choices using information on their differences. Guides are often designed to first lead readers to the appropriate section of the book, where the choices are not so overwhelming in number.
Guides for students often introduce the concept of identification keys. Plant field guides such as Newcombs frequently have an abbreviated key that helps limit the search. Insect guides tend to limit identification to Order or Family levels rather than individual species, due to their diversity.
Many taxa show variability and it is often difficult to capture the constant features using a small number of photographs. Illustrations by artists or post processing of photographs help in emphasising specific features needed to for reliable identification. Peterson introduced the idea of lines to point to these key features. He also noted the advantages of illustrations over photographs:
A drawing can do much more than a photograph to emphasize the field marks. A photograph is a record of a fleeting instant; a drawing is a composite of the artist's experience. The artist can edit out, show field marks to best advantage, and delete unnecessary clutter. He can choose position and stress basic color and pattern unmodified by transitory light and shade. ... The artist has more options and far more control .... Whereas a photograph can have a living immediacy a good drawing is really more instructive. —R T Peterson 1980 :9-10Field guides aid in improving the state of knowledge of various taxa. By making the knowledge of experienced museum specialists available to amateurs, they increase the gathering of information by amateurs from a wider geographic area and increasing the communication of these findings to the specialists.
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