  Conventional light focusing, i. e. concentration of an extended optical field
within a small area around a point, is a frequently used process in Optics. An
important extension to conventional focusing is the generation of the annular
focal field of an optical beam. We discuss a simple optical setup that achieves
this kind of focusing employing a phase plate as unique optical component. We
first establish the class of beams that being transmitted through the phase
plate can be focused into an annular field with topological charge of arbitrary
integer order q. Then, for each beam in this class we determine the plate
transmittance that generates the focal field with the maximum possible peak
intensity. In particular, we discuss and implement experimentally the optimum
annular focusing of a Gaussian beam. The attributes of optimum annular focal
fields, namely the high peak intensity, intensity gradient and narrow annular
section, are advantageous for different applications of such structured fields.
