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The strict definition of an afocal system is a system in which both object and image conjugates are at infinity. Such systems would include, for example, a laser beam expander in which both input and output beams are collimated. Another example is a system like binoculars where light is brought to focus by the eye, and the binocular design itself relays light from an infinite object conjugate to an infinite image conjugate, with some angular magnification.

The term "afocal" is also used sometimes to mean any system in which the image conjugate is at infinity. ZEMAX uses the term "afocal image space" to describe any system in this category or in the full afocal category. The major consequence is that the units we use to describe optical performance in the image space change from spatial units to angular units. Different units are used in different applications, and the choice of units is made on the Units tab of the System > General Dialog box:

As a result, the various ZEMAX analysis features will report in different units:

Whether ZEMAX uses focal or afocal units is set by a control on the General > System> Aperture tab, "Afocal Image Space":

Other than the change of units, most ZEMAX features work exactly the same with focal or afocal image spaces. Some features are specific to focal systems: relative illumination, for example, has no physical meaning in an afocal system. In addition there are default merit functions for either mode: spot radius, spot in x only, spot in y only for focal system and angular radius, angular x only angular y only for afocal. Wavefront error can be used in either mode.

In this article we will design two simple systems: a laser beam expander which is a true afocal system, and a cylindrical lens which is focal in one direction and afocal in the other.