The Boolean object works by converting each parent object into a NURBS based representation, and then a series of Boolean trimming and combining operations are performed to yield the resulting object.  Some loss of precision is possible when converting from ZEMAX native, high precision representations to NURBS representations; this is not a limitation of ZEMAX but is inherent in the NURBS representation of arbitrary surfaces. However, precision can be increased arbitrarily using the Spline and Tolerance parameters described in the manual. When extreme optical precision is required, the user should verify that the Boolean object traces to a suitable accuracy. We have tested this feature with sensitive aspheres and received very good results.

However in most cases where this feature is used, the default values of spline and tolerances will give very good results, as with the parabolic mirror in the previous example, but care should be taken with highly-balanced aspheres.

The other consequence of using NURBS is that more memory is required to hold the object, and ray-tracing is slower, than a comparable parametric object if one is available. It will be faster to trace, and require less memory to hold, the Standard Lens object than a Boolean combination of spheres and cylinders that yields the same shape and volume!

Built-in objects will always perform better than Boolean objects and should always be used when available. The user-defined object is also available to let you write your own parametric objects, if required. The Boolean object, however, gives great flexibility in defining unusual shapes easily and quickly.