Selection of the design parameters for an adaptive optics system is normally based on consideration of a set of three basic errors referred to by the terms "measurement noise error" , "servo lag error" , and "fitting errorS" In this paper I introduce another basic error, which I refer to by the term "reconstructor formulation error!' This fourth basic error has previous'y been overlooked in analysis of adaptive optics system performance. In this paper the concept of reconstructor formulation error is presented and then the magnitude of the error is developed. It is found that for the forms of wave front sensor and reconstructor formulation considered the mean square value of the reconstructor formulation error is proportional to (?/ro)5/3, with the constant of proportionality having values ranging from about 0.2 to 1.0, times the reconstructor noise gain factor—the relevant value depending on the form of the sensor and the nature of the reconstructor formulation. It is noted that although the (?/ro)5/3-dependence is the same as for the fitting error, reconstructor formulation error is distinct from and in addition to fitting error.