Mid-spatial frequency on freeform optical elements induces small-angle scatter and affects the optical performance. Fabrication techniques involved in making freeform surfaces leave tooling marks on the surface due to the sub-aperture nature of the fabrication process. In recent years, there has been a growing need for specification and characterization of the mid-spatial frequencies for freeform surfaces. There is a range of methods to consider for representing the mid-spatial frequency content: the power spectral density (PSD), the structure function (SF), and a polynomial basis representation such as Zernike and Forbes polynomials. Using research discussed here and published discussions from the literature, we compare a Zernike representation to a PSD representation, and structure-function representation in terms of ability to connect to optical performance, ability to connect to the fabrication process, and other practical considerations. Our focus is on investigating a Zernike polynomial representation for quantifying the mid-spatial frequency content in the height maps. We will illustrate how this polynomial representation captures certain characteristics of the mid-spatial frequency texture. A filtering aspect of these polynomials is also explored and optical performance is predicted based on very large orders of Zernike polynomials.