This is the reconstruction of the 5° off vertical wedge found in the top middle of the ISO 12233 resolution chart. To obtain it a line is drawn on the image parallel to the edge and then the pixels are read horizontally across the edge. The distance of each pixel from the edge is calculated and its intensity (RGB data is converted to luma) is plotted against this distance to make this plot. When the entire edge is scanned the data are least squares fit with 30 Chebychev polynomials. The solid line on the plot is this fit and the residuals, calculated from it, are plotted on the top of the graph. The slope of the hypothesized edge is then changed by moving the end point 1 pixel and the process repeated. This is done a couple of times and the plot with the least residuals is taken to represent the best approximation to the actual edge. Note that the reconstructed edge here clearly shows that the camera applies sharpening

The Chebychev polynomials are computed by a simple recursion and as their derivatives are also easily computed in the same way the fitting coefficients are used with these derivatives to calculate the first derivative of the edge at equally spaced points. This is the system's line spread function. It is FFT'd to compute the MTF estimates shown in the next image.