Focus plot for one of the CRISP narrowband cameras. The plot shows data from all 121 pinholes within the FOV. A few of the pinholes are partly blocked, resulting in the outliers shown. The remaining data are fit to a fourth-order polynomial that results in a best-fit focus position of −0.16 mm. A second-order polynomial provides a much poorer fit to the data, but the resulting focus position is almost the same, −0.04 mm. It is estimated that the accuracy of the method is better than ±0.3 mm.