Summary: A previous experiment indicated that the effective size of the diffraction blur circle is substantially smaller than customarily assumed. Smaller size can be accounted for in optimal aperture calculations by incorporating a diffraction blur coefficient, α. The present paper describes a simple procedure to estimate α. Although simple in concept, the described experiment can be difficult in practice because results are sensitive to focus accuracy. The value of the diffraction blur coefficient was determined to be α ≈ 0.70. That is slightly larger than the value 0.60 from the previous experiment. Additionally, Nmin(Blur) was shown to be an optimal aperture under experimental conditions.
Key words: optimal aperture, diffraction blur coefficient, defocus blur, blur equivalence, Sigma DP3 Merrill, Iridient Developer