**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, *N** _{min(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*