**Summary: **The fundamental requirement of optimal aperture calculations is that diffraction and defocus blurs be interchangeable — that they make equivalent contributions to total blur. Examples are shown in which images that contain exclusively diffraction blur are indistinguishable from images that contain mostly defocus blur. However, the conditions under which equivalence is obtained imply that the formulas previously used for blur calculation are approximate. Experimental determination of equivalence suggests that the effective diffraction blur circle may be as little as 60% of the size customarily assumed. Smaller diffraction blur means smaller optimal aperture, in some cases more by than one f-stop. That results in a corresponding increase in depth of field at optimal aperture. If the goal of optimal aperture calculation is to minimize blur of an object at a known distance from the plane of focus, the effect of this correction may be minor — the decreased defocus blur obtained by having a smaller optimal aperture is partially offset by the attendant increase in diffraction blur. However, if the goal is to maximize depth of field, or to determine hyperfocal distance, while controlling total blur to a specific value, the effect of the diffraction blur correction may be important.

*Key words: optimal aperture, diffraction blur coefficient, blur equivalence, Sigma DP3 Merrill, Iridient Developer, Photoshop, defocus blur, Airy disk, Leica Disto D5, depth of field, hyperfocal distance** *

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