Effective aperture
From The Glossary of Digital Photography
The size of a noncircular aperture expressed as the size of a circular aperture. Variable apertures are adjusted with a series of interwoven blades used to reduce the size of the largest opening. Because the blades do not make a perfect circle, the actual aperture is expressed as an effective aperture size, as if it were a circle of a certain size. The more blades, the closer the aperture is to a circle, and the closer it is to the effective aperture size. See also aperture
