Abbreviation: RD = "Rounded Diaphragm" (Nikon)
The abbreviation RD means "Rounded Diaphragm" and is seen on Nikon products.
Explanation from Nikon
Soft-focus shots of point light sources create regular polygonal shapes that reflect the shape of the opening created by the diaphragm blades. A rounded diaphragm uses blades designed to create a circular opening for a more beautiful soft-focus effect.
