EMD (Electromagnetic Diaphragm) is a diaphragm drive control actuator, which is a component integrating both a stepping motor and a diaphragm blade unit in a single unit, used for fully-electronic data transmission EF mount system. A conventional diaphragm mechanism controls diaphragm transforming driving power from the body mechanically and resulted in;
1. a long time lag between shutter button release and aperture diameter control.
2. low reliability and durability.
3. limited placement of diaphragm unit in the lens barrel.
EMD controls diaphragm blade by a stepping motor and aperture diameter by electronic pulse signal from the camera body to overcome the above mentioned problem of conventional mechanical diaphragm units, while also provides benefits that conventional mechanism does not offer;
1. high-precision digital diaphragm control is much more accurate than with conventional mechanical linkage systems.
2. extremely quiet operation because of no shock of mechanical levers. 3. compactness thanks to the integrated diaphragm control components.
4. automatic aperture is possible with a lens whose diaphragm unit moves along with the optical system like TS-E series lenses.
Every EF lens incorporates an EMD unit as a basic component of the EOS system.