Complex sentence: All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed; either a manual ring control or an electronic motor controls the angle of the diaphragm leaves and thus the size of the opening.
Split into simpler sentences: All modern lenses use a multi-leaf diaphragm so that at the central intersection of the leaves a more or less circular aperture is formed. Either a manual ring, or an electronic motor controls the angle of the diaphragm leaves and thus the size of the opening.