A stage lighting fixture has a casing having a closed first end and an open second end; a light source housed inside the casing, close to the first end, to emit a light beam along an optical axis oriented longitudinally with respect to the casing; an objective optical system located along the optical axis, at the second end of the casing, and having a focus position; a circular-aperture diaphragm located along the optical axis, between the light source and the objective optical system, to intercept the beam; and an iris diaphragm located along the optical axis, between the light source and the objective optical system, and which can be set to the focus position.