An iris diaphragm comprises a stationary ring, a rotatable adjusting ring, and a plurality of shutter blades. The plurality of shutter blades forms a diaphragm aperture inside the stationary ring and rotation of the adjusting ring moves the shutter blades inside the stationary ring whereby the size of the diaphragm aperture can be changed. The shutter blades are formed as a flat opaque surface and at least one of the shutter blades comprises at least one transparent region formed in said opaque surface. A light fixture comprises such iris diaphragm arranged between a light source generating a light beam and an optical assembly configured to project the light beam along an optical axis.