What is an iris diaphragm?

And how do they work?

Iris parts

An iris diaphragm, for the uninitiated, is a mechanical device used traditionally for regulating the passage of light. It achieves this by blocking excess light, allowing only a controlled amount through a variable aperture mechanism.

Sometimes just called an ‘iris’ for short (or irises if referring to more than one) they can typically be found in all sorts of devices, such as: cameras, microscopes and theatrical spotlights. But are also used in non-optical applications such as; gas flow control, orientation/gripping mechanisms and even used to ensure the perfect portion of spaghetti!.

Irises are mesmerising when first seen in operation, appearing almost organic in the way their aperture can change size so fluidly. As such it may not be immediately obvious as to how they achieve this, as their mechanism is hidden away in the outer ring. But surprisingly there is a very simple mechanical movement going on.

The Mechanism

The mechanism functions by the use of an array of movable blades or leaves (leafs?). These are tucked away in an overlapping fashion inside of the circular body of the iris.

The blades are all hinged at one end and sweep towards the centre of the iris in unison, thus creating an adjustable aperture.


Most irises follow a similar geometric pattern, and as such designing one is just like a puzzle that needs solving. Something the Iris Calculator, we feel, does rather well!