Why refer to "aperture" as "iris"?

Question

With common lenses the aperture is controlled by a mechanism called an "iris."

But, especially with cine/video-oriented lenses, I often see the term "iris" used as an apparent synonym for aperture. E.g., "smooth iris control."

Is there a reason they refer to "iris control" instead of "aperture control?" If a lens uses an iris for aperture control wouldn't that always be the same thing? Or, for example, are there other non-iris aperture controls in modern use so that it would make sense to specify that the aperture control on a particular lens is via iris?

Answer 1

Aperture is the actual opening, the device which creates the opening is called diaphragm, if diaphragm is adjustable, then it's called iris, because it has resemblance of an iris of an eye. So using "iris control" instead of "aperture control" gives you more information about what kind of device you're going to operate.

There are other types of diaphragms of course, like brass strips with a hole in it, used in old Pertzval lenses. And some cameras might not have diaphragm at all, having ND-filters to adjust amount of light hitting the sensor instead.

Answer 2

It is mainly just a difference in terminology used by what was once separate communities of artists using vastly different equipment. Those using movie cameras have historically used the term iris to describe the setting of the diaphragm that determines the size of the aperture, which is the opening itself. Those using still cameras have historically referred to the same setting as aperture.