https://www.nfsa.gov.au/sites/default/files/2023-11/Chapter%201%20Block%201000%20x%20745.jpg
Iris
Adjustable diaphragm controlling amount of light transmitted by a lens (1985).
The diaphragm in the lens which controls the intensity of light reaching the film emulsion, tube or CCD.
References
Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston