Where is the aperture on an enlarger?

"Aperture" means opening. In a lens it is called "iris" because it resembles the petals of a flower. Like this: www.google.com/search?site=&tbm=isch&source=hp&biw=1024&bih=739&q=enlarger+lens+aperture The iris is controlled by a ring around the outside of the lens. Enlargers that I have worked with did not have one, since you control exposure by turning the light off.