How does a manual lens know when to adjust the aperture?

Question

When I look through a viewfinder before I take the photo, the lens is "wide open" - at its largest possible aperture (unless the depth of field preview is used). The aperture is adjusted to whatever I set it to just before the shot.

I understand it this way: the camera tells the lens to adjust the aperture, makes a shot, and resets the aperture back to the max.

But what about manual lenses like, e.g., the Samyang 8mm FishEye f/3.5? It does not have any connection to the camera (in fact, my D5000 shows No lens attached when I mount it). I set the aperture directly on the lens and the camera has no idea about its settings. Still, the viewfinder shows the scene very brightly, even if I set f/22.

How do such lenses know when to adjust the aperture? Am I missing something?

Answer 1

Nikon has a very noticable lever just inside the lens mount on the left side (viewing the body without the lens from the front).

^{Photo source: Nikon D5000 DSLR: Announced and Previewed}

On the lens there should be a matching lever. You can move it and see the diaphragm close.