This is done:
- to reduce the exposure (or to compensate when changing to a slower shutter speed, keeping the exposure constant),
- to increase the depth-of-field,
- to improve image quality by lessening lens aberrations, or,
- in an SLR camera, to switch from the aperture fully open (for viewing & focusing) to the taking aperture; whether to check depth of field, for stopdown metering (see), or to take the photo.
On the earliest SLRs, the lens had to be manually stopped down to the taking aperture; later cameras coupled the diaphragm to the shutter release to stop down automatically — the meaning of "Auto" in the context of older lenses.
Stop-down or stopdown metering is a technique used in TTL systems where light measurements are made through the lens while closed down to its taking aperture. This was used on all early SLR TTL systems, and has the flaw of temporarily dimming the viewfinder image. The alternative—open-aperture metering—requires the metering system to "know" the taking aperture and compensate for it when determining the shutter speed, which requires a coupling to communicate the selected f-stop from the lens to the camera body. This was was not generally available on SLRs until the 1970s. Many cameras with open-aperture metering still feature a stop-down metering mode to allow use of lenses or accessories without this coupling.