Stop down metering for manual Pentax lenses
Pentax Spotmatic SP was the original
M42 Pentax body and lots
of classic M42 lenses can still be
used on a new Pentax DSLR
The words “stop down metering” are often mentioned when usage of older manual focus glass (M42, Pentax-K, Pentax-M series of lenses) is discussed on camera forums. To stop down a lens means of course to change the used aperture, but you may wonder how it relates to light metering, at least if you’re not an old fart who grew up with film cameras (in other words, a person like me).
With modern SLR cameras, the lens is kept at a wide open aperture up until the shutter is pressed, at which point the lens is stopped down to the selected aperture, an exposure is taken and the aperture is again set wide open. In automatic exposure modes, for the camera to know the correct exposure time, it needs to know the f-stop to which the lens will be stopped down. The aperture can be selected either from the camera or the lens, but there needs to be a way (contacts and electronic circuits) to communicate the selected aperture between the lens and the camera body.
Older manual focus Pentax glass (M42 lenses with adapters, Pentax-M and Pentax-K series) lenses do not have the necessary contacts or electronics. This is why the lens needs to be stopped down to the desired aperture before the light measurement is made. This is called stop down metering. To make a stop down measurement on a K10D, you select the desired aperture from the aperture ring and press the Green button to set the shutter speed according to the metering. You can also use the lenses in Aperture Priority (Av) mode, but the metering will only be correct at the widest aperture.
