The Pentax K-m (K2000 in the US), a compact digital SLR camera with a 10.2MP CCD APS-C image sensor, was introduced on September 22, 2008. The entry-level camera was fitted with a KAF2 lens-mount system and was supplied, for the twin lens kit, with the lightweight and KAF2 specific SMC Pentax-DA 18-55mm f/3.5-5.6 AL and SMC Pentax-DA L 50-200mm f/4-5.6 ED lenses.
The KAF2-mount, a follow-up from the KF, KA, and KAF mounts, was Pentax's development sequence for adding auto-focus features to its lenses. The first KA-mount, for example, allows the lens's aperture opening to be set by the camera body. This enables shutter priority and program auto exposure modes.
The KAF2-mount, however, does not have the aperture coupling lever to tell the camera what aperture the lens is set to, which means that older generation of manual focus lenses, when fitted to the camera, will not be able to communicate its aperture setting to the camera. Vise versa, the camera does not have the capability to put the lens into auto-aperture mode either.
A legacy Pentax-K lens, say the SMC Pentax 50mm 1:1.4, can still be used on the Pentax K-m digital SLR, both in fully manual mode and stopped-down manual mode, with the proviso of a couple of caveats. First, for image stabilization functionality, the lens's focal length must be indexed to the camera's lens registry, done on power-up where the 'Input Focal length' menu is displayed and the focal length (from 8 to 800mm) is selected from the display bar.
Second, for the stopped-down manual mode, the metering function of the camera needs to be activated to measure the scene and record the aperture setting for the lens before releasing the shutter, and when the shutter is released, set the lens aperture to the reading automatically.
This is done by first pressing the 'Green' button, or in the case of the Pentax K-m, the 'Exposure Compensation' button before releasing the shutter button. When observing the scene through the camera's viewfinder, you will notice a slight darkening of the viewfinder when you press the 'Exposure Compensation' button as the camera runs the metering option for the correct lens aperture opening.
The steps may sound rather complex initially, but once you get the hang of the routine, getting the correct exposure for your shots with the stopped-down metering method is all but a breeze. All the best!
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