The SMC Pentax 50mm 1:1.4 (1975 to 1977), introduced by Pentax with the launch of Pentax K-series 35mm SLR film cameras, is a robustly built all metal and glass 7 elements in 6 groups (with SMC coating) 35mm full frame lens. The lens is more than often wrongly misrepresented as the later version SMC Pentax-M 50mm 1:1.4 (1977 to 1984), which is lighter, more compact, and with similar image characteristics. The lens (and series), just as well, was not officially referred to as a K-series lens by Pentax, a designation was given to later K-mount lenses (such as the M, A, F, FA, and DA series) produced with later model cameras.
Using the lens on the Pentax K-m (K2000 in the US), which has a 'crippled' KAF2 mount, is with a couple of caveats. First, for image stabilization functionality, the lens's focal length must also be indexed by the camera. This is 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, as the lens mount of the K-m does not have the aperture coupling lever to tell the camera what aperture the lens is set to, metering the SMC Pentax 50mm 1:1.4 can be done stopped-down, with the camera's exposure mode set to manual ('M' on the mode dial), and exposures made fully manual (you may use the Sunny 16 Rule for this), or stopped-down, which is executed by pressing the AV (on the K-m) / Green button located on the top plane of the camera (circled red in the image above) before releasing the shutter.
When the AV button is pressed, the viewfinder (depending on how small the lens aperture is set to) will darken slightly as the diaphragms are stopped down for the exposure measurement to be taken, and the value converted to the shutter speed setting.
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