Contax RTS III Instruction Manual page 25

The Contax RTS lll offers the following exposure modes:
aperture-priority auto exposure, shutter-speed-priority auto
exposure (with MM type lenses only), manual exposure
and bulb exposure. Set your desired mode with the
exposure mode selector lever while pressing in the
exposure mode lock release button.
For details on the exposure modes, see pages 58 -76.
< Exposure Check Button >
When the main switch is at ON, depress the exposure
check button. The display in the vieMinder will turn on to
let you check the exposure. You can also check the
exposure by depressing the shutter release halfway.
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