The Canonet QL17 GIII is generally described as using a trapped-needle auto-exposure mechanism. In this type of system, the light meter moves a needle according to the measured light level. When you take the picture, the camera mechanically clamps or “traps” that needle at its current position.

A spring-loaded exposure-setting mechanism then moves toward one end of its travel, but its movement is limited by the trapped meter needle. That limited travel determines the exposure setting—typically the aperture, shutter, or a linked cam that controls them.

So the camera doesn’t need a separate electronic motor or solenoid to position the iris directly. The electronics drive the meter reading, and the needle’s position is then used as a mechanical stop for the exposure mechanism.

The shutter lockout in Auto mode is likewise tied to whether the meter reading falls within the camera’s controllable range. If the needle indicates a value outside that range, the linked mechanism prevents release.