With a Canon FL lens, the Canon Reflex 500mm or TS 35mm lens or any other non-FD lens, full aperture metering is not possible. The lens must actually be closed (stopped down) to the shooting aperture for metering. This called stopped-down metering. In stopped-down metering the lens diaphragm will open or close as you turn the aperture ring (5)

Push the stop-down slide (1) towards the lens until it locks
Press the shutter button halfway or press the exposure preview switch (2) and turn the aperture ring and/or the shutter speed dial until the meter needle is in line with the battery check / stopped-down metering index. A red "M" will blink in the viewfinder to indicate manual aperture control. Press the shutter button all the way down for exposure.