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A full stop difference in metering between lenses of different design and the same maximum aperture is unusual, but not unheard of. That being said, it is possible that your new lens is not fully open when mounted. As an initial screen, compare the metering using center-weighted mode and an evenly-lit blank wall as a subject. Is there still a 1 stop difference in meter reading? If so, with the DA 16-50 off the camera:
- Carefully inspect the mount and aperture actuator lever for evidence of damage
- Using your finger, slowly move the aperture actuator from the fully stopped down position (resting state when off-camera) to fully open. The movement should be smooth with no roughness or catching. Confirm (looking from the front) that that aperture is fully open when the lever is full depressed.
- Also using your finger, flick the actuator lever and note the blade action. The action should be snappy through its full range with no hesitation or slowness.
- With the lens mounted on the camera and looking from the front, confirm that the aperture is fully open.
As a final test, put the camera in Av mode with the DA 16-50 mounted and take two photos of an evenly lit blank wall at both f/2.8 and f/4 (one stop difference in taking aperture). The brightness for both photos should be the same (similar, if not identical histograms). If the f/2.8 version is darker, there is a problem.
Steve