Can you turn the aperture ring beyond the "2.8" marking?
Turn the aperture ring; make sure you see the iris; might require to set the shutter to B. Bring a ruler to the front of the lens. With the aperture ring turned as far as it will go, try to measure the apparent size of the iris as seen from the front; to avoid (as much as possible) parallax, move your sighting eye so that your line-sight remains parallel to the main axis. At f1.9, the aperture (entrance pupil, technically) should be 45/1.9=23.7mm, give or take. For f:2.8, same equation, different numbers.