One likely possibility is that the adapter ring you're using doesn't have a pin to hold the DoF preview lever on the lens in place, so that the lens actually stops down, and despite setting the aperture with the lens's aperture ring, you're still shooting wide open. You may need to hold the DoF preview button down while you take the shot to get the aperture you've set.

The other possibility is that your adapter is chipped and fools the camera into thinking a Canon lens is mounted so that the aperture is not displayed as "00", and the camera is not performing stop-down metering. This will bias the metering by whatever you've set the aperture to in the camera, which can have no effect on the lens.

Remember that by default when an EOS lens is mounted and communicates with the camera body, the lens is kept wide open all the time you are composing and adjusting settings. The camera takes a meter reading, and then adjusts the reading based upon changes you make to the settings, but the amount of light coming in doesn't actually change. When the camera cannot sense a lens through the electronic contacts, it reverts to "stop-down" metering, and the meter simply reflects the amount of light coming in, and the camera does not adjust that reading based upon changes to the exposure settings, expecting that the aperture is already stopped down, rather than the lens being wide open, and the aperture only closing down to the set f-stop when the image is taken.

If this is the case, meter with the lens wide open, figure out what aperture you want to use, and then understand that the meter will have to bias in the opposite direction to get the right exposure. I.e., if you want to use the lens at f/2.8, then you'll need to underexpose by 1.3EV on the meter (f/2.8 is 1.3 stops smaller than f/1.8).