Yes - it's essential to actually take the still picture to register the aperture. The iris does not move until you click the shutter.
You can put the camera in S and then adjust your exposure using exposure compensation (+/- 5 stops), until the desired aperture is reached. Then click the shutter and the aperture is set. Before clicking the shutter, the image is darkened and lightened by the camera to simulate the stop you'll be using (so you can visually dial in exposure) but the stop won't actually register until you click the shutter button.
