The whole system is designed to keep the viewfinder bright by keeping the lens open to its widest aperture until the shutter button is pressed. The preview button is there simply to allow you (the photographer) to judge just how limited or extensive the depth of field will be when you take the image. If your lens is set to 1.8, nothing will happen when you press the DoF Preview button. If the lens is set to minimum aperture (16, 22 on this one?), the viewfinder will get dark but you should also notice that the DoF will deepen substantially. Intermediate results at intermediate aperture settings.