The aperture ring is controlled by the camera on the AF bodies, it's opened to the max of the len's aperture on the body once mounted (ie f/3.5 at 18mm in this case), once the shutter is released, it will adjust the aperture to the setting on the camera via a mechanical coupling. If you noticed, the aperture can be f/3.5 f/4, f/4.5, f/5.6 f/6 etc. These apertures are achieved by the mechanical coupling within the body & the len.shutterfly said:Curious to know why must you to do that to use the camera.
If you have a len with aperture ring, you'd note that the aperture values on the aperture ring is only limited to 1 stop each, ie f/2 f/4 f/5.6 f/8 f/11 f/16, aperture inbetween the values is not possible. AF bodies make it possible.
On manual bodies, the aperture ring is controlled by the photographer, so you can mount it at any F-stop, f/22 f/16, f/14 but you cannot set any aperture at half or 1/3 or 2/3 stop.
Thus on AF bodies it must be set to the min aperture before you can fire, else you'll get Fee on the display indicating aperture error.