How to Adjust Aperture for Long Exposure Shots
For long exposures, there are three factors to consider: speed setting, ISO and aperture. Normally, long exposures would depend greatly on the first two factors--speed to determine how much motion you wish to capture, and the ISO setting depending on your lighting conditions. Normally, long exposure shots are used for night photography. So this would translate to a slow shutter speed, and an ISO speed on the high side, such as ISO 400 or even 800.
For Long Exposure
Choose the "A" priority or Aperture priority on your camera. A rule of thumb: the longer the exposure, a smaller aperture is recommended. Determine the depth of field you want to achieve without compromising your shutter speed too much. A lens aperture of f/5.6 will provide a good sharp subject, and a background in soft focus.
From hereon, your camera can automatically adjust your shutter speed. If you wish for a slower or faster exposure, adjust your aperture accordingly. For wide shots, it's advisable to go for a smaller aperture or higher f-stop number. A lens aperture of f/8 or f/11 is recommended, and this will also give you more leeway with the length of your exposure.