To set the aperture for a Canon EOS lens to a position other than the widest aperture is relatively simple:
- With the EOS camera powered on and the lens mounted select the aperture you wish in either Manual (M) or Aperture Priority (Av) exposure mode.
- Press the Depth of Field Preview button and continue to hold it down. Not all Canon EOS models include a DoF Preview button, but some will allow you to custom map another button to that function.
- Press the lens release button and remove the lens before releasing the DoF Preview button.
The aperture will remain in that position until it is reconnected to a powered up EOS camera.