The mode is effect for controlling the depth of field in you flash pictures. You can also obtain a correct exposure for the subject and background. You set the aperture and the camera sets the shutter speed automatically for the correct exposure of the background. The TTL autoflash system obtains the correct flash exposure based on the aperture you set

Set the camera shooting mode to Av and set the aperture Set the MR-14EX's flash mode to TTL Focus and take the picture Check that the flash icon (flash ready) is displayed in the viewfinder before taking the picture