The only way to change shutter speeds in aperture priority mode is to change the aperture, causing the shutter speed to change accordingly. If you want to change shutter speeds manually, you need to use either manual or Shutter priority mode. In Aperture mode, the camera gives flash synch from 1/60 to 1/250, according to the aperture set, unless you change the flash synch mode to Slow Synch, in which case the camera will set any shutter speed slower than 1/60. I have used speeds as slow as 1/5 (still on aperture priority auto) to get blur effects along with the flash. You cannot, of course, use a synch speed faster than 1/250 with the F100.