Okay somebody help me understand this. While playing around with my 420ex on top of my rebel XT, I was alternating between aperture-shutter-program mode. Using program mode the camera was happy with 1/60 & F5.6, shooting at something on my wall about 8 feet away. When using either aperture or shutter priority, the camera wasnt happy until the shutter speed was around 0"3. Why such a difference in shutter speeds. I mean the flash went off everytime, the subject did not change nor did the distance.