can someone explain the following please. i was taking some pictures of a band this evening and had the camera in a/v mode aperture 2.8 the camera was giving me a shutter speed of around 30th of a second not very quick and lots of movement on the shots, but i then set the camera to manual mode chose 2.8 aperture and 1/250 as the shutter speed and got very well exposed images, is there somethng i am missing ?