When to use Av mode
There will be times when the aperture used for an image is more important than the shutter speed. When shooting landscapes, for example, you will often want everything from the foreground to the horizon to appear as sharp as possible. This requires a small lens aperture, such as f16 or f22, to give a wide depth-of-field. At other times you might want a narrow depth-of-field so the subject stands out sharply against an out-of-focus background.
That’s why the best shooting mode for these effects is aperture-priority, shown as Av (for ‘aperture value’) on the shooting mode dial. By using this mode, you can concentrate on the aperture setting for your subject, whilst leaving the camera to set a suitable shutter speed that takes your aperture into account, whilst ensuring that the shutter speed is fast enough to avoid the effects of camera shake.
If the shutter speed flashes in the viewfinder, then the exposure is out of range.