In “Shutter Priority” mode, you manually choose the camera’s shutter speed and the camera choosing aperture depending on the amount of light entering the lens. This setting is supposed to be used to freeze or artificially blur motion. If there is an extreme quantity of light, the camera will adjust the lens aperture to a larger number, which will result in a reduction in the amount of light that is allowed to enter the camera. If there is not enough light, the camera will adjust the aperture to the smallest value possible, which will allow lighter to enter the camera via the lens. In Shutter Priority mode, the shutter speed stays constant (what you’ve set it to), but the aperture adjusts automatically dependent on the quantity of light. In addition, there is no control over subject isolation, since the depth of field is controlled by the camera.