The Vivitar Battery Grip provides the power you need for the Canon EOS 40D and 50D SLR digital cameras. It adds to the power by using 2 BP-511A rechargeable, high-capacity batteries. In addition, the battery pack provides a vertical shutter release, that makes holding the camera in the vertical orientation as easy and secure as in the horizontal position.The versatility of using the Battery Grip is three fold. Two BP-511A rechargeable batteries will allow you many hours of uninterrupted shooting. Plus the vertical shutter release will make vertical shooting as comfortable as the horizontal format.This grip also comes with an adapter that allows you to place six "AA" batteries for even longer extended shooting times.