8dbb5d98_Film_Cameras__3
[0] Aperture Priority Automatic Exposure [1] Very useful, especially for telephoto work. Most cameras have this. [4] The main batteries in many cameras are expensive. An external battery pack uses standard or rechargeable batteries. This adds weight, but gives you more power - sometimes the only power in remote locations. [3] 
Accessory Battery Packs
Film Cameras
3