The optional Power Booster PB-E1 or Battery Pack BP-E1 attach to the base of the camera. The booster (PB-E1) holds 8 AA batteries or an optional Ni-Cad pack, and boosts the standard drive from 3 frames per second to 6(AI Servo AF: 5). The Power Booster E1 has its own shutter release and input dial for use when shooting in the vertical format. The battery pack (BP-E1) is a lighter and simpler accessory. This holds 4 AA cells and a lithium battery, and the user can select between the two power sources with a switch. It provides a grip for vertical shots but no additional controls. The major appeal of the Battery Pack BP-E1 is that is lighter and less expensive than the power booster. There is also a PB-E2 (has AF-field-button, a wheel around the shutter-button). ATT: Statement about input dial on PB-E1 is wrong. PB-E1 has 2 buttons (shutter button, AE-lock) and vertical control switch (on/oFF) and nothing else. Input Dial is then only on PB-E2!Also called Power Drive Booster PDB. AA-Lithium -Batteries only if AE-Lock-button has a star * imprinted.