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The Canon BG-E2N battery grip is compatible with the Canon EOS 20D, 30D, 40D and 50D digital SLR cameras and replaces the older BG-E2. It was introduced when the 40D came out and has improved sealing around the battery compartment to be more weather resistant inline with the 40D and 50D cameras.
I have two 40D cameras and I use a battery grip on each of them, in fact it was the first accessory I bought after I bought the camera. Once the battery door of the camera has been removed the grip fits easily into the bottom of the camera and screws in to tighten in place. But fear not, those clever people at Canon have thought of everything and the grip has a little space inside it to store the battery door of your camera. I was mightily impressed by this, as I'm sure I would have lost the door by now otherwise!
The purpose of a battery grip is twofold - firstly it doubles the shooting capacity of the camera by accepting two BP-511 batteries instead of the just one which usually fits into the camera. Although I only ever use one to keep the weight down.
Secondly it gives for more comfortable and easier vertical shooting. If, like me, you shoot a lot of your photos on the portrait position, then it's invaluable as you don't have to adopt that awkward shooting position with you elbow stickling out to the side. Instead you simply rotate the camera and use the shutter release button on the grip which enables you to keep the same stable shooting stance you would use if shooting in the landscape position. Much more comfortable!
As well as the shutter release button the BG-E2N also has the main dial for changing shutter speed or aperture, the AF selection point button and the AE/FE lock button. These last two button also double up as zoom buttons when viewing images on the 40D camera screen. So you can access the major functions without having to keep rotating the camera back to landscape.
The grip is comfortable to use and actually make the whole camera feel more balanced, especially if using large or heavy lens. Changing a battery is a doddle, the battery compartment pops open easily and the battery pops out at the press of a button. Another advantage, although I admit to never having used this, is the cartridge tray that comes with the grip and accepts 6 x ordinary AA batteries. This could be a useful tool to get you out of trouble if you run out of power in the rechargeable BP-511 - most shops will always stock AA batteries! As you would expect the bottom of the grip has a screw thread so you can still mount your kit onto a tripod.