It's a question we get often. "What focal length should I use?". The answer is not clear cut. As with anything photography/cinematography related there's never one true answer or rule. Generally speaking though you'll find that you will be shooting most often between 16-50mm Any longer than 50mm and you'll start to find you need a faster shutter speed for photography and for video you'll start to see the effects of motion blur and instability. The best shots seem to come in around 15-26mm.
We're speaking of 35mm equivalents here so if your camera is a 1.5x crop (Nikon DX for instance) and you want to shoot at 20mm you'd divide 20mm by 1.5 putting you at shooting 13mm on your crop camera. For full frame sensor cameras like Canon 5D no conversion is required.