Use this chart to determine what focal length you should use depending on the horizontal angle of view you need to capture. The longer the focal length/narrower angle of view, the more flattening that occurs in the image, meaning that objects that are at a distance from one another appear to be in much closer proximity and within a flatter space than on a wide angle/short focal length. A wide angle lens will increase perceived spacial depth and at the same time increase distortion.
*The following is for full-frame/35mm cameras. You will need to do some conversions if you shoot with a different sensor size such as a Canon with a X1.6 crop factor or a Nikon with a x1.5 crop factor you will need to multiply the focal length indicated on the lens by the crop factor before referring to the chart [50mm x 1.6 = 80mm]. There are ways to convert for other image sensor sizes. Another popular one is for the Four Thirds System. The Four Thirds website lists the 35mm equivalents of all their lenses here: http://www.four-thirds.org/en/microft/lens_chart.html