I've always heard it said that the 50-60mm lens (all 35mm format) approximates the part of vision that you mentally 'focus' or concentrate on, the 35mm lens approximates what you just 'see' in a glance, and the 21-24mm is about what your visual field will cover with peripheral vision. Personally, if you held a gun to my head and told me I could have only one lens (and my wife would probably reward you for this) I would carry a fast 35mm lens.