Cheap narrow field lens to detect red laser light?
You'll want a lens at least as big as your laser spot size at 50m.  (How are you holding everything steady?  You don't want the beam bouncing around.)  If this is in daylight you'll also want a tube/ shroud around the photo diode to reduce all the stray light getting in. (lens well inside the tube)  A filter could help also.  If money is no object an interference filter centered around the laser wavelength is nice.    

What's the application?