Really depends on what yeast you are using. For a lager, 50 would usually be acceptable for fermentation, but 50 would be way too low for say a Belgian Ale strain.
Thats exactly it... the "normal range" depends on the strain of yeast. For example: Wyeast 1007 German Ale likes to ferment at 55-68 degrees. Whereas Wyeast 1214 Belgian Abbey likes to ferment at 68-78 degrees and Wyeast 2001 Urquell Lager likes 48-56 degrees.