i was looking at this lens new to photography have a xti and i was wondering if this lens is good for a new lens its got good reviews on bhphotovideo and it cheap but all the reviews said good is it good for a beginner lens http://www.bhphotovideo.com/c/product/391074-REG/Sigma_508101_70_300mm_f_4_5_6_APO_DG.html