I'm thinking of buying my husband a D7100. Now I've never fully understood the 1.5 crop factor and there always seems to be disagreements about what this means to the length of your lens. I noticed in the D7100 specs you can have a crop factor of 1.3 (I assume by means of a switch). One retailer is advertising this effectively changes your 70-300 to 140-600. In relation to shooting wildlife what changes would he notice?