The Federal Reserve reports more banks are tightening their lending standards for subprime mortgages. That's the type associated with a wave of foreclosures that have rattled financial markets. The survey found that nine of the nation's 16 largest banks, which accounted for more than half of all home loans, upped their lending criteria for things such as adjustable rate mortgages. That's the type often offered to borrowers with weak credit. The Fed even found some banks are tightening lending standards for the traditional fixed-rate mortgages that they offer to their best customers.