With interest rates near rock bottom and home prices down, this ought to be a great time to buy a home. But for most people, it's a lousy time to get a mortgage.

Years after the collapse of the real-estate market and resulting financial crisis, it takes nearly pristine credit scores and hefty down payments to get the best rates.

"Since 2009, credit has become a lot tighter," says Greg Reiter, who follows mortgage-backed bonds at RBS Global Banking&Markets.

For borrowers, this highlights the need to pay close attention to credit scores. New rules unveiled last week should make it easier for consumers to see how their credit scores affect the interest rates they pay. These rules, the result of last year's Dodd-Frank financial-services legislation, require banks and other lenders to disclose to consumers the scores used to determine interest rates charged borrowers, or to deny credit.

The new reality for borrowers can be seen in the FICO credit scores on the loans that banks are giving out and that are backed by government agencies Fannie Mae and Freddie Mac. These days the two agencies essentially finance 75% of all mortgages by purchasing the loans from the banks. In the process, they shape how much it costs to borrow.

FICO scores range from 300 to 850. Pre-crisis, a score of 700 to 725 was deemed solid and a borrower could expect to get a "conventional" mortgage at the lowest rates.

From 2003 through 2006, 82% of Fannie Mae mortgages were for borrowers with a score between 700 and 750, according to data compiled by RBS.

But so far in 2011, only 13% of Fannie Mae mortgages carry that score, and just 1.7% have a score of 700 to 725, according to RBS. This year, 75% of Fannie Mae mortgages are for FICO scores of 750 to 775, up from less than 5% before 2005.

Meanwhile, the median score is 711, according to FICO.

"Half the population is locked out" from the best mortgages, says Mr. Reiter.

The upshot is that borrowing costs more even with a 730 score and a 20% down payment, says Norman Calvo, president of Universal Mortgage in Brooklyn, N.Y.

"Three years ago, if you had 730 it was excellent," Mr. Calvo says. Today, he says, it could cost an extra 0.125 percentage point per year on a mortgage, "just because you have one little nick on your credit report."

For more typical scores, the premiums are even bigger. At 700 to 725, it's usually an extra quarter percentage point, and at 630 -- if a borrower can find a loan -- the additional cost is 1.5 percentage points, Mr. Calvo says. "If you have a credit score of less than 680, you've got to be worried about approvability."

The news is also grim for those looking to refinance. Based on the level of interest rates, RBS estimates 60% of agency-backed mortgages should be eligible to refinance. But once home values and credit scores are factored in, just 12% are eligible.

These trends show the importance of understanding credit scores. Mr. Calvo says borrowers sometimes unintentionally make matters worse. For example, closing an unused credit card can actually lower a score in the short term, he says.

Check your credit scores at AnnualCreditReport.com. And to learn more about scores, visit the education section of myFICO.com.