If you're a first-time home buyer, you're probably thrilled that prices are falling in two out of every three cities.

But have they fallen enough to make homes in your area truly affordable?

Maybe not.

"In spite of the downturn in the housing market ... affordability continues to be the No. 1 housing challenge," Rachel Drew, research analyst at Harvard University's Joint Center for Housing Studies, recently told the Associated Press.

The median resale price for existing homes has dropped about 9% from $230,100 to $208,600 over the past two years, according to the National Association of Realtors.

Yet a recent study by Moody's Economy.com found households making 120% of the median income were unable to afford the median-priced home in nearly half of the 40 major metro areas.

Economists usually define an affordable home as one that doesn't cost more than 28% of your monthly, pretax income, and that includes mortgage payments, property taxes and condo fees.

Lower median prices have helped make many real estate markets more affordable, but experts say they're not deep enough in many major metro areas to narrow the affordability gap for police officers, teachers, nurses, restaurant and retail workers, and many other vital service jobs.

The 10 areas with the biggest affordability problems include San Francisco, Los Angeles, Miami and Stamford, Conn.

"In many metropolitan markets, certainly in California, you can earn 120 percent of the median [income] and still not be able to find anything affordable, or that's in a reasonable commute distance," Barbara Lipman, research director of the Washington-based Center for Housing Policy told the AP. "There just isn't a sufficient supply of housing for moderate-income people."

Here's how to tell how much home you can afford.