After the close last Tuesday, AFLAC (NYSE:AFL) reported fourth-quarter earnings of $1.48 per share. Make no mistake: this was a solid quarter for the duck stock, and it was squarely in line with what they told us to expect. Three months ago, AFLAC said Q4 EPS should range between $1.46 and $1.51.

But here’s the issue for us: Since most of AFLAC’s business comes from Japan, their bottom line can be adversely impacted (or helped) by fluctuations in the yen/dollar exchange rate. Lately, the government in Japan has aggressively stated its intention of pursuing a pro-inflation policy. That’s caused the yen to tank against the dollar. In response, the Nikkei Index has soared.

As I said, AFLAC as a business is fine and dandy and as strong as it’s ever been. I want to make it clear that I’m not overly worried about the exchange rate, but I have to say that it’s an issue for investors. AFLAC said the falling yen cost dinged their Q4 by four cents per share. Not fun, but not a disaster either. Bear in mind that AFLAC’s full-year earnings for 2012 were actually helped by one penny per share, thanks to the exchange rate. So it works in both directions.

For all of 2012, AFLAC made $6.60 per share in operating earnings. The company said it sees operating earnings growth of 4% to 7% for this year. On a currency-neutral basis, that means operating earnings of $6.86 to $7.06 per share.

Now here’s the tricky part (warning: math ahead). Each move in the exchange rate of one yen from 78.5 will cost AFLAC 4.3 cents per share for the year. So if the exchange rate averages 90 for the entire year, that will cost AFLAC 49.45 cents per share (11.5 times 4.3).

That stings, but it’s roughly 50 cents per share out of $7 of earnings. It’s not enough for me to change my opinion that AFLAC is a very solid stock to own. And of course, I have no idea what the exchange rate will do this year. However, I suspect that most of the damage to the yen has already been done.

Shares of AFL pulled back after the earnings report, and stand right around $50 per share as of mid-day Friday,  but the stock is basically where it was three months ago.

AFLAC remains an excellent company. Due to the recent pullback, I’m going to lower my Buy Below to $54 per share.