Warren Buffett continues to add to his already large stake in Apple Inc. (AAPL).

In an interview with CNBC on his birthday, the famous investor said that Berkshire Hathaway Inc. (BRK.A) had bought “just a little [more]” shares in the iPhone maker since the end of the June quarter.

According to its last regulatory filing, Buffett’s firm had increased its stake in Apple to 252 million shares in the second quarter. According to Bloomberg, Berkshire's investment in Apple is worth more than $50 billion, making it the third-biggest shareholder in the Cupertino, California-based company with a more-than 5% stake. (See also: Warren Buffet's Berkshire Portfolio is Now 24% Apple: 13F.)

Buffett Believes the iPhone is "Enormously Underpriced"

Buffett told CNBC that he continues to buy Apple shares because of the strength of the company's brand and ecosystem. The investor was particularly enthusiastic about the iPhone, describing Apple’s flagship product as an indispensable device for a large portion of the world’s population.

"I do not focus on the sales in the next quarter or the next year," he said. "I focus on the ... hundreds, hundreds, hundreds millions of people who practically live their lives by it [iPhone]."

Buffett also brushed off criticism that the latest iPhone models are too expensive. Rather than balk at paying $1,000 for the device, he said people should put the cost into perspective and appreciate that the iPhone is so important that it surpasses the value of many more expensive objects. That observation led him to describe Apple’s flagship product as “enormously underpriced.” 

“I have a plane that costs me a lot, a million dollars a year or something of the sort,” he said. “If I used the iPhone -- I use an iPad a lot -- if I used the iPhone like all my friends do, I would rather give up the plane.”

To further emphasize his point about the iPhone being undervalued, the Oracle of Omaha said that some people spend $1,000 on a dinner party. "Now it's got competition so you can't push the price, but in terms of its utility to people and what they get for a thousand dollars...you can have a dinner party that would cost that, and here this is, and what it does for you, it's incredible,” he added. (See also: Apple Buys Maker of Augmented Reality Lenses.)