Strolling into the vice presidential mansion yesterday afternoon, Fritz Mondale encountered a 4-by-3-6 foot camera occupying much of his reception room.

"Well, I guess this is a big enough camera to capture the egos in this town," he said.

Perched on a step ladder, the grand old man of American photography, Ansel Adams, was fine-tuning the composition for the first of two official portraits he is making here this week. Yesterday it was Mondale's. Today he will photograph President Jimmy Carter at the White House.

Official portraits have often been touchy endeavors, particularly since President Lyndon Johnson called the oil painting of him by Peter Hurd "the ugliest thing I ever saw." (Didn't hurt me one," Hyrd responded.)

Carter put a new twist on portrait controversies when he suggested that photographs rather than oil portraits of himself, Mondale and the cabinet might be a good way to cut costs and symbolize frugality. Adams' work is even more in keeping with that request: He is doing it for free.

Adams, who is known, for the awesome detail of his work, frequently spends days refining his compositions. Yesterday he had 45 minutes to set up his cameras and one hour to get his shots.

"I want you to move just a little bit this way," he said to Mondale, who was standing on the main stairway of his house.

"I hate to move to the right," came the response. "Do you think you can capture my beauty, Ansel?"

"If not, we'll bring in a bigger camera."

Admas gently ordered Mondale about: "Stand a little straighter, but lean forward." "Move the hands up just an inch on the railing." A little bit over now so that painting doesn't slip under your arm."

Mondale responded in kind.

"My office is good for this kind of work," he said. "I stand where I'm told."

Unlike Johnson, who didn't see Hurd's oil portrait until months after his sitting, Mondale got a unique chance to see Adams' work envolve. The artist made two sets of photos yesterday: his usual outdoor black and white on the mansion's porch; and a series of color shots with the huge Polariod in the reception room. It delivers 20-by-24 inch finished prints in 75 seconds.

"Prepare for an Armageddon of light," Adams cautioned Mondale, just before a huge bank of strobes fired off for the first shot.

The photography squeezed the camera's shutter release, and then stepped away, as five assistants took over the camera.

"All I do is push the buttom," he said. "This is a union shop."

The paper sandwich containing the print rolled slowly from the camera's belly. One assistant cut the print off from the roll, and then taped it to a plexiglass easel. After 75 seconds a buzzer in the camera sounded, and the technician peeled away the top of the print.

"Not quite right," said Adams, pointing to a few shadows around the eyes that no mortal man would notice. Mondale looked at the print with his wife Joan, who suggested that he smile and show some of his teeth.

"Don't worry about the eyes," said Adams. "We can fix that.Everybody's got different eyes."

"These are the only two I've got," said Mondale.

Adams snapped another. He examined it, and then coached Mondale into a different position.

"You're a wonderful subject," he told the vice president. "I think the worst subject I ever had was Conrad Hilton, who was 82 years old. He refused to allow me any more than five minutes, so we had to find some little guy to sit for him while I composed.

"Well, we finally got the old boy in the chair for the shot, and he was supposed to hold up a Carte Blanche credit card. Only he was so old that he had the shakes, and he couldn't hold the card still. We tried to tape his arm to the chair, but even that didn't work."

Mondale countered with a political tale.

"When I was in Ames, Iowa, somebody asked me how Jerry brown would change the presidency I said he'd substitute a glider for air Force One. To know where he's coming from you'd check the prevailing winds."

Adams snapped several more Polaroids, refining the portrait with each successive shot. The final picture was a rich, vibrant visage of a smiling Mondale.

Adams and his subject then headed outside for the black and white portraits.

Mondale walked past an assistant drying the Polaroids with an electric hair blower, and said, "I hope you're not getting any asbestos on me."

Outside, Adams shot six or seven sheets of black and white film, again moving Mondale after each shot. Finnally, Mondale announced, "You've got the one you're going to get," and headed for his waiting limo.

A minute later Ansel Adams wiped the brow under his Stetson.

"Whew," he declared. "I think it came out pretty good."