The photo opportunity was invented only a few years ago. But already -- such is the speed of modern technology -- there comes a diplomatic version: the photo summit, to which importunate leaders of West Germany, Britain and Japan are being exposed.

The treatment consists of an official state visit replete with highly visible marks of personal esteem. The visitors go away beaming, and while they haven't exactly influenced U.S. policy, the decisive fact is that they don't seem to care.

The stage for the photo summits was set by the landslide election victory in November. The sweep of 49 states staggered the world and raised President Reagan's international prestige to a record high. Leaders of friendly countries lined up to be seen with the new World Champ. Those with heavy burdens especially sought the solace of his touch.

So it was child's play for the White House staff to invite in only the very best friends. It was equally easy to impose conditions that preserved the lead part for Reagan while shuffling the visitors among the spear carriers. What ensued has been a paying of tribute by the richest of his vassals to the Emperor of the West.

First came Helmut Kohl of West Germany. A spreading bribery scandal besets the chancellor, his Christian Democratic party and its partners in rule, the Free Democrats. The opposition Social Democrats are not immune. So it is easy picking for those radical ecologists- cum-anarchists -- the Greens. In a moment of such wobbliness, simply being seen with the Emperor of the West at the White House has a steadying effect on the chancellor.

To be sure, Kohl had a few thoughts of his own. He was keen for the Emperor of the West to get together early in a meeting with he Emperor of the East, Konstantin Chernenko. He saw a scheduled session between their grand viziers -- George Shultz and Andrei Gromyko -- in Geneva on Jan. 7 as a good beginning. He feared, however, that if the dialogue dipped to lower levels, it would be lost in a swamp of nasty, technical recrimination.

But in all, Kohl had about 12 working hours in Washington. His message was all but lost in the briefness of the encounter. Shultz and Gromyko will both bring with them to Geneva delegations heavy with expert nay-sayers. A long journey through the swamps of discord is far more likely than the leap toward the East- West summit Kohl desires.

Just as short was the visit made by Margaret Thatcher a couple of eves before Christmas itself to the president's sylvan retreat at Camp David. The prime minister needed those signs of special favor. She had just come halfway round the world from a meeting in Peking, marking the cession to China of the remaining jewel in the crown, Hong Kong. She also faces a coal-mining strike at home, a sinking pound and an economy that is not yet singing. So there was reason to lay a weary head on the broad shoulder of the president.

For all her troubles, though, Thatcher also had a message. The Emperor of the East had sent his likely successor, Mikhail Gorbachev, on a visit to London before she left for Peking and Washington. Thatcher deduced from their chats that the Russians were prepared to talk about limiting their offensive nuclear missiles if the United States in return restrained its new program for an anti-missile defense based in space -- the so-called "Star Wars" project. So she cautioned Reagan that "Star Wars" had best be kept as a research project -- and not moved to development except after extensive consultation.

A receptive ear semed to catch that message. Thatcher's man at the Court of St. Ronald, Defense Secretary Caspar Weinberger, and Robert McFarlane, the national security adviser, both declared that "Star Wars" was not excluded from the Geneva talks with the Russians. But hardly was Thatcher out of the country when Weinberger, prompted by the president, took it back. He said Reagan "will not give up" the "Star Wars" program "or the opportunity to develop it."

Early next week, the White House welcome mat will be out in Los Angeles for Yasuhiro Nakasone. The Japanese prime minister has just been reelected chief of his party after a surprisingly nasty fight. His enemies sit all around him -- in the foreign ministry, the finance ministry and the chief party offices. Rapport with Reagan is essential to foil these foes.

But Nakasone also brings words of counsel. He wants the United States to take the lead in keeping the world trading system open. He even thinks it is time to renew some of the trade ties with Russia, ruptured in deference to political feelings in Washington about nasty Soviet doings in Poland and Afghanistan. But he, too, will have only a few hours to get those views across.

Which suggests that the visiting leaders are far more interested in being seen with Reagan than in influencing his policies. The foreigners are talking for the record against the day when they may want to get serious. But for the time being they are giving Reagan a free hand, and he is probably less constrained by allies than any president in the postwar period.