The Smithsonian Institution's National Collection of Fine Arts has acquired a major Western landscape by the painter Albert Bierstadt (1830-1902).

Grandiose and dramatic, "Western Landscape With Lake and Mountains" may be the most important picture given to the gallery since it opened in its present home in 1968.

The 6-by-10-foot panorama was left to the museum by the late Helen Huntington Hull, whose great-grandfather, William Brown Dinsmore, bought it from the artist in 1868. Bierstadts of this sort and size might fetch as much as $700,000 in today's inflated market for 19th-century scenes of the Old West.

Part travelogue, part opera, the picture perhaps prophecies both Big Sky western movies and the intimidating, mural-sized abstract expressionist canvases painted after World War II. With its vertiginous peaks and swirling skies, its ducks and deer and lonesome pines, the painting seems to typify that blend of awe and fear which 19th-century esthetes described as "the sublime."

Though Bierstadt was born in Germany and later studied at Dusseldorf, he is best known to art history for his half-invented scenes of the newly explored West. In 1858 he joined a government surveying team sent out to map an overland route from St. Louis to the Pacific. Returning with his sketches and photographs to his New York City studio (another famous landscape painter, Frederick Edwin Church, worked in the same building), Bierstadt composed huge and costly landscapes which he sold with ease to the Victorian rich.

If "Western Landscape With Lake and Mountains" is at once comforting and frightening, it also entertaining. Those who study it with care will discover with delight a secret rainbow trout swimming underneath the gentle ripples at the lower left.

Dinsmore glued the painting to a wall of his Hudson River mansion in New York's Duchess County. Since 1977, when it was given to the museum, a team of NCFA conservators has spent over 600 hours cleaning, relining, mounting it on an aluminum honeycomb panel, and returning it to its frame, which they luckily discovered in one of Dinsmore's barns.

Its original title, "Dream Lake, Estes Park, Colorado," baffled NCFA scholars, for it looks nothing like that spot. Newly, though more ambiguously titled, the Bierstadt is now hanging among other 19th-century landscapes on the museum's second floor.

Bierstadt's landscapes made him rich enough to build a palatial 35-room mansion at Irvington-on-Hudson. The contemporary success of the style called photo-realism suggests that Americans are just as fond of light effects, reflections and highly detailed paintings today as they were then.

Another branch of the Smithsonian, the National Air and Space Museum, meanwhile has been given the Stuart M. Speiser Photo Realism Collection, a set of 22 recent paintings whose common theme is aviation.

In 1973, attorney Speiser asked well-known photo-realists - among them Richard Estes, Ralph Goings, Robert Bechtle, Malcom Morley and Audrey Flack - to paint pictures on the theme of flight. Estes did a reflection-laden view of an Alitalia office; Noel Mahaffey did a scene of a TWA hanger; Charles Bell, Audrey Flack, Jerry Ott and Tom Palmore responded with pictures of planes that are toys.

Most of the paintings given Speiser look more like detailed magazine illustrations than museum works of art.

The photo-realists stress the bland - city windows, parking lots and snap-shot-sterile views of men posing beside planes. Though their paintings hint at deeds of derring-do, at flight and exploration, in spirit they seem small.

The Speiser Collection is the largest collection so far given to the Air and Space Museum. It will go on view in March 1980 in a show titled "Flight and the Arts."

Though few will find them beautiful they belong in that museum. "Ninety percent of the people who come in here," observes James Dean of the Air and Space Museum, "are looking for airplanes, not for art."