Since it started acquiring photographs five years ago, the National Portrait Gallery has amassed quite a collection: about 7,500 images at last count. A bit of the breadth and quality of the enterprise can be judged in a summer exhibition of 61 selected photographs that bounces lickety-split along the bumpy road of American history.

Mainly the exhibition makes lightning stops at familiar, if widely dispersed, gateposts of the nation's political, cultural and social story, and it even touches lightly upon its criminal record. The show encompasses a hundred years or so of presidents, presidents' wives (and daughters), generals, warriors, actors, actresses, dancers, poets, painters, dreamers and assorted crooks.

In this context there are no stars, but Lincoln, again, comes close. The first president to take advantage of the medium, he was photographed nearly 120 times, and these pictures almost always tell us more than painted portraits. No one would want to exchange the rarity included in this show--an original 1858 photograph that had been lost for a century--with the tidied-up painted image of the man by George Alexander Healy, on view elsewhere in the gallery.

One of the minor physical imperfections that shows in photographs of Lincoln, but not in the painting, is the mole on his lower right cheek. Moles-and-all verisimilitude was, of course, one of the attractions of photography before retouching and gauzy artfulness became art forms in themselves toward the end of the 19th century.

It is odd to see the point made again in an 1860 studio photograph of the landscape painter John Frederick Kensett, an image whose studied pose and elevated tone owes much to the conventions of painted portraiture. Still, the realism of the medium wins out: A tiny mole on the painter's face, by total happenstance in about the same place as the blemish on Lincoln's impressive countenance, refuses to go away.

Most of the portraits in this exhibition were posed, in one way or another, and it is tempting to say that the earlier, sometimes anonymous, studio hacks did it better in their unself-conscious ways than later,more polished photographers. William Tecumseh Sherman strikes a pose, for instance, in the portrait of him (a recent print from an original Mathew Brady studio negative), but his hard look is fearsome and real, while Arnold Newman's 1948 portrait of Alice Roosevelt Longworth is too artful at least by half, what with the sitter's left arm being mainly a diagonal prop leading to a background print of her father's charge up the San Juan hill.

It is a funny picture, maybe even intentionally so (the president's daughter is aptly identified in the label simply as a "socialite"), but her glazed eyes tell us almost nothing of her personality--except perhaps that she was justifiably bored with being pushed this way and that by the artist-photographer. Then again, as Hegel said, all things exist in order to be contradicted (or something like that), so that the smart spark of Anna Mary Robertson Moses ("Grandma Moses") comes through in Clara Sipprell's 1940 photograph, despite its vague artificiality.

A lot of these photographs confirm our expectations: We look for a steely stare from Gen. Sherman or a mirthful smile from Grandma Moses. The exhibit occasionally provides less predictable glimpses of the famous: Thomas Eakins and his sister Frances as children; Isadora Duncan without veils (although exotically clothed), relaxing at home.

And not all the famous folks are good guys--the Pinkerton Detective Agency recently gave the Portrait Gallery a selection of images, including a riveting look at the real Butch Cassidy and the Sundance Kid with friends--and not all the folks are famous. One of the more appealing early photographs in the show gives us a look at a little-known 19th-century artist named Vinnie Ream Hoxie, a study in contrasting impulses from her ringlet curls to her sharp, observant eyes.

The show in short plays an entertaining game of historical hopscotch on the way to demonstrating the wisdom of including photographs in the collection of the nation's gallery of portraits. Strangely, this was prohibited by law until the fall of 1977. Selected by William Stapp, curator of photography at the gallery since the law was changed, the exhibition continues through Aug. 22.