The Portland Museum of Art’s retrospective of the work of Richard Estes illuminates one of the enduring questions in painting — the photograph. The camera, in one form or another, has been around since before Degas and even Vermeer. Nearly every artist who has relied on the contraption has felt ambivalent about it. This astonishing exhibition suggests that Mr. Estes might be the first painter to conquer the photograph.

Like many other photorealist paintings, Mr. Estes’ canvases — there are 50 of them in the exhibition in Maine — are, at first blush, hard to tell from a photograph. Yet when one gets to pause in front of them, to bend over and peer closely, to step back and squint, one sees a painterly quality. There is an efficiency to Mr. Estes brush (or knife) strokes, a sense of color, and a commitment to his motifs, particularly, but not exclusively, in the city.

Mr. Estes has, in effect, made photographs not his tool or his prop or his shortcut but his subject, though he himself likens them to preparatory sketches. What a breakthrough his painting is in an age when photographs — snapped by the millions each day through the I-phone and Instagram and I-pad and Coolpix — have become so ubiquitous. Mr. Estes uses what the museum calls “amalgamations of multiple photographs” for a single painting.

It calls them “dramatic and complex to the point of ambiguity.” One can see this in a painting of cars parked beside a park in Manhattan. The curators of this show have mounted it adjacent to one of the photographs on which the painting is based. At a glance it is hard to tell which is which. After a few moments one begins to see that the painting contains subtle improvements, a barrier removed, license plates with no numbers, better color.

Many of Mr. Estes’ urban masterpieces are here. The bus beside a car with a reflection of the Flatiron Building, the painting of the buildings reflected in the window of a Horn and Hardart, and, to my eye, a particularly beautifully painted street scene in Paris. The phone booths in front of a diner. There’s a memorable painting of the Pont Neuf at Paris. These are too many masterpieces in this show to comment on one-by-one.

What astonished me were the less urban landscapes. This began to dawn on me with a painting of Central Park, where the buildings are but in the far background and another of an ancient Roman bridge at Cordova, Spain. “Sea of Marmara” is a magnificent paint of a sunset over calm water. There are marvelous paintings of Mount Katahdin, a beaver damn on Mount Desert, and the Hubbard Glacier.

My own favorite, though, is a smaller painting of a trail at Acadia National Park. It is just a simple scene of the cerulean sky reflected in a rill. It discloses not only his sense of abstract forms but his capacity to capture color —and the deftness of his brush strokes. A tree or a twig put down with, seemingly, a single, probably swift but unerring movement.

Or not. I found myself thinking of the line from Yeats, “A line will take us hours maybe; Yet if it does not seem a moment’s thought, our stitching and unstitching has been naught.”

This is another summer that I’ve stopped by the Portland Museum to discover a show that is truly special. A few years ago it was “Winslow Homer and the Poetics of Place.” Last year it was “The William S. Paley Collection: A Taste for Modernism.” These exhibitions are a reminder of the glory that a medium-sized museum can be.

From the Estes exhibition one can stop by a separate gallery within the Portland Museum to see a showing of some of the museum’s masterpieces from an earlier time — Winslow Homer, Albert Bierstadt, Thomas Cole. It makes clear how well the modern master compares. It’s an optimistic thought, a reminder that even with such timeless tools as paint, oil, brushes, and canvas, each generation can hand up its own heroes.

Richard Estes’ Realism is up through September 7 at the Portland Museum of Art, 7 Congress Street, Portland, Maine, (207) 775-6148, portlandmuseum.org.