A well-designed tool does the job for which it is intended efficiently. But some tools incorporate such brilliant engineering, perform so well, and appear so ineffably beautiful to the people who get to use them, they take on the status of works of art themselves. One thinks of Stradivarius violins, or Beretta shotguns, or the 12,736 cc engines of the Bugatti Type 41 “Royale.”

The Leica 35 mm camera is that type of tool. Without the images photographers have taken with various model Leicas, our sense of what the 20th century looked like would be sadly diminished.

The Leica Gallery has opened a two-part exhibition, “Witness to a Century,” and W.W. Norton has published a book with the same title, written by Alessandro Pasi, to celebrate this achievement. This year, for the first time, digital cameras will outsell film cameras, and there is the sense of an era passing. So it is appropriate that Leica – the apotheosis of the 35 mm rangefinder camera – reflect on its accomplishments.

Not surprisingly, since no tool can perform any better than the craftsmen who use it, the list of Leica photographers is a list of greats. Consider our collective store of images of World War II: Robert Capa’s pictures of American troops landing in Normandy on D-Day 1944 are in the exhibition, as is Joe Rosenthal’s picture of six Marines raising the flag on Mt. Suribachi, Iwo Jima (1945) – the most replicated photograph of all time.

That picture inspired Yevgeny Khaldei’s photograph of two Soviet troops raising the hammer and sickle on the Reichstag over a devastated Berlin (1945). (Obscure fact: The soldier in the lower left corner was wearing two – looted? – wristwatches. Stalin had one removed.) Carl Mydans photographed the Japanese surrender on board the USS Missouri in Tokyo Bay (September 2, 1945) with a Leica.

Also in the exhibition are five frames from the roll of film (accidentally loaded into the cassette backwards by Life technicians) that Alfred Eisenstaedt used to shoot his picture of the “VJ Day celebration, Times Square” (1945): the sailor bending a hapless woman over double in an exuberant, amateur parody of a Hollywood embrace. For Americans who lived through the anxious years of war, that kiss is the kiss of peace. They loved it when Life magazine first ran it; they love it still.

The current Leica Gallery show covers 1914-59 and includes works by Andre Kertesz, Arthur Rubenstein, Ilse Bing, Erich Salomon, Humphrey Spender, Leni Riefenstahl, David Seymour (“Chim”), Gordon Parks, George Rodger, Cornell Capa, Elliott Erwitt, Marc Riboud, Inge Morath, Leonard Freed, William Klein, Bruce Davidson, and Henri Cartier-Bresson. What a list! What super photographs!

“Leica: Witness to a Century” (159 pages, $35) is a handsome book with wonderful illustrations and a disappointing text, scrimpy and shallow. No translator is credited, so Mr. Pasi will have to bear the blame. (A similar book, “The Leica Story: A History of the Leica Camera” by Emil G. Keller, a former employee, contains much interesting information, but is too amateurish.) The Leica story deserves a proper historian, one who can do justice to the talented, ambitious and, in many cases, idealistic people who made a small optical firm founded in Wetzlar, Germany, in 1849 into the manufacturer of one of the world’s most respected and commercially successful products.

Many, but by no means all of those people were descendants of Ernst Leitz I, who took over the optical firm in 1869 and changed its name to his. It was Ernst Leitz II who gave the go-ahead to begin manufacture of the first Leica (from Leitz and camera) which became an instant hit when it was introduced at the Leipzig fair in 1925. But the indispensable man was Oskar Barnack, a technician who came to Wetzlar in 1911 to help develop motion-picture projectors. Barnack was a talented photographer who had asthma, and hauling around the large camera and heavy glass plates in their big leather case was a problem for him. He thought, “There must be a better way.”

Using smaller glass plates would not work, because enlargements were too grainy. Still, Barnack kept thinking, as a sort of mantra, “small negative, large picture.” “Well, when I changed employment in 1911 (at age 31) and came to Leitz Wetzlar, my activities included designs for cine photography and it soon occurred to me that cine film might contain the answer to my problem because its grain size was much smaller,” he is quoted as saying “The cine format (18 mm x 24 mm) was simply too small … but it had become an international standard, and I decided on doubling its width to 24 mm x 36 mm – and the Leica format was born.”

By 1913 Barnack had designed and built two prototypes of a camera to use movie film – the Ur-Leica. Production did not begin for another decade because of World War I, but the advantages of the Leica I were immediately appreciated. The Leica was lightweight and inconspicuous. It could be carried in your pocket and take 36 exposures before the film had to be changed. The lens produced im ages that remained sharp even when greatly enlarged. The talented Max Berek designed many of the early lenses, including the Elmar 50 mm f/3.5, the original standard lens.

Other companies soon made similar cameras, and some of them (for example, Zeiss Ikon and Voightlander) were very good cameras. But none was ever as sturdy as the Leica. Photojournalists especially relied on their Leicas to perform in the most horrendous environments – African heat, Arctic cold, the mud and abuse of warfare. Leica earned and maintained its reputation by continually making improvements: In each model series, there are modifications as this or that mechanism was tinkered with and incrementally changed for the better. The two current models, the M7 and MP, are probably the best 35 mm rangefinder cameras ever built.

And maybe the best that ever will be built. Leica recently issued more EU100 million of debt to finance the development of a digital body to use the M series lenses. Introduction is scheduled for 2006. Someone should tell the story up to now with the depth and detail it deserves. In the meantime, I look forward to the second half of the Leica Gallery’s show, which will open next spring.

Until January 15 (670 Broadway, Suite 500, between Great Jones and Bond Streets, 212-777-3051).