The Association of International Photography Art Dealers’ annual fair has been getting bigger – literally.

The 25th annual “Photography Show” begins today and the images being shown far exceed the 8-by-10-inch prints that were common when the show was founded.

Boston dealer Robert L. Klein (who is also Aipad’s president) will be featuring Paul Ickovic’s “Chambermaid, Prague, Czechoslovakia” (1978), a 6-by-4-foot, black-and-white print. (He’s also bringing an early Carleton Watkins, a mammoth plate from the 1870s.)

Photography is big today, and the Aipad fair is the can’t-miss event in the photo world. The largest show of photography in the United States, it attracts a worldwide audience of collectors and artists, and 75 exhibitors from a half-dozen countries. The work for sale spans the medium’s history, from masters like Roger Fenton and Walker Evans to contemporary artists like Beth Yarnelle Edwards and Mr. Ickovic.

The importance of the fair is underlined by the fact that it kicks off a de facto photography week in the city. Swann galleries is holding its seventh annual Valentine’s Day sale. The 113 lots include early experiments, Modernist prints from 1920s and 1930s, and more modern photojournalism.

Then on February 15 Christie’s has a photography auction emphasizing affordability and accessibility, aimed at bringing in first-time buyers. Estimates range from $150 to $30,000. It’s possible to get a William Eggleston or a Henri Cartier-Bresson for $2,500.

The field of fine-art photography is still a relative newcomer to the world of collecting and connoisseurship. Hybrid of art and science, photography, with its mechanical process, has always suffered as a poor relation of painting and the other plastic arts.

A century ago this year, Alfred Stieglitz opened his gallery, 291. There and in the pages of Camera Work, founded the year before, he set about securing a place for the photographic medium on the art map. Speaking in New York around that time on the means of assessing fine art photography, Stieglitz said, “The result is the only fair basis for judgment. It is justifiable to use any means upon a negative or paper to attain the desired end.”

His bold statement started a battle that continues today, namely, how to establish the value of photographs as collectible art forms.

Stieglitz understood that a dialogue between the artist, collector, and curator was imperative to the valuation process; at times he himself embodied all three roles simultaneously and single-handedly. The Aipad show is now the arena where just such a dialogue takes place. As Mr. Klein puts it, “It’s a crossroads for photography connoisseurship.”

Aipad began with conservative collectors and traditional works; the show is today seeing more high-end contemporary works and competitive prices. Some big-ticket items this year include works by Loretta Lux (Milo Yossi Gallery), Sarah Anne Johnson (Julie Saul Gallery), and Lalla Essaydi (Laurence Miller Gallery).The photo market has grown tremendously since the 1970s, but high Modernist photographs are getting ever rarer, and the contemporary photography market – like all other contemporary art – is booming.

Sotheby’s three auctions last April brought in $8,738,600, more than $2 million above the presale estimate. Diane Arbus’s “Identical Twins (Cathleen and Colleen), Roselle, N.J.,” brought in $478,400 – a record for the artist. The top sale by far in 2001 was Andreas Gursky’s “Paris, Montparnasse,” which sold for $600,000.

Six-figure photographs used to be exceedingly rare and the high-price trend has altered collecting and curating. A curator of photography at the Cleveland Museum of Art, Tom Hinson, said that size and conceptual intent are two areas in which there have been great shifts recently, as more artists make “photobased” works.

Aipad establishes the rules of the game. Buying a photograph can be a cause of real anxiety. Has the image been manipultaed? Who printed it? What is the provenance? For museums, too, there is the worry of deaccessioning. Aipad encourages dialogue in these areas. The editional aspect has always been a bone of contention for many would-be collectors, and Mr. Klein feels it’s important for buyers to have faith in the works they are considering.

He believes Aipad’s stringent code of ethics, by which all member-exhibitors must abide, assures the highest levels of quality. For the novice that means an accurate statement of edition size and print date. Also available from Aipad’s Web site is a useful guide, “On Collecting Photographs” ($10), which includes a photography timeline, a full glossary of terms, a bibliography, and other valuable tools for the photography collector.

The $30 ticket gets viewers return admission, entry to all the scheduled lectures and panel discussions, as well as the illustrated catalogue, and a compendium of Aipad members and their collections, cross-referenced by artist (this directory alone is worth the price of admission). Keep in mind that exhibitors are limited in the number of works they can show, so don’t hesitate to ask what other works are in their collection.

In terms of prices, this show is as broad as it gets, from contemporary photos starting in the $800 range to several hundred thousand. Among the notable photographs on view at the booth of Upper East Side dealer Hans P. Kraus Jr., Inc. are a Roger Fenton landscape, “Kadikoi, From Camp of Horse Artillery”; a 10-by-14-inch 1855 salt print from a glass negative, priced at $30,000; an 1839 William Henry Fox Talbot leaf study, a photogenic drawing in pink and purple; and a Lewis Carroll, “Portrait of Young Frederica Harriett Morrell,” recently discovered, dating from 1874.

There will also be events for both exhibitors and collectors. Aipad board member William Hunt, of Ricco Maresca Gallery, has put together several lectures, as well as a series of films and videos. Mr. Hunt said his goal was to find “people in midcareer who could articulate a debt to Bresson – people who could talk longer than usual and say something personal.” Panelists Bruce Davidson, Susan Meiselas, and Joel Meyerowitz are “three street photographers of different stripes,” according to Mr. Klein.

During his lifetime, Stieglitz ardently lobbied the trustees and curators of the Metropolitan Museum, and his ardor opened the door for photography to enter the world of fine-art collecting. He might be gratified by the Met’s spring schedule, which includes two major photography shows: one of Fenton and one of Arbus. In the span of one century, not to mention a quarter-century, photography has come a long way.

Until February 12 (53rd Street at Avenue of the Americas). Ticket price: $30 for four days or $20 for one.