How can one art photograph have four very different price tags?

ANYONE making a tally of the most important fine art photographs of the 20th century would probably include Henri Cartier-Bresson's 1932 image of a man jumping into a puddle; Robert Doisneau's 1950 photo of two people kissing in front of Paris's Hôtel de Ville; László Moholy-Nagy's 1928 shot from the Berlin Radio Tower; and, somewhere near the top of the list, Alfred Stieglitz's The Steerage, which he took while on a cross-Atlantic trip in 1907.

The image is widely considered one of the world's first modernist photographs, and Stieglitz did everything he could to further that perception, publishing it alongside a cubist painting by Pablo Picasso. "It's truly one of the five or 10 most important pictures ever made," says the New York photo dealer Howard Greenberg. "It's a memorable, timeless image, and it's very important in photography in terms of its modernism."

Stieglitz, no stranger to self-promotion, printed at least several hundred copies of the image over the course of his lifetime, and since it was first published in his journal, Camera Work, in 1911, the copies have been resold hundreds, if not thousands, of times. (The Artnet database of public auction sales, which goes back to 1987, lists 187 results.) Still, April's coming New York photo auctions will feature a nearly unprecedented confluence of five Steerages up for sale: Copies of the work will be sold at Christie's (April 6), Phillips (April 9), Sotheby's (April 10), and Swann's (April 19).

Stranger still, each auction house has placed a different valuation on its image. At Christie's, The Steerage is estimated to sell for US$15,000 to US$25,000; at Sotheby's, US$12,000 to US$18,000; and at Phillips, the work is expected to fetch from US$60,000 to US$80,000. Swann Auction Galleries will sell two copies - one is estimated at US$12,000 to US$18,000, while the other, which is being offered with another Stieglitz image, is estimated at US$5,000 to US$7,000.

Each of the images was printed by Stieglitz from 1907 to 1916, and each (obviously) depicts the same thing. The price differential between the images boils down to minute differences in chronology, dimension, provenance, and signatures; in other words, it's about connoisseurship and collectibility rather than artistry. "I think that photo collectors take these issues, which might seem like small differences, quite seriously," says Christopher Mahoney, the senior international specialist for photographs at Phillips.

And that, in turn, represents some of the hurdles facing the historical photography market, where the logic behind valuations can require either serious research or a serious leap of faith. Anyone hoping to invest in, rather than merely enjoy, the medium has to do their homework. "It's quite a sophisticated market," says Darius Himes, head of photographs at Christie's. "It's not for people who just want to jump in like day traders and bag a few bucks, and then get out. You have to understand what you're looking at."

Because The Steerage is so famous, and so many copies have hit the market so consistently, it's actually a rare example of a fairly predictable segment of the photo market. Stieglitz first printed the image as a photogravure (an early photographic process) on Japanese tissue paper.

"He didn't keep records," says Mr Greenberg, "but there are ostensibly a lot, more than 100 works printed for Camera Work." Then, for his gallery at 291 Fifth Avenue in New York, Stieglitz printed more of the images on vellum, this time larger than the initial images. There was also a still-larger size on Japanese tissue, "and I believe there were 150 made of that one," he goes on to add. "All of these forms exist." (All the collectible versions of The Steerage, to be clear, were printed in Stieglitz's lifetime.)

The value of a Steerage image comes not from its size (the Christie's and Sotheby's versions, which are comparatively cheap, are slightly larger than the one at Phillips) but from chronology and, most important, whether Stieglitz actually signed it. The Christie's, Sotheby's, and Swann examples do not bear his signature; Phillips's does.

"The variances we think about are the condition, relative dating, and whether or not there's something unique or remarkable about the work," says Emily Bierman, head of Sotheby's photography department in New York. "Is the work inscribed? Does it have a special provenance? When you look at more significant prices (for The Steerage) outside of the US$10,000 to US$30,000 range, you're looking at works that have something extra." The Phillips copy is not only signed in three places - it also has a unique provenance.

"We've gotten it from the direct descendant of someone who bought it from Stieglitz in 1941," says Phillips's Mr Mahoney. "You really get a sense that Stieglitz had his hands on this. It's something that met his approval and that was acquired directly from him."

Since there's no authoritative report on the overall photography market, determining its relative strength (or weakness) is contingent upon anecdotal evidence, and trends can differ widely from artist to artist. Yet, there's enough auction data from The Steerage to argue that its market has crept slowly upwards, though anyone looking for a money-making opportunity will probably come away disappointed.

In 2005 a Steerage the same size as Phillips's upcoming image, signed and inscribed, was estimated to sell for US$40,000 to US$60,000 at Sotheby's New York. It went for US$96,000. On the same day, an even rarer version sold at the same auction for US$180,000, eking its way over a low estimate of US$150,000. (That work was one of two silver gelatin prints of The Steerage believed to be in private hands. BLOOMBERG