If you paid $40 for the DVD of "Lawrence of Arabia" when Columbia released it with great fanfare in April 2001, you probably felt hoodwinked when, this autumn, the studio brought out a new, improved version for two-thirds the price and (sorry) no factory trade-in for your not-so-old but flawed copy.

It turns out the original discs had problems with color accuracy. Look at that first desert battle scene: in each shot the sky is a different color — magenta, green, reddish-blue. Those grand, gorgeous desert expanses look pale tan instead of the golden brown of the real sand. Occasionally, Peter O'Toole looks either out of focus or artificially sharpened, as if someone had traced his body with a felt-tipped pen.

All these problems have been fixed on the new DVD, but you'll have to spend $27 more to add it to your collection.

The video market these days is flush with fabulous-looking DVD's, for great and lousy movies alike. But bad-looking ones keep trickling out, too. So what goes wrong when bad DVD's happen to great films?

The tale is not a new one. In June 1999, Warner Home Video issued "The Stanley Kubrick Collection," a seven-disc boxed set of the director's films, for $149. Two years later, Warner came out with a remastered edition for $199. The new box had much better picture quality (and two extra discs, for "Eyes Wide Shut" and a documentary about Kubrick), but, again, no refund for the earlier, shoddier goods.

The first Warner DVD of Clint Eastwood's "Unforgiven," released in 1997, had blurry images, skewed colors and a woozy choppiness whenever the characters or camera moved. It took five and a half years before a good version hit the video bins.

At least these films were sent through rehab. Many other great films were turned into substandard DVD's — and, so far, have been left that way.

Bernardo Bertolucci's "Last Emperor" is a gorgeous film. It won nine Oscars, including the 1987 Academy Awards for best picture, best director and best cinematography. And the DVD, from Artisan Home Entertainment, is one of the most dreadful ever made. Colors are faded, images blurry; if a shot is filled with lines (say, the slats of a roof), they shimmer like hula hoops. It is tragically, agonizingly unwatchable.

Francis Ford Coppola's "Godfather" and "Godfather, Part II," which take up three DVD's in a five-disc boxed set, don't look quite that bad, but — given that they're among the greatest, most beautifully photographed films of all time — the results are dismaying. Images are faded in some scenes, way too dark in others and often speckled with weird distortions. For instance, in the opening shot of "Part II," the close-up of Al Pacino against a dark backdrop, it looks as if mosquitoes are swarming down his face. The movie looks better when it's televised on HBO.

Paramount is making new digital masters of the "Godfather" films for reissue, as single-film discs, late next year. Steve Beeks, Artisan's president, says a new version of "The Last Emperor," mastered from Bertolucci's personal print, will come out next year as well. Neither re-do is likely to carry a banner boasting vast improvement. That would be tantamount to admitting that something was wrong with the lavishly promoted originals.

Digital technology seems, on the face of it, a preposterously inadequate medium for storing movies, and we should gape in wonder that DVD's yield coherent pictures at all, much less the gloriously sharp, detailed images they churn out under the best of conditions. Consider: A DVD stores only 17 gigabytes of data. A two-hour film, transferred to digital data and otherwise untreated, would take up more than 150 gigabytes.

So the data must first be massively compressed, mainly by digitally sampling a frame, then sampling only the information that changes in subsequent frames. This is no big deal for a scene of someone standing still against a blank wall. But it's a major challenge for a scene of someone running through traffic surrounded by dozens of flashing lights and moving objects. If a film is old and damaged, the compression machine will "read" random dirt and scratches in the same way it reads motion. If the machine's operator doesn't pay attention and make adjustments, or if the machine is sub-par, the digitized image will be full of waves, zigzags and other distracting distortions.

Similar problems can plague color or, if it's a black-and-white film, the gradations of gray. When transferring film from a negative to a print, someone has to practice the fine art of "color timing." The same thing has to be done, though electronically, when transferring it to DVD. The job can be done well or it can be done badly.

"The main reason a lot of DVD's are so bad," says Robert A. Harris, president of the Film Preserve, one of the top film-restoring companies, "is that the people making them don't know what they're doing and don't care what they're doing."

Several years ago, when Mr. Harris restored the film of "My Fair Lady" — which had faded badly — he tracked down many of the original costumes, so that he could replicate their precise shades of pink or white. When he restored Alfred Hitchcock's "Vertigo," he asked Jaguar to send him a paint chip from a 1957 car — like the one Kim Novak drove in the film — so he could match the shade of green.

Mr. Harris also restored the 1989 theatrical rerelease of "Lawrence of Arabia," and one problem with the first DVD of that film is that the technicians who made it did not consult with him. One virtue of the new version is that they did. Grover Crisp, Sony Pictures' vice president for film restoration, who supervised the new "Lawrence" transfer, admits that the original discs had problems. "The people doing our DVD's back then," he says, "may not have paid as close attention as they should have."

Doing a DVD right takes time and money. A good Telecine machine, which transfers film to an image suited for television, costs about $2 million. Use of an outside lab's Telecine facilities can cost up to $1,000 an hour.

The Criterion Collection, which produces some of the finest DVD's of classic films, routinely takes months to make a digital transfer. Lee Klein, Criterion's chief technician, says: "If there's a scratch, we draw it out frame by frame. When there's 12 pieces of debris on each frame, it takes a long time."

Most studios don't bother. Some simply take the master that was made for laser disc, or even for VHS videotape, and transfer it to DVD. This was an especially common practice in the infancy of DVD, four to six years ago. It explains the problems with the first Kubrick set. Kubrick had just died. Years earlier, he had approved masters of his films for VHS and laser disc. Warner executives knew these masters had much less detail than DVD's could reveal, but felt nervous about selecting new masters on their own. Not until the company was pelted with complaints from home-theater enthusiasts did they bring in Leon Vitali, Kubrick's assistant, to authorize better masters for new DVD's.

"The Godfather" is a sadder case. The negative and all existing prints were, and still are, in horrible condition. When a film is a big hit, studios put the negative through the wringer, churning out print after print after print. With each new churning, the negative deteriorates. (Now that awareness of film preservation has grown, studios usually make a back-up negative.)

One independent film archivist says that Paramount "horribly mishandled" the negatives for the first two "Godfather" films, not just by making so many prints but also by moving the job among several printing labs, some of which were "grossly careless." An executive at Paramount blames American Zoetrope, Mr. Coppola's company, which made the digital transfers, for the quality of the DVD's. Kim Aubry of American Zoetrope — which, for what it's worth, has made several excellent DVD's — blames Paramount for providing him with poor film materials.

"We spent more time in the compression lab on `Godfather' One than on anything we've done," said Mr. Aubry, who doesn't think the resulting DVD's are so bad anyway. "But at the end of the day, you can do only so much."

In any case, according to a Paramount executive, who asked not to be named, the studio has hired an outsider, John Lowry, to re-do the "Godfather" discs. Mr. Lowry has worked on such brilliant DVD's as "North by Northwest," "Sunset Boulevard" and the new double-disc "Casablanca" — all of which were made from prints or negatives in dreadful condition. He holds patents on film noise-reduction techniques. Decades ago, he cleaned up moon photos for NASA's Apollo project. A few years ago, he devised a program that harnesses the computational power of 400 Macintosh G5 computers to clean up the damage that's been done to film by age, wear and handling. This program detects flaws like scratches or dirt, erases them, and — by comparing flawed frames with those in better shape — "draws" in a facsimile of what an unflawed version of the image should look like.

Cleaning a film, though, poses its own problems. Mr. Lowry's otherwise wondrous DVD of "Citizen Kane" (for which no negative exists; it was destroyed in a fire decades ago) has been criticized for being too clean, for wiping away the natural film grain along with the scratches, for looking more like video than film. "I'd agree with that," Mr. Lowry says. "We were just getting our feet wet in the business when we did `Kane.' It was our third or fourth film. We've done 70 films now. Even that's not enough to raise the bar for the entire industry. You have to remember, this is still a very new art."

Fred Kaplan is a columnist for Slate.com and a film critic for The Perfect Vision.