LONDON, England - Turning a great novel into a film should be a no brainer.  So, why are there so many disappointing movie adaptations of good novels?

Brazilian Fernando Meirelles' 'Blindness' is an ambitious adaptation of a classic novel that doesn't quite work.

"City of God" director Fernando Meirelles' new film "Blindness" is the latest example: an ambitious adaptation of a classic novel that has fallen short of the mark.

On paper there is a lot to anticipate: it is a silver screen version of Portuguese Nobel Prize-winner José Saramago's celebrated novel "Ensaio sobra a cegueira," directed by hot Brazilian director, Meirelles and starring Julianne Moore, Mark Ruffalo and Gael Garcia Bernal starring.

And yet, the film, which premiered at Cannes Film Festival earlier this year, was not well-received by critics. It is certainly affecting and beautifully shot, but despite profund themes the plot is muddling and lacks coherent direction, which is where it falls down.

Perhaps it is because the film tries too hard to do justice to its rich source material, and while a reader may readily accept a wide range of complex messages and themes delivered in written prose, films require more cohesion.

There is also specter of the "unfilmable novel," those works considered impossible to translate for the big screen for whatever reason: overtly sexual content, nebulous plot or complex structure among others.

It is a notion that has been strengthened by questionable adaptations of classic works by accomplished filmmakers: David Cronenberg's attempt at J.G. Ballard's highly erotic "Crash" which managed to convey the sex but was dull and Michael Winterbottom turning "Tristram Shandy" into the highly forgettable "A Cock and Bull Story" to name two.

Are simple storylines just easier to translate? Francis Ford Coppola managed to create an award-winning work of art from Mario Puzo's pulp crime novel, "The Godfather."

Streamlining novels with complex narratives can also work well. Last year's three big awards season movies, the Coen Brothers' "No Country for Old Men," Paul Thomas Anderson's "There Will be Blood," and Joe Wright's "Atonement," are all based on rich and intricate novels.

But, in "No Country for Old Men" very little is made of Llewelyn Moss or Ed Tom Bell's military past, which is a big part of the book; and much of Robbie Turner's torrid and soul-searching journey through France to Dunkirk in Ian McEwan's "Atonement" is omitted from the film version.

But just cutting out the hard bits can't be the whole story. Mary Harron's successful adaptation of another "unfilmable book," "American Psycho" succeeded because she injected humour into Brett Easton Ellis' cold vision of an 80's murderer and transformed it into a satire of the yuppie lifestyle of the time.

As the cinema world waits for the upcoming film adaptations of Cormac McCarthy's "The Road," Jack Kerouac's "On the Road," and Haruki Murakami's "Norwegian Wood" let's hope that these challenging books will unleash the director's creativity.

What do you think? Can a great novel become a great film? What are the best adaptations of novels? And, do you think there are some novels that are unfilmable?