It is easy to see a stylistic contrast. On the fifth floor of the Museum of Modern Art, a three-canvas set of Monet’s water lilies spreads across a gallery wall in dazzling homage to the artist at the height of his brilliance. Off to one side is a painting of the Japanese bridge at Giverny from the early ’20s, when Monet’s cataracts were at their worst. It is a disturbing mix of dark reds and browns, much darker than the water lilies, yet just as compelling, perhaps, in its brooding intensity.

Monet, terrified by Cassatt’s example, put off surgery, but finally had a successful operation on one eye in 1923. His last paintings before his death three years later harked to his earlier work. He also destroyed many cataract-period canvases, but it is unclear whether they surprised him. He had ruined paintings at other times in fits of pique.

Degas first noticed eye problems as a national guardsman in the Franco-Prussian War in 1870-71, when he could not aim his rifle because of a blind spot in his right eye. By 1890, his left eye also began to deteriorate. Light dazzled him. He tried to use peripheral vision to compensate for his loss of central vision.

Dr. Marmor used computer simulations to gauge the problem. Retinal disease, unlike cataracts, does not cause major difficulties with color perception. But Degas had blurred vision, affecting his ability to perceive form and line.

An ophthalmologist in Toledo, Ohio, Dr. James G. Ravin, who has collaborated with Dr. Marmor in the past, suggested that Degas’s return to pastels and his interest in sculpture might have arisen from seeking an easier-to-control medium.

The simulations showed that the draftsmanship became less detailed and the shadowing coarsened as Degas’s sight deteriorated. Even so, Dr. Marmor said, his work would have looked smoother to him than it actually was.

An art historian and a Degas scholar who taught at Columbia, Theodore F. Reff, wrote in an e-mail interview that retinal disease was a factor in Degas’s late style. “Bitterness and growing isolation,” caused, in part, by the infirmity, may have led him “to paint and draw and sculpt more brusquely and summarily,” Mr. Reff said. But eye disease in no way compromised his art, he added. “What his draftsmanship lost in fullness of realistic description and refinement of execution, it gained in grandeur and expression.”