Seymour was especially adept at composing powerful images, even of fleeting events. Whenever possible, he carefully prepared to cover a story; he learned all he could about the issue or subject and thought about the story both conceptually and pictorially, leaving as little to chance as possible, as though he imagined photographs in his mind before actually taking them.
Well-versed in math, science, and modern printing technology, Seymour quickly mastered the medium's technical aspects. In addition to his instinct for composition he was particularly attentive to lighting and depth of field. Many of his images are sharp from the immediate foreground to the distant background, providing a plethora of information and indicating the context for a scene. Seymour often incorporated signs, banners, or posters into his images that functioned like internal captions. He also had a scrupulous eye for encapsulating details.
Nearly all of Seymour's photographs focus on people. Occasionally a subject will look directly into the lens, but more often Chim preferred to shoot people absorbed in what they were doing. The sympathy and authenticity that radiate from his most arresting photographs derive from his up-front working method. Rather than trying to be invisible, he engaged with his subjects and gently earned their trust, which in turn encouraged them to act naturally around the camera. Seymour saw his work as a collaborative effort between himself and his subject.
He favored two camera models, Leica and Rolleiflex. The revolutionary Leica was small and unobtrusive, perfect for working quickly; the Leica proved that the small negatives (24 x 36 mm) produced with 35mm film could be used professionally. He opted for the more deliberate Rolleiflex—a twin reflex camera that produced remarkably sharp, large, square-format negatives (6 x 6 cm)—when he had more time to compose his images (such as in his series on children in post-World War II Europe). Seymour was well known for his expertise in developing and printing and also for his excellent eye in laying out a photo story.