Patients with simultanagnosia can only pay attention to a small part of their field of vision. Afflicted patients generally say that they can see only a small part of the world clearly, with the rest appearing out of focus or mired in fog. Consequently, they can deal with single objects (or a few small objects close together), but can't keep track of a visual field as a whole, and generally can't follow movement. Moreover, their inability to keep track of multiple objects makes it hard for them to navigate through their environment--they tend to bump into tables, smack into walls, etc. In some cases, simultanagnosics are letter-by-letter readers; that is, they can read only one letter at a time, and have to mentally combine those letters to understand a single word. Weirder still, their area of clear vision is not stable, but can shift from point to point without warning.
Simultanagnosics are not blind, and they don't suffer from hemianopsia; the disorder arises from damage to the parietal and occipital lobes of the brain. The disorder may appear as part of Balint's syndrome, along with ocular apraxia and optic ataxia.
Some people have speculated that there are really two forms of simultanagnosia. Dorsal simultanagnosics can't navigate and can't see more than one object at a time; ventral simultanagnosics can navigate, and can see (but not recognize) more than one object. Other people believe that the difference is in degree, not in kind. These patients are fairly rare, so it's hard to know for sure.