You Can't See Where You're Blind
This is a simple concept, but worth explaining a bit deeper in relation to visual field loss. A person with two fully functional eyes has a visual field of 190 degrees. They can see everything in front of them. But the 170 degrees behind them...they're blind.
A person with just one eye loses 35 degrees of visual field on one side. They have 155 degrees of sight, and 205 degrees of blindness.
A person with homonymous hemianopia loses a full half of their vision. They have 95 degrees of sight, and 265 degrees of blindness.
Wherever a person's blind area, they are prone to encounter unexpected surprises. A person with a larger blind area will encounter more unexpected surprises.
Reading with Homonymous Hemianopsia
When a person with normal vision reads, their peripheral vision guides them along the line of reading, and to the beginning of the next line so they do not lose their place. Patients with hemianopia have challenges reading. The challenge is different depending on the side of their field loss. A patient with left hemianopsia will often have trouble finding the beginning of a reading line. They may start reading the next line of text without going all the way to the beginning of it. A patient with right hemianopia may read just the beginning of a word without reading the entire word. Both of these scenarios make reading very difficult for a person with hemianopsia.