Here is Jeanette Gurney using a homemade viewfinder to frame a composition for her sketch. The red arrows show the lines from the apex of her eye through the corners of the viewfinder to the scene beyond.
The “pyramid of vision” is the sector of the entire field of view that you have selected to represent in your picture.
In perspective class, they used to call it “cone of vision,” but I like “pyramid” better, because most pictures are rectangular, so a pyramid better describes the volume of space reaching out from your eye. "Pyramid" is also used now more frequently in the CGI field.
Even though the overall visual field takes in an angle of about 60 to 90 degrees, the ideal pyramid of vision for figure drawing or landscape painting in my experience is about 20-30 degrees. Any more than that, and the drawing will look distorted, because you have to turn your head to see from top to bottom or side to side. An angle much less than that puts the subject a bit too far away to see clearly.
Here is Jeanette’s watercolor painting from the scene.
To see other views of the motif, visit the Dalleo’s Deli post.