Watch this slide show to see the process of setting out a one-point perspective of a Renaissance building in the field
The Importance of the Picture Plane Perspective is predicated on just a few parameters: a viewer in a fixed position, with a fixed gaze, and a constant picture plane (always perpendicular to the axis of the gaze). Imagine looking through a glass and tracing what you see on the glass; now, move the glass closer or farther from the subject, or move your head closer or farther from the glass (you can also do this looking out a window, moving closer and farther from the window). Notice how the size of the image changes. Then, turn your head at a different angle to the subject, keeping the glass in front of you. Notice that, while your horizon line remains constant, the image's vanishing points have changed.
Working Without a Picture Plane (or, Using a Virtual Picture Plane) Few of us go out in the field with a plane of glass to draw on. And, we are often working on a piece of paper at a different orientation than the subject we're drawing. But it's still important to keep the idea of the picture plane in your mind, as you look out at the subject and look down at your drawing. You shouldn't turn your head left or right to see the image (if you do, the picture plane moving with you means you're now seeing the image with different vanishing points). While this is challenging, it's also of real value: not only are you transcribing what you see, you're training your memory to recall what you've seen in the time between looking and drawing. That's the original Old Master method; and very different from "sight size"...
The Downside of Sight Size
As we say on the Disegno page, the purpose of drawing in the Renaissance was to train the memory, so the artist would be freed from the model to invent. Pietro Bellori criticized those artists who were "slaves to the model." By learning to observe without measuring, the artist is training her memory, and also her judgment: she is inculcating not just the subject, but its principles, its proportion, and its structure. Those things can then build a storehouse of memory for the process of invention. By relying instead on direct measuring and transcription at the same size as the subject, the sight size method privileges accurate documentation over training the judgment of the mind and hand. As Michelangelo said, It is necessary to keep one's compass in one's eyes and not in the hand, for the hands execute, but the eye judges.