But perspective is just illusion. It's a product of how the lens of our eye projects light onto the "picture plane" of the retina and of how our brain interprets the resulting pattern. Railroad tracks don't really converge three miles away to a single point.
The rules of perspective are a framework we painters impose on what we see in order to make our paintings look real. The rules of this "grand illusion" were first invented by goldsmith Filippo Brunelleschi (1377-1446) and later codified in a treatise by humanist Leon Battista Alberti (1404 – 1472). (For more on the history of perspective, see http://www.math.nus.edu.sg/aslaksen/projects/perspective/theory.htm.) Artists have been using these rules ever since.
But you don't really need to apply these rules to make your paintings look real. Like almost everything else, getting your paintings to look real comes down to observation. Observe the illusion, and simply paint what you see. Compare the relationships of lines and shapes, angles and directions, to record the slope of a roof.
The painting below, of the Roosevelt Cottage in the Roosevelt-Campobello International Park, is an example in which I purposely did not draw a skeleton of perspective lines before painting. I simply painted shapes, and compared what I painted with the actual cottage before me. Surprisingly, one doesn't have to take out a ruler and compass to measure line and angle, since the trained eye is pretty good at estimating these things. Just draw and compare -- constantly.
"Roosevelt's, Morning" 9x12, pastel, en plein air(click on image for larger version)
