Perspective ratio The following image illustrates a simple question - is there a simple ratio to a, b and c if all distances are equal?

 A: One can imagine that you want to render with a correct perspective a line of regularly spaced  poles along a straight road extending to infinity on a flat plateau.
One could think that there is a relation of preservation of ratios like $c/b=b/a$. This is false. What is preserved in this vast domain called "projective geometry" if you change the point of view is the "ratio of ratios" called "cross ratio" 
(https://en.wikipedia.org/wiki/Cross-ratio). 
Let us be more precise.
Let $A,B,C$ be the points such that $AB=a, BC=b$ in your figure. Besides, let $D$ be the point at infinity (intersection with horizon line).
Then 
$$\text{Cross ratio} \ (A,B,C,D) = \text{Cross ratio} \  (0,1,2,\infty)\tag{1}$$
meaning that 
$$\dfrac{CA/CB}{DA/DB}=\dfrac{((2-0)/(2-1))}{((\infty-0)/(\infty-1))}=2/1=2\tag{2}$$
Why that ? Because we refer to the position of points/poles that, in, say a satellite view, would be regularly spaced (at "coordinates 0,1,2,3,...").
Out of relationship (2) one can deduce all distances in your figure.
Connected, on an artist point of view : http://teresabernardart.com/using-linear-perspective-to-create-depth-in-your-paintings/
