According to this article about making a simple raycasting engine (like the one that was used in Wolfenstein3d), the distance between the player and the projection plane will be constant.
For example, suppose we are using a projection plane of dimension 320 X 200, and player's field of view (fov) is 60 degrees. Then the distance from the player to the projection plane is going to be
160 / tan(30 degrees) = 277 units
I can understand the trigonometry, but I can't understand why would the distance remain constant througout the game? Why wouldn't the distance change when the player moves closer or farther away from the projection plane?