I am working on a virtual camera inside a game engine, which provides me with an FoV control, but no Focal length control (and no filming sensor dimensions short of aspect ratio), and I am less than a beginner in photography.
Now when I position the Camera in a specific location in the world, and I position an object in front of the camera such that the object covers 50 degrees of the camera field horizontally. The camera FoV is 100 degrees horizontally.[see image please]. Then I measure the object in the image and I find it covering around 40% of the image width instead of 50%.
What is the most expected mistake that I am making here?
Relevant information: I don't know the position of the virtual camera is the position in the middle of the lens itself, the position of the sensory retina, or the position in between.