Sorry, time zone differences.
I set it like this:
float fov = 70;
float aspect = (float)camera.getWidth() / (float)camera.getHeight();
camera.setFrustumPerspective(fov, aspect, camera.getFrustumNear(), camera.getFrustumFar());
Though I’m not certain I’ve solved my issue, probably because I’m working from presumptions. As you say the camera does do the calculations I presented previously.
By that line of code, it is indeed setting the leg size of the distance between A and B (diagram below). This was the part I did not understand.
As the field of view expands, the screen has more of the world visible in the same space, so understandably it results in being able to see through walls I previously couldnt.
I guess what I was trying to do was increase the length of a which doesnt make sense now because that would alter the field of view. From what you are saying, or from what I now understand, the nearPlane actually sets the distance between a and b. and not the length of a