Linearity of world angles from image pixels

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This may be a naive question, but I'm hoping to get a heads up or other directions from you.

So if I have somehow calibrated/measured my camera to know that a certain angle of view is a certain amount of pixels, I am wondering, if I can assume that as linear. More concrete: Let's say from where my camera is, the angle to 2 known objects is 60°. On the camera image, the two objects distance is 1000 pixels. (let's consider only the x-axis). From that, can I assume that 500 pixels is 30°?

Furthermore, how could I calibrate/measure this angle-to-pixels ratio? Is there a name for it? Or can I even find that property in the camera matrix, which I have obtained during camera calibration?