Im currently trying to understand the cone of vision and I was wondering why the cone of vision is recommended to have 60 degrees? I mean where does the number come from? I always though the vision span was about 120 degrees for our eyes?
The problem isn't in our eyes. Its a geometric problem when 3D shapes are drawn on 2D paper having perspective.
If one draws objects with perfect perspective, they start to be increasingly distorted and look out not plausible if they are more than 30 degrees off from the midpoint of our field of sight. That makes the 60 degrees wide cone to be the area for shapes that can be drawn with exact perspective and the result also looks out plausible.
Of course there's no strict limit. The 60 degrees limit is based on experience. It's only a thumb rule. A mathematician probably could find a good measure for the apparent distortion of perfectly in perspective drawn shapes and would show that about 60 degrees width is some kind of well definable culmination point of the apparent distortion. But this is beyond the scope of my answer.
Here the phenomena is explained simply without math: