The studio thought of offering "three different FOV presets" but it would demand "significant work to re-do every animation," adjusting the HUD, UI and other odd bits. So they compromise.
Despite everything being designed with the current FOV in mind, if you really want to adjust the angle then go ahead, but expect to see a number of weird things happening.
"We're aware the community have been asking for a 'Field of View' slider for Metro: Last Light. Unfortunately, this was not as simple a thing to implement as it might appear!" began their update on Steam.
It's those pesky 3D elements going haywire that are the problem, like ammo counters etc. "In addition, all the game's first-person cut scenes and cinematics and each and every animation involving Artyom's hands - idle weapon animations, reloads, ladder climbing, melee attacks etc, - were created assuming the same, fixed field of view."
"Changing the FOV would break all the cut-scenes and animations - you would be able to see inside Artyom's arms, or they would appear to float in the air in front of you. Or worse," they continued.
"We had considered offering three FOV pre-sets, but this would still require significant work to re-do every animation, adjust the HUD and UI and other seemingly small but incredibly time consuming tasks."
"Even with a wider but still fixed field of view, Artyom's hands would look too far away. We know - we tried."
4A Games also caution that game performance is tied to the FOV, as allowing more into the screen will change how much more geometric data is displayed. In other words you might lose some fancier settings visually to compensate.
"We understand this issue is important to many people, and we have been looking at possible solutions," they said. "In the immediate term - the next title update, due in the next few days will allow you to directly change the FOV in the .cfg files." While it might break the game, 4A will at least let PC gamers decide for themselves if it’s worth it.