<camera> lacks a way to specify the vertical field of view

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/166)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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sdf spec 1.6

The `` allows specifying the horizontal field of view, image width, and image height, but not the vertical field of view. Gazebo calculates the vertical field of view using ` vfov = 2.0 * atan(tan(hfov / 2.0) / ratio)` where `ratio = width / height`.

The vertical field of view of a simulated camera doesn't have to obey that formula. For example Ogre allows changing the vertical field of view [directly](http://www.ogre3d.org/docs/api/1.9/class_ogre_1_1_camera.html#aa6f83f76980139d6a9c36abf635e0c75) or by [setting an aspect ratio](http://www.ogre3d.org/docs/api/1.9/class_ogre_1_1_frustum.html#a3235cda8bbc2ae127082e191ae11bcb0). The `` tag could have an `` or `` so it could be specified in SDF. I have not checked if there are real cameras that would require this setting to be simulated correctly.

The issue identifies the element in SDF spec 1.6 but names no implementation files or tests. Start by locating the camera parsing and serialization entry points, then clarify whether vertical_fov or aspect_ratio is supported. Done requires an agreed SDF representation and coverage showing vertical field of view can be specified independently.

Written by the indexing model from the issue text.