Obviously, to be able to produce an image of your scene, you need to have a camera. Unlike Cinema 4D, Thea cameras don't operate in a hierarchy. Even if there isn't any camera object in your Cinema 4D hierarchy, Thea Render will always create one employing the Viewport's camera as default. To properly adapt a Cinema 4D camera object into a Thea Camera, we have created the Thea Camera Tag that allows you to extend its functionality as well as apply Display Settings to it. So, the correct way to setup your camera is to create a camera object and apply a Thea Camera tag to it.
In Thea Camera Tag you can find all the properties related to Thea Camera. Please refer to Thea Render Manual (link) for more details on Camera parameters. If override Darkroom Display is activated, the camera's display settings will be used instead of the Darkroom's ones.
As happens with Thea Lights, Camera objects can be converted even if they are untagged. In this case, the camera's properties that are supported by Thea will be read and transferred in the engine's active camera, while the default values will be used for those parameters that are requested but don't exist in a Cinema 4D camera object. The default viewport camera always operates without a tag.