Field of View
This horizontally measured angle defines the camera’s field of view. Smaller Focal Width values will lead to a larger Field of View (Horizontal), larger Focal Width values will lead to smaller fields of view. Hence, the Focal Width and image size values are directly linked to the formula used to calculate the Field of View.
Increasing the Focal Width value will result in a smaller field of view, while increasing the image size will lead to a larger field of view. In the end however you should orient yourself according to the view through the camera and not the numeric values.