The focal length of the lens: d, also known as the number of millimeters of the lens, refers to the distance from the center of the lens to the focal point of the light when the parallel light incident.
The focal length of the lens determines the size of the image, the Field Of View (FOV), the depth of field and the perspective of the picture.
Whether you really understand the physics or not, just remember that the smaller the focal length, the wider the Field of view(FOV), the larger the range of field, and the smaller the object in the picture;On the contrary, the larger the focal length, the narrower the Field of view(FOV), the smaller the range of view, and the larger the object presented in the picture.
