The main advantages of prime or fixed focal length lenses are their size and weight as well as their maximum aperture or f/stop prime lenses tend to be more compact and lightweight than zoom lenses prime lenses also tend to have a larger maximum aperture (f/14 to f/28.
Principal focal length for a thin double convex lens, refraction acts to focus all parallel rays to a point referred to as the principal focal point the distance from the lens to that point is the principal focal length f of the lens.
Focal length is a calculation of the distance between your subject and your cameras digital sensor learn to use the correct lens size. Focal length is measured in millimeters, but the measurement is not of the actual physical length of the lens, but rather the magnification properties of the lens the actual measurement of focal length is the distance, in millimeters, between the convergence point and the imaging sensor.
The focal length of a convex lens is the distance between the center of the lens and the focal point here, f is the focal point and f is the focal length different lenses converge light at different focal points, and thus, have different focal lengths.
Focal length is measured in millimeters (mm) and it represents the distance from the optical center of a lens to the digital camera sensor when the subject of the photo is in focus this is the standard textbook definition, but it's still not entirely obvious why you need to know about it before your purchase a new lens. For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens for a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot.
The focal length of a lens defines the lens’s angular field of view for a given sensor size, the shorter the focal length, the wider the angular field of the lens additionally, the shorter the focal length of the lens, the shorter the distance needed to obtain the same fov compared to a longer focal length lens.
The focal length is an indicator of the distance from the subject: the images are all framed the same differences arise because the focal length is getting longer (zooming in) as the camera moves further away from the subject remember, the distance from the subject is changing the perspective the focal length is just used to compensate for this.