The plate scale is the reciprocal of the focal length of the camera lens lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.
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 for a double concave lens where the rays are diverged, the principal focal length is.
Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens it is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera.
A 400-cm tall light bulb is placed a distance of 830 cm from a double convex lens having a focal length of 152 cm (note: this is the same object and the same lens, only this time the object is placed closer to the lens) determine the image distance and the image size. 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. A zoom lens is one where the photographer can vary the focal length within a pre-defined range, whereas this cannot be changed with a prime or fixed focal length lens the primary advantage of a zoom lens is that it is easier to achieve a variety of compositions or perspectives (since lens changes are not necessary.
The focal length of a lens, expressed in millimeters, is the distance from the lens’s optical center (or nodal point) to the image plane in the camera (often illustrated by a φ on the top plate of a camera body) when the lens is focused at infinity the image plane in the camera is where you will find your digital sensor or film plate. The principal focal length of a lens is determined by the index of refraction of the glass, the radii of curvature of the surfaces, and the medium in which the lens resides it can be calculated from the lens-maker's formula for thin lenses.
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.