For a spherically curved mirror in air the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. So if youre calculating the focal length using a full-frame camera which has a sensor of 36 x 24mm the equation should look like this. The higher the number the more zoomed your lens will be.
H 2 f.
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 case of simple lens designs like Double-Gauss. This is the standard textbook definition but its still not entirely obvious WHY you need to know about it before your purchase a new lens. The convex lens is thicker in the middle and thinner at the edges and is also known as the converging lens.