If the light be placed beyond the principal focus of the lens, the rays will converge to a point some distance ahead of it.
The former of these two has a lateral focus, besides the principal focus or pole.
The magnifying power is greater as the lens is more convex and the object near to the principal focus.
By aperture is meant the angle obtained by joining the edges of a lens with the principal focus.
To avoid confusion, this focal length is known as the principal focus, and is denoted by the symbol f.
The focal length of the lens was 150 feet, and the mirror was placed about 15 feet within the principal focus.
This point is termed the “first principal focus” (German der objektseitige Brennpunkt, the object-side focus).
The strip is made to run in a groove across the principal focus.
The point midway between the vertex V and center of curvature C is called the principal focus, F.
(B) principal focus, focal length, virtual focus, conjugate foci.