Focal length of the lens medium. The quantity is also known as the optical power of the lens. The corresponding front focal distance is: and the back focal distance: In the sign convention used here, the value of "R" will be positive if the first lens surface is convex, and negative if it is concave. The value of "R" is negative if the second surface is convex, and positive if concave. Note that sign conventions vary between different authors, which results in different forms of these equations depending on the convention used. For a spherically curved mirror in air, the magnitude of
Lens (optics) real object at the location of that virtual image. Likewise, it appears to a subsequent lens as if it were an object at that location, so that second lens could again focus that light into a real image, "S" then being measured from the virtual image location behind the first lens to the second lens. This is exactly what the eye does when looking through a magnifying glass. The magnifying glass creates a (magnified) virtual image behind the magnifying glass, but those rays are then re-imaged by the lens of the eye to create a "real image" on the retina.