Far-sightedness
Far-sighted individuals are unable to focus on near objects. The desired normal near point is 25 cm. If an individual's near point is beyond that, a converging (convex) lens is used to modify light from an object at 25 cm to form an image at the individual's near point.
To solve for the required focal length to produce an image at the individual's near point, given that the object is at the normal near point,
The negative sign appears because the image is behind the lens. By including an extra distance x (2.0 cm) for the distance between the eye and the glasses (not needed for contacts), the equation becomes
