Final answer:
Selecting the best free form lens for a patient requires determining if the lens is converging or diverging, understanding the patient's specific vision issues such as nearsightedness or farsightedness, and the necessary corrective power based on their far point and lens-to-retina distance.
Step-by-step explanation:
Choosing the best free form lens for a patient involves understanding their specific vision needs and requires testing. First, determine if lenses are converging or diverging by checking if they are thicker in the center or edges, respectively. Using converging lenses, one can find the focal length by focusing sunlight onto paper and adjusting the distance for the brightest spot, which also represents an image of the Sun. Backward and forward inspection of eyeglasses can reveal if they behave like thin lenses. For those with a lens-to-retina distance smaller than 2 cm, glasses are needed; typically converging lenses for nearsightedness.
Nearsighted individuals required correction for seeing distant objects clearly. For example, if a nearsighted person's far point is 30.0 cm, the power of the corrective lens needed can be calculated accounting for the glasses being 1.50 cm from the eyes. In the case of laser vision correction, diopter adjustments are calculated based on the reduced power of the eye post-procedure.