What would be the required power of a soft contact lens for a patient with +8.50D spectacles sitting 12 mm from the cornea?

To determine the required power of a soft contact lens for a patient with +8.50 diopters (D) spectacles, it is important to account for the vertex distance, which in this scenario is 12 mm from the cornea.

The vertex distance is the distance from the back surface of the lens to the front surface of the cornea. When calculating the contact lens power needed for a patient who wears spectacles with a given refractive error, it is necessary to adjust the spectacle power based on this vertex distance.

For a spectacle prescription of +8.50D, we need to consider the formula used for adjusting the power when transitioning from glasses to contact lenses: [ P_{CL} = \frac{P_{SP}}{1 - (h * P_{SP})} ] where:

• ( P_{CL} ) is the power of the contact lens.
• ( P_{SP} ) is the power of the spectacle lens (+8.50D).
• ( h ) is the vertex distance in meters (12 mm is 0.012 meters).

Plugging in the values: [ P_{CL} = \frac{+8.50D}{1 - (0.