Suppose that the eye is farsighted; the person cannot see clearly objects that are closer than 1.0 m. Find the

Question

Suppose that the eye is farsighted; the person cannot see clearly objects that are closer than 1.0 m. Find the power of the contact lens you would prescribe so that objects as close as 20.0 cm can be seen clearly.

Answer 1

6.5D for near only - assuming the patient has little or no accommodation and no astigmatism.

Answer 2

OK, I'll answer that question.  

If the eye is farsighted it cannot see clearly closer than infinity except by accommodating.  To come into one meter the patient would have to accommodate an additional 1.00 Dioptor.  That means that the patient has no more than 1.00 Dioptor of accommodation.  If this is true it would take an additional 100/20 dioptors to bring him in to 20 cm.  100/20 is 5.00 Dioptors.  I would have to give him +5.00 Dioptors.

This is not something I would be expecting to do in practice.

Tell me honestly, did you know that farsighted (hyperopic) patients have to use accommodation to see anything clearly at any distance near or far?

Answer 3

I think one can assume the question asks what power, in addition to the person's own 1 diopter obtained by accommodation, is needed.

power(corr. lens), in diopters = 1/f(corr. lens) = 1/d(corr) - 1/d(unc)

power(corr. lens) = 1/0.2 - 1/1 = 4 diopters