Simon needs glasses for reading, without glasses the nearest he can focus is 1m away. He would like a reading distance of 0.25m.

First it asks for the difference in curvature which is 1/0.25 - 1/1 = 3D.

Then it asks for the power needed for his reading lenses. In the answers it gives 3D, but for a focal length of 0.25m shouldn't it be 1/0.25 = 4D rather than the difference?

Question

Simon needs glasses for reading, without glasses the nearest he can focus is 1m away. He would like a reading distance of 0.25m.

First it asks for the difference in curvature which is 1/0.25 - 1/1 = 3D.

Then it asks for the power needed for his reading lenses. In the answers it gives 3D, but for a focal length of 0.25m shouldn't it be 1/0.25 = 4D rather than the difference?

vincent-lyon.fr · Answer

It is because the necessary focal length is not 0.25m.
You must adapt his nearest focus ability to his practical reading distance.

anonymous · Answer

Ah I get you, thanks.