A patient wears +5.00 D contact lenses. He now gets +5.00 D spectacles and wears them with a vertex distance of 20 mm. What is the change in magnification in using these spectacles?
Ophthalmic Optics / Spectacles
This is a question regarding lens effectivity. Specifically, this question tests whether you know how the power of a lens will change as you move the lens further from the eye. The basic premise is this:
Moving a lens (either plus or minus) away from the eye gives it more effective plus power
To calculate the new effective power of this plus lens, you need to calculate the new focal length:
new focal length = old focal length - distance moved from eye
new focal length = 0.2 m - 0.020 m = 0.180 m
Thus, the effective power of this +5.00 diopter lens is really 1 / 0.180 = +5.56 diopters. You know you have to subtract the distance moved from the old focal length since the new effective lens power has to be more positive.
The change in magnification is simply the ratio of the new effective lens power to the original lens power:
change in magnification = 5.56 / 5.00 = 1.11
Please note that this question is NOT asking you to calculate the power of the spectacles that would be needed given his contact lens prescription. That type of question is also a very common question that is asked on board exams. In that latter case, you would need a weaker plus power spectacle since it is effectively stronger further from the eye. The power of this spectacle would be: 1 / (0.2 + 0.02) = +4.55 diopters.