What is the magnification of a +4.00D reading cap at a standard working distance of 25 cm?

• A. +1x (+4.00D/4, assuming 25 cm standard working distance)
• B. +2x
• C. +0.5x
• D. +4x

Answer: (A)

The idea being tested is how lens power translates into magnification when you’re viewing at a standard reading distance. At 25 cm, the eye requires about 4 diopters to focus. For a simple near lens, the angular magnification is roughly the lens power divided by that distance demand. So with a +4.00 D lens, the magnification is about 4 ÷ 4 = +1x. That’s why this reading cap yields roughly 1× magnification.

If the lens were stronger, you’d get more magnification (proportional to the power), and if it were weaker, you’d get less. For example, a stronger lens would push toward higher magnification, while a weaker one would give below 1×.