What is the equation for Near Point?

What is the equation for Near Point?

• A. NP = 1 / (RE + Accommodation)
• B. NP = RE + Accommodation
• C. NP = 1 / RE - Accommodation
• D. NP = 1 / (Accommodation - RE)

Answer: (A)

Focus near vision depends on the total optical power the eye must produce to bring a near object into focus. That total power equals the sum of the eye’s resting refractive error (RE) plus the amount of accommodation the eye makes for the near target. Since power is in diopters (1/meters) and near point distance is a length in meters, the distance to the near point is the reciprocal of that total dioptric demand. Therefore Near Point = 1 / (RE + Accommodation). For example, if the eye’s resting refractive error is 2 D and it needs 4 D of accommodation to see a near object, the total demand is 6 D, giving a near point at 1/6 m ≈ 16.7 cm. If the eye increases accommodation to 5 D, the near point shifts to about 1/7 m ≈ 14.3 cm (closer). The other forms would mix units or omit the reciprocal, so they wouldn’t yield a valid distance.

Focus near vision depends on the total optical power the eye must produce to bring a near object into focus. That total power equals the sum of the eye’s resting refractive error (RE) plus the amount of accommodation the eye makes for the near target. Since power is in diopters (1/meters) and near point distance is a length in meters, the distance to the near point is the reciprocal of that total dioptric demand. Therefore Near Point = 1 / (RE + Accommodation).

For example, if the eye’s resting refractive error is 2 D and it needs 4 D of accommodation to see a near object, the total demand is 6 D, giving a near point at 1/6 m ≈ 16.7 cm. If the eye increases accommodation to 5 D, the near point shifts to about 1/7 m ≈ 14.3 cm (closer). The other forms would mix units or omit the reciprocal, so they wouldn’t yield a valid distance.