According to the Half Amp Method, the theoretical add is calculated as which formula?

According to the Half Amp Method, the theoretical add is calculated as which formula?

• A. ADD = Working distance - 1/2 Amplitude of Accommodation
• B. ADD = Working distance - Amplitude of Accommodation
• C. ADD = 1/2 Amplitude of Accommodation - Working distance
• D. ADD = Working distance + 1/2 Amplitude of Accommodation

Answer: (A)

The Half Amp Method estimates how much plus add is needed at near by looking at how much the eye must focus (the working distance) and how much accommodation the eye can still supply. The idea is that part of the near task can be handled by the eye’s remaining accommodation, roughly half of its amplitude of accommodation, so the external add should offset only the remaining demand. In practice, you take the dioptric demand of the working distance (the reciprocal of the distance to the near target) and subtract half of the amplitude of accommodation. The result is the theoretical near add power. If this calculation comes out negative, it means no additional add is needed; if positive, that amount of plus power would be prescribed. So the correct formula is: ADD = Working distance - 1/2 Amplitude of Accommodation.

The Half Amp Method estimates how much plus add is needed at near by looking at how much the eye must focus (the working distance) and how much accommodation the eye can still supply. The idea is that part of the near task can be handled by the eye’s remaining accommodation, roughly half of its amplitude of accommodation, so the external add should offset only the remaining demand.

In practice, you take the dioptric demand of the working distance (the reciprocal of the distance to the near target) and subtract half of the amplitude of accommodation. The result is the theoretical near add power. If this calculation comes out negative, it means no additional add is needed; if positive, that amount of plus power would be prescribed.

So the correct formula is: ADD = Working distance - 1/2 Amplitude of Accommodation.