What is the intermediate power of a 50% trifocal: + 2.00 - 1.00 x 180 add + 2.50?

To determine the intermediate power of a trifocal lens, one must understand how trifocal lenses are designed. Trifocals consist of three distinct sections: distance, intermediate, and near. The "add" value represents the additional power required for near vision beyond the distance prescription.

In this case, the distance prescription is +2.00 with a cylinder of -1.00 at 180 degrees, and the add power for near vision is +2.50.

To find the intermediate power, we usually calculate it as a value that is between the distance and near additions. The common practice for trifocal lenses is that the intermediate power is typically half of the add power added to the distance prescription.

So, with the given information:

1. Distance power: +2.00

2. Add power: +2.50

The intermediate power can be calculated as follows:

• Take half of the add power: +2.50 / 2 = +1.25

• Add this to the distance prescription: +2.00 +1.25 = +3.25

Thus, the intermediate power becomes +3.25 with the same cylinder of -1.00 at 180 degrees, which corresponds to the