Which of the following is NOT an appropriate lens choice for Jackson Cross Cylinder (JCC)?

• A. +1.00, -1.00
• B. +1.50, -1.50
• C. Plano, -1.00
• D. +0.25, -0.25

Answer: (C)

Jackson Cross Cylinder works by presenting equal powers in two perpendicular meridians, one positive and one negative, so rotating the cross cylinder lets us test and refine astigmatic correction while keeping the same magnitude of cylinder power. This symmetry is what makes the cross useful for pinpointing both the axis and the cylinder strength.

A pair like +1.00 and -1.00 has equal magnitude in opposite signs, which is appropriate. The same goes for +1.50 with -1.50, and +0.25 with -0.25. The reason plano, -1.00 isn’t appropriate is that one meridian would have zero power while the other has -1.00, breaking the required equal-magnitude, opposite-sign relationship of a Jackson Cross Cylinder.