A patient has 10/25 vision at 10 feet. What is the cylinder power?

• A. 0.25
• B. 0.50 (doubled JND)
• C. 0.75
• D. 1.00

Answer: (B)

When we talk about cylinder power, we're dealing with astigmatism—the eyes have different focusing power along two principal meridians, and the cylinder value in a refraction captures the difference between those meridians. The smallest change a patient can reliably notice in cylinder power is about a quarter diopter in each meridian. Because the cylinder power represents the difference between the two meridians, the detectable amount of correction is roughly twice that, around half a diopter.

So, for a patient whose acuity is reduced to 10/25 at 10 feet, a small cylinder correction on the order of about half a diopter is the most plausible adjustment to test and optimize vision. Smaller increments (a quarter diopter) would reflect changes in one meridian but not the full cylinder correction, while larger increments (like more than half a diopter) would likely produce a more pronounced change than suggested by the acuity level.