What is the power step size for common powers?

• A. 0.25D
• B. 0.50D
• C. 1.00D
• D. 0.75D

Answer: (A)

Power step size is the smallest practical increment used when prescribing lens powers. For common powers, manufacturers typically offer quarter-diopter steps, meaning 0.25 diopters. This finer granularity lets you match a patient’s refractive error more precisely—so someone who needs −2.75 D can be prescribed exactly that value rather than rounding to −2.50 or −3.00. That accuracy improves visual acuity and comfort.

Other step sizes are less suitable for routine corrections. A 0.50 D step is coarser and still common, but it sacrifices some precision. A 1.00 D step is even cruder, making it harder to fine-tune prescriptions. A 0.75 D step isn’t a standard, widely used increment for most common powers, so it doesn’t provide the same reliable precision for matching refractions.