For near visual acuity, 2M is twice as small as what?

For near visual acuity, 2M is twice as small as what?

• A. 1M
• B. 8M
• C. 4M
• D. 2.5M

Answer: (C)

Near visual acuity in M units is defined by the distance at which a letter subtends 5 minutes of arc. That means letter size grows in direct proportion to the viewing distance. A 2M letter read at 2 meters and a 4M letter read at 4 meters subtend the same angle, so the 4M letter is twice as large as the 2M letter. Therefore, 2M is twice as small as 4M. Other options don’t show that doubling of size: 1M would be smaller than 2M, 8M would be much larger, and 2.5M isn’t a clean doubling.

Near visual acuity in M units is defined by the distance at which a letter subtends 5 minutes of arc. That means letter size grows in direct proportion to the viewing distance. A 2M letter read at 2 meters and a 4M letter read at 4 meters subtend the same angle, so the 4M letter is twice as large as the 2M letter. Therefore, 2M is twice as small as 4M.

Other options don’t show that doubling of size: 1M would be smaller than 2M, 8M would be much larger, and 2.5M isn’t a clean doubling.