A common problem encountered in using projected visual aid is:

• A. Blurriness
• B. Overlap
• C. Keystoning
• D. Flicker

Answer: (C)

Keystoning is the distortion that occurs when the projector isn’t aimed straight at the screen. If the lens isn’t perpendicular to the screen, the rectangular image becomes skewed into a trapezoid—the edges that should be parallel diverge or converge as you move away from the lens. This happens frequently because projectors are often placed on a desk, shelf, or mounted off to the side, so the angle isn’t perfectly level with the screen. To fix it, adjust the projector so the lens is perpendicular to the screen, or use keystone correction if available, or physically reposition the projector or screen. Blurriness comes from focus or resolution issues, overlap from two images or slides, and flicker from lamp or signal timing—these aren’t the same geometric distortion and are less consistently observed than keystoning in typical setups.