Mid-density is usually measured around what optical density above the base-plus-fog level?

• A. 0.25
• B. 0.50
• C. 1.0
• D. 2.5

Answer: (C)

Optical density measures how dark a film becomes after exposure, and base plus fog represents the film’s unexposed state. Mid-density sits in the middle of the useful exposure range, and it’s typically about one optical density unit above that base-plus-fog level. This position puts the image in a balance point where enough blackening exists to distinguish tissues and structures without losing detail from being too dark. If the density is lower than this, the image is underexposed and features may be hard to see; if it’s higher, the image becomes too dark and subtle differences can be washed out.