The density difference is usually measured between which optical density values above base-fog level?

• A. 0.25 and 2.50
• B. 0.5 and 1.5
• C. 1.0 and 3.0
• D. 0.0 and 1.0

Answer: (A)

The measurement uses the full useful density range of the film, avoiding the limits posed by base fog and saturation. By taking the density difference from about 0.25 above base-fog to about 2.50 above base-fog, you sample a wide portion of the film’s response where changes in exposure produce meaningful density changes. This range captures enough dynamic range (roughly 2.25 OD units) to assess film speed and contrast reliably. Choosing endpoints lower than 0.25 would be dominated by base fog and noise, while endpoints much higher than 2.50 approach saturation and lose sensitivity to exposure differences. Hence, 0.25 to 2.50 above base fog is the standard.