The d/1500 rule is a practical convention, not a fundamental constant. It assumes a “standard” final viewing condition: roughly an 8×10 inch print, viewed from about 10 inches, with normal human visual acuity.

Why 1500? The idea is that blur on the sensor/film gets enlarged in the print, then judged by the eye at a typical viewing distance. A common visual-acuity standard is about 2 minutes of arc, which leads to an allowable blur on the final image of roughly 1/1800 of the print diagonal. In practice this is often rounded into the familiar d/1500 rule.

To adapt it, scale the CoC by:

• print diagonal relative to the standard 8×10 diagonal (12.8 in / 325 mm)
• standard viewing distance relative to your actual viewing distance

So, starting from the standard CoC: new CoC = standard CoC × (new print diagonal / 325 mm) × (254 mm / new viewing distance)

Implications:

• Larger print: smaller acceptable CoC on the sensor
• Greater viewing distance: larger acceptable CoC
• Closer viewing distance: smaller acceptable CoC

So the “1500” is just a shorthand for specific assumptions about enlargement, viewing distance, and eyesight.