To truly verify a light meter’s accuracy, you compare it either to a known-good calibrated meter or to a calibrated light source. That’s how meter calibration is normally done, often by a metrology lab with traceability to a standards body such as NIST.

For a Weston Master II, the rear zero adjustment only sets the needle’s no-light mechanical offset. It does not fully calibrate the meter across its working range. If the meter is inaccurate under actual light, there usually isn’t much user adjustment available beyond basic maintenance such as cleaning contact or potentiometer surfaces if applicable.

Comparing it to different cameras is not a reliable calibration method, since camera meters can differ and may themselves be out of spec.

In practice, your options are:

• compare it to a trusted calibrated handheld meter
• have it checked by a calibration/metrology service
• accept that professional calibration may cost more than the meter is worth