External meters work because exposure is standardized around scene brightness, aperture, shutter speed, and ISO—not around the hidden details of a specific sensor.

A handheld meter measures the light level and converts it to exposure values using standard calibration. You then set the same ISO on the meter that you set on the camera, and the meter calculates the matching shutter speed or aperture. In that sense, the camera is expected to conform to the same exposure standard.

Sensor quantum efficiency and electronics do matter internally, but the camera’s base ISO and gain are designed so that a given light level at a given ISO produces the expected exposure. Higher ISOs are then additional amplification from that baseline.

So an external meter does not need to know the sensor design directly. Small differences between cameras can exist, and some meters can be calibrated to match a particular camera more closely, but usually the differences are minor.

Lens choice generally does not affect an incident handheld meter reading. With reflective TTL metering, lens transmission and scene reflectance can matter more. That’s one reason external meters are often considered very consistent and precise.