Every exposure meter answers the same question with one of two fundamentally different measurements. A reflected meter reads the light leaving the subject; an incident meter reads the light falling on it. The distinction is not academic. The two methods can disagree by several stops on the same scene, and understanding why explains one of the most common sources of mis-exposure in black-and-white work.

What Each Meter Actually Measures

A reflected meter, including every in-camera meter and every spot meter, samples luminance: the light bouncing off the scene toward the lens. Because that quantity depends on how reflective the subject is, the meter cannot know whether a dark reading comes from dim light or a dark surface. To produce a single exposure value it assumes the area it reads is of average reflectance.

An incident meter sits at the subject position, usually behind a translucent dome, and measures illuminance: the light arriving at the subject regardless of what the subject later does with it. A white wall and a black coat under the same lamp return the same incident reading, because the meter never sees either surface.

Both methods are codified in ISO 2720:1974, the standard governing general-purpose photoelectric exposure meters. It defines separate calibration constants for each. The reflected constant K is recommended in the range 10.6 to 13.4; manufacturers use 12.5 (Canon, Nikon, Sekonic) or 14 (Minolta, Pentax). The incident constant C depends on the receptor: roughly 250 for a flat cosine-responding cell, and 320 to 340 for the hemispherical dome.

The Middle-Gray Assumption

The reflected meter’s averaging assumption is the heart of the matter. It is calibrated so that whatever it reads is rendered as a middle tone, the Zone V gray that Ansel Adams placed at the center of the exposure scale in “The Negative.” For a scene of typical reflectance this works. For one that is not, it fails predictably: a snowfield metered reflectively is reproduced as gray, underexposed by roughly two stops, while a dark subject filling the frame is overexposed.

The familiar “18% gray” shorthand is an approximation rather than the defining figure. Working back from the standard’s constants, the implied reflectance for the common K = 12.5 calibration is closer to 12 percent, which is why a reflected reading off an 18% card and an incident reading of the same light can differ by something near half a stop.

When Each Method Excels

An incident reading sidesteps the trap entirely. Because it ignores reflectance, it places a middle-gray subject on Zone V, a white one near Zone VII, and a black one near Zone III, exactly where the eye expects them. This makes incident metering reliable for portraits, products, and any scene dominated by unusually light or dark tones, provided the meter can be placed in the same light as the subject.

Reflected metering reclaims its advantage where that placement is impossible, such as distant landscapes, stage lighting, or any subject the photographer cannot approach. A narrow-angle spot meter also enables tonal placement: reading a specific value, then opening or closing from the meter’s middle-gray indication to set that value on a chosen zone. The incident meter measures the light; the spot meter measures the scene. Knowing which question is being asked is the whole of the technique.