Incident Light Metering

Incident light metering measures the light falling on a subject instead of the light reflected from it. A handheld meter with a white dome is placed at the subject and aimed at the camera. The meter then suggests an aperture and shutter speed for the chosen ISO. Because the reading ignores the subject's own brightness, it stays accurate for pale, dark, or mixed subjects in the same light.

A reflected meter, including every built-in camera meter, reads light bouncing off the scene and assumes the scene averages middle gray. An incident meter reads the light arriving at the subject and never sees the subject at all. Reflected readings shift with the subject's tone, so snow and black fabric fool them. Incident readings stay constant as long as the light does.

For a normal exposure reading, the dome faces the camera lens from the subject's position. This averages all the light landing on the side of the subject the camera sees. To measure one light on its own, for example when checking a ratio between key and fill, turn the dome toward that light and shade it from the others. Both methods are correct for different jobs.

Camera meters are calibrated to render whatever they read as middle gray, about 18 percent reflectance. A white snow field reflects much more, so the meter cuts exposure and the snow turns gray. A black coat reflects much less, so the meter adds exposure and the coat turns gray. An incident reading sidesteps the problem because it measures the light, not the subject.

Not directly, since it only sees reflected light. The closest substitute is to meter off a gray card placed in the subject's light. The card reflects a known 18 percent, so the reflected reading matches what an incident meter would report. Some phone apps use the front camera with a small diffuser to approximate an incident reading as well.

Most flash meters are incident meters with an extra mode. In flash mode the meter waits for the strobe to fire, measures the brief burst arriving at the dome, and returns the aperture to use at the set ISO and shutter speed. Many models also measure continuous light and show how much of the exposure comes from flash versus ambient light.

It fails when the meter cannot reach the subject's light: distant landscapes, wildlife, stage performers, or anything behind glass. It also ignores bright light sources inside the frame, such as a sunset sky behind a person, so those areas can clip. And it assumes a normal rendering, so deliberately bright or dark images still need compensation from the reading.

Take one reading with the dome facing the key light, shading it from the fill, then a second reading facing the fill light with the key blocked. The difference between the two in stops is the ratio. One stop apart is a 2:1 ratio, two stops is 4:1, and three stops is 8:1. Portrait photographers often aim for two to three stops for a soft but defined look.

Only when the photographer stands in the same light as the scene. On a clear day with the sun on both the camera position and the distant hills, a reading taken at the camera with the dome facing the lens is valid for the whole view. When the scene sits in different light, such as a sunlit peak above a shaded valley, spot metering is the better tool.

Yes, mainly in the studio and on set. It lets a photographer set flash power and ratios by number before the first frame, repeat a lighting setup days later, and match several lights without trial and error. For handheld digital work outdoors, the camera's own meter plus the histogram usually covers the job, so many photographers reserve the incident meter for controlled lighting.