Light Metering for Film: Reflected, Incident and Spot Meters

A light meter measures light and turns it into an aperture and shutter speed for your film. It is a powerful tool, but it doesn't know what it is pointed at. Learning what a meter assumes, and when that assumption is wrong, is the core skill of exposing film well.

Why meters aim for middle gray

A meter that reads light reflected from a scene cannot tell a white wall from a gray one in dim light. So every reflected meter makes the same assumption: whatever it reads should come out as a middle gray, the tone often described as 18% gray. Most ordinary scenes average out close to that, which is why meters are usually right.

When the scene is mostly light, such as snow or a beach, the meter makes it middle gray and the film is underexposed. When the scene is mostly dark, such as a black cat or a night street, the meter brightens it to gray and the film is overexposed.

Reflected light metering

Every in-camera meter is a reflected light meter: it measures light bouncing off the subject toward the camera. Cameras weight that reading in different ways:

Averaging reads the whole frame equally.
Center-weighted gives more importance to the middle of the frame. Most manual film SLRs work this way.
Matrix or evaluative divides the frame into zones and compares them with a pattern library. Common on later autofocus SLRs.
Spot or partial reads a small circle in the center.

Incident light metering

An incident meter measures the light falling on the subject, through a white dome, held at the subject's position and pointed back toward the camera. Because it ignores how light or dark the subject is, it gives the same exposure for a bride in white and a groom in black standing in the same light, and both render correctly.

Incident metering is excellent for color negative and slide film, portraits and any situation where you can stand in the same light as the subject. It can't measure a distant landscape in different light, or tell you the scene's contrast.

Spot meters

A spot meter is a reflected meter with a very narrow angle, often one degree, so you can measure individual tones: a shadow under a tree, a face, a bright cloud. Measuring the darkest and brightest areas you care about tells you the scene's contrast range in stops, which is the foundation of the Zone System.

Metering tricky scenes

Scene	What the meter does	What to do
Snow or white sand	Underexposes	Open up 1.5 to 2 stops, or meter incident
Backlit portrait	Underexposes the face	Meter close on the face, or open up 1.5 stops
Dark subject on dark background	Overexposes	Close down 1 to 2 stops
Stage or spotlit subject	Overexposes the subject	Spot meter the subject
Sunset	Varies with what's in frame	Meter the sky beside the sun, not the sun

Metering for negative versus slide film

Expose for the shadows with negative film. Negatives record highlights over a wide range, but shadows that receive too little light are simply empty. If unsure, give more.

Expose for the highlights with slide film. Blown highlights on a slide are clear film with nothing in them, so meter the brightest area that must hold detail and place it about two stops above middle gray.

Substitute readings and gray cards

When you can't reach the subject, meter something in the same light that is close to middle gray: green grass, weathered asphalt, a blue north sky, or a gray card. The palm of a hand is roughly one stop brighter than middle gray, so meter it and open up one stop.

Checking an old meter

Selenium cell meters in old cameras can weaken; CdS meters may have been designed for discontinued mercury batteries, reading slightly off with modern cells. Compare the camera against a known-good meter or the Sunny 16 rule before trusting it with an important roll.