Metering, histograms and exposure compensation

Lessons 3 to 5 showed how aperture, shutter speed and ISO combine to set exposure. But what exposure is "correct"? The camera's light meter proposes an answer, and most of the time it is good. It can also be badly wrong in predictable situations: a snowy field turned grey, a bride's dress without detail, a black cat lost in murk, a backlit face in silhouette. This lesson explains how meters work and why they are fooled, the metering modes and when to use each, exposure compensation and exposure lock, the histogram and highlight warnings that let you judge exposure objectively, how to handle scenes whose contrast exceeds what the sensor can record, and how to work in manual exposure with steady light.

What a meter measures

A camera's built-in meter measures the light reflected from the scene through the lens, so it is called a reflected-light meter. It cannot know whether it is looking at a bright subject in dim light or a dark subject in bright light; both reflect the same amount of light. So it assumes every scene averages to a middle grey, a tone reflecting about 12 to 18 % of the light falling on it, roughly the brightness of weathered concrete, green grass or average skin in shade. It then sets an exposure that renders the measured area as that middle grey.

For most scenes, which contain a mix of light and dark tones, this works well. It fails when a scene is mostly lighter or darker than average:

• Bright scenes (snow, sand, white walls, a bride in white) are rendered too dark, as grey, because the meter pulls them down to middle grey. You must add exposure.
• Dark scenes (a black dog, dark clothing, a night street with a few lights) are rendered too light, washed-out and noisy. You must subtract exposure.

An incident-light meter, a handheld meter with a white dome, avoids the problem by measuring the light falling on the subject rather than the light reflected from it. Because it ignores the subject's own brightness, it renders white as white and black as black, which is why studio, portrait and film photographers use them. You can mimic it by metering a grey card placed in the subject's light.

Metering modes

Cameras offer several ways of weighting the meter's reading across the frame:

• Matrix, evaluative or multi-zone metering divides the frame into many zones, evaluates brightness, contrast, colour, focus point and sometimes detected faces, and compares the pattern with a database of scenes. It is the best general-purpose mode and handles many awkward scenes, though not all.
• Centre-weighted metering averages the whole frame but gives most weight to the central area; predictable and long-established.
• Spot metering measures only a small circle, often 1 to 3 % of the frame, usually at the centre or the active focus point. It is the precise tool for metering a particular tone, such as a face in a backlit scene or a performer under a spotlight.
• Highlight-weighted metering, on some cameras, sets exposure to avoid clipping the brightest areas, useful for stage performances and backlit subjects.

Know how your camera's default mode behaves, and switch modes deliberately. A good habit: use matrix metering by default and spot metering when the subject's brightness differs strongly from its surroundings.

Exposure compensation and exposure lock

In program, aperture priority and shutter priority modes, exposure compensation (the +/− button or dial) tells the camera to make the image brighter or darker than the meter suggests, in thirds of a stop. The camera then changes the shutter speed, aperture or auto ISO accordingly. +1 means one stop brighter. Typical corrections:

Exposure lock (AE-L) stores a meter reading so you can recompose without the exposure changing. A common technique is to spot-meter a face, lock the exposure, recompose the frame, and shoot. On many cameras, half-pressing the shutter also locks exposure, and some photographers set a separate button so that exposure and focus can be locked independently.

The histogram

A camera's screen is a poor guide to exposure: its brightness depends on the screen setting and the surrounding light, and a picture that looks fine on a dim screen at night may be badly overexposed. The histogram is objective. It is a graph of how many pixels have each brightness value, from black at the left to white at the right. A well-exposed image of an average scene spreads across most of the range without piling up against either end.

• Pixels piled against the right edge are clipped highlights: pure white with no detail, which cannot be recovered.
• Pixels piled against the left edge are clipped shadows: pure black.
• The shape depends on the scene, not on "correctness". A snow scene should be weighted to the right, a night scene to the left. There is no ideal histogram shape, only histograms that do or do not suit the subject.

The highlight warning ("blinkies"), which flashes clipped areas on the playback image, shows exactly where detail is lost. Many cameras also show RGB histograms, one per colour channel: a single channel can clip, such as the red in a sunset or a red flower, while the overall brightness histogram looks fine, leaving that area flat and posterized. Mirrorless cameras can display a live histogram and zebra stripes over areas near clipping before you take the picture.

The camera's histogram is calculated from a JPEG preview with the camera's picture style, so it is slightly conservative compared with what a raw file actually holds (Lesson 10). A small amount of "clipping" on the camera may be recoverable in raw processing, but treat that margin as a safety buffer, not a target.

Exposing for the highlights, or to the right

Digital sensors behave differently from film: once a photosite is full, it records pure white, and nothing can bring detail back. Shadows, on the other hand, can be brightened in editing, at the cost of noise. Hence two related strategies for important images, particularly raw files:

• Protect the highlights: expose so that important highlights (a face, a white dress, clouds) are not clipped, and brighten shadows later if needed.
• Expose to the right (ETTR): in low-contrast scenes, give as much exposure as possible without clipping important highlights, pushing the histogram to the right, then darken in processing. Because shadows collect more light, noise is reduced (Lesson 5).

Specular highlights, such as the sun, reflections on chrome and street lamps, are normally allowed to clip; they are brighter than any medium can show.

When the scene has too much contrast

A sunlit landscape with deep shadows, an interior with a bright window, or a backlit portrait may span 15 to 20 stops, more than a sensor can record in one frame (Lesson 5). The photographer has several options:

• Choose what matters and let the rest go: expose for the face and let the sky burn out, or expose for the sky and let the foreground fall into silhouette. Silhouettes against a sunset are made this way, exposing for the sky.
• Wait for different light, change the viewpoint, or move the subject into shade (Lesson 9).
• Add light to the shadows with a reflector or flash (PHOT-102).
• Use a graduated ND filter to darken the sky (Lesson 4).
• Bracket and blend: take several exposures at different settings and merge them into a high dynamic range (HDR) image or blend them by hand. Bracketing is also insurance in tricky light.

Manual exposure with steady light

Automatic modes adjust exposure for every frame, which is a disadvantage when the light is constant but the subject or framing changes. Photographing a person in a white shirt and then in a black jacket, or framing more or less sky, makes the meter change its reading even though the light has not changed, so the exposure jumps around. In steady light, such as a studio, a consistent overcast day, or a sports arena, manual exposure gives consistent results: set the exposure once from a meter reading of a middle tone or a grey card, check the histogram, and leave it. Professional sports and event photographers often work in manual with auto ISO switched off for exactly this reason.

The camera's exposure indicator, a scale with a marker showing how far the current settings are from the meter's suggestion, still works in manual mode: treat it as advice, and decide for yourself, guided by the histogram.