How to measure light

Light must be measured before it can be used creatively, and over time you will learn to ‘read’ the light before ever bringing the camera to your eye.

You can build this expertise in a practical way by making use of a light meter—a tool for numerically quantifying the intensity of light re􏰍flecting off or falling on a given area. These are built into digital cameras, wherein they measure through-the-lens (TTL) the light refl􏰍ecting off a framed scene. Separate, handheld meters are also available, and work by directly measuring the light falling on a subject.

Using your camera’s light meter

Light meters built into digital cameras are an essential and advanced tool, but they are not a silver bullet; it is up to the photographer to use the meter appropriately for a given scene, and to interpret its measurements accurately. This is relatively easy with evenly-lit scenes wherein all parts of the frame are re􏰍flecting the same general amount of light. Many scenes, however, are more complicated, being composed of varying elements, each of which requires its own particular exposure. While the metering systems in digital cameras vary in their sophistication, they generally all have three distinct modes of operation, each optimised for particular lighting conditions.

Metering with your camera is as simple as choosing an automatic mode and taking a picture. Refl􏰍ected light is measured from various areas of the framed scene, and the camera sets a corresponding ISO􏰏/shutter speed􏰏/aperture (depending on the exposure mode in use). Even in Manual mode, digital cameras will still meter the scene, displaying those measurements as a guide for your exposure settings.

Matrix metering

This metering mode goes by many different names depending on the camera manufacturer—Multi-Zone, Evaluative, Electro-Selective Pattern (ESP), etc.—but they all work according to the same principle. A framed scene is divided into multiple segments, each of which is metered separately. The system then calculates an average exposure for the composite scene by comparing its readings to an image library programmed in its memory 􏰐consisting of some 30􏰆􏰄,000􏰄􏰄􏰄􏰔 images in the case of Nikon). It may even take into account focusing information, giving exposure preference to the area of sharpest focus.

Centre-weighted metering

Centre-weighted metering mode still measures the full scene, but gives exposure preference to a circle in the central zone of the frame—the general idea being that what is in the centre of the frame is more important than the elements around the edges. The precise weight that this centre circle is given depends on your particular camera model, and some models allow you to adjust the degree of exposure preference and size of the circle.

Spot metering

Spot metering measures the light only within a tiny circle that may cover as little as 􏰃􏰕 of the total frame, and completely ignores everything outside of that circle. It is used for precise measurements in complex lighting conditions, and the circle can often be placed anywhere in the frame.
Learn to “read” light in this complete course on the building blocks of photography. Michael Freeman teaches not only how to see the subtle nuances of light and shade, but also how your camera views and records them, to make sure you approach each scene with the optimal settings and appropriate technical know-how.

Photo School: Light and Lighting
Michael Freeman and Catherine Quinn
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