A light meter is a tool that measures light intensity and aids photographers in setting the appropriate exposure. Photographic light meters are available as off-camera handheld devices, and they’re built into virtually every digital camera.
The only thing a light meter can do is measure the actual brightness of an object; it can make no assessment of whether the object is light or dark, in deep shade or in bright sun. It’s up to you to evaluate and interpret the nature of your subject according to the information provided by the meter. By understanding how light meters work and are calibrated, you’ll be better able to use them to determine the appropriate exposure for a scene.
– John P. Schaeffer.
The 18-percent grey standard
The 18-percent, or middle grey, standard is the mathematical average of all tones evenly distributed across a scale from absolute black to absolute white. In digital photography, the middle grey tone lies at the precise middle of the luminance histogram, and there are corresponding middle tones for the red, green, and blue colour channels. (All future references to 18-percent grey or middle grey refer to the tone independent of its colour). When printed, a middle grey tone reflects 18 percent of incident light (i.e., light falling upon it). Photographic exposures derived from metering the average of all tones in an average scene are remarkable at obtaining average results.
Two types of light meters
There are two types of light meters: incident-light meters and reflected-light meters.
Incident-light meters. Incident-light meters measure the amount of light falling on the subject and are only available as off-camera, handheld devices. The most prominent feature of incident-light meters is the translucent white hemispherical dome (the “lumisphere”) that both encloses and provides even illumination to the photocell within, which measures the light’s intensity. To obtain an exposure reading, you hold the incident-light meter at the position of the subject, ensuring that the dome is in the same light, point the lumisphere towards the camera, and take a measurement.
Since incident-light meters measure the intensity of light striking the subject, they provide accurate exposure information regardless of your subject’s inherent brightness; so long as the subjects in your scene are evenly illuminated by the same source of light, dark tones will be rendered as dark, grey tones as grey, and light tones as light. Furthermore, incident-light meters are incredibly useful for making accurate measurements in a controlled-lighting environment, such as a studio, and are especially practical when used to calculate precise contrast ratios between different lights.
Despite the neutrality of incident-light measurements, there are several inherent disadvantages you should know about. First, measuring the intensity of light falling upon a distant subject may be difficult, impractical, or impossible. For instance, an incident reading of the night sky is impossible, of a sunlit mountain from an outlook covered by the canopy of trees is impractical, and of a subject under complex or mottled lighting is imprecise. Secondly, since incident-light meters measure subject illumination, the exposure values they provide for the aperture are in theoretical f-stops. If the transmission value of your lens is significantly different from the theoretical f-number it’s set to, the exposures won’t be accurate (see F-Stops and T-Stops).
Reflected-light meters. Reflected-light meters work by measuring the light reflected off the subject, thereby measuring the subject’s brightness. Reflected-light measurements are taken from the intended position of the camera regardless of whether the meter is on- or off-camera. Virtually every digital camera meters exposure using reflected-light entering the lens, which is known as through-the-lens (TTL) metering.
Handheld reflected-light meters come in two broad varieties. Wide-angle reflected-light meters take an average reading from across a relatively large area. Spot meters are reflected-light meters that read the brightness from a relatively small portion of the scene—typically one degree of your field of view. All handheld spot meters feature a magnified viewfinder with a clearly marked circle that outlines the metering zone. Spot meters tend to be more expensive than both incident- and wide-angle reflected-light meters.
Reflected-light meters and 18-percent grey
Every reflected-light meter is calibrated to provide an exposure reading that renders the subject it’s pointed at as middle grey. If you take a reflected-light reading off a black square of paper, the meter will propose exposure values that will render the paper middle grey in the photograph, thereby overexposing it. If you take a reflected-light reading off a white square of paper, the meter will propose exposure values that will render the paper middle grey in the photograph, thereby underexposing it. The only tone for which reflected-light meters provide objectively accurate exposure values is middle grey.
On-camera exposure metering
Every digital camera that features either exposure compensation or a manual exposure mode will feature an exposure meter either in the viewfinder, the top LCD panel, or both. The exposure meter is a simple linear scale of relative exposure values marked by notches, dots, or numbers, and on some cameras, all of the above. The standard exposure index, characterized by the big central notch, or a zero, always represents the optimal exposure as determined by the camera’s programming. Flanking the standard exposure index are marks that represent increments of 1/3 EV and 1 EV. The marks on the right side represent added (or positive) exposure, and the marks on the left represent subtracted (or negative) exposure. The exposure level indicator is the needle or marker beneath the notched scale.
Together, the exposure meter scale and exposure level indicator serve one of three functions depending on the exposure mode you’re using.
Exposure compensation amount. Exposure compensation allows you to increase or decrease image brightness beyond what the camera determines is optimal exposure. (See Exposure Compensation). When the camera is set to the Program, Shutter Priority, or Aperture Priority modes, the exposure meter shows the amount of compensation applied in units of EV. When the exposure level indicator is set to zero, there’s no exposure compensation, and the camera reverts to its standard exposure programming.
Exposure level indicator. In Manual mode, the exposure level indicator shows how much your set exposure values deviate from the levels the camera believes are optimal, which always reside at the standard exposure index. (See Manual Exposure).
Auto-exposure bracketing range. Exposure bracketing is the technique of taking multiple exposures of the same scene while slightly varying the amount of exposure between the individual photos. This technique is incredibly useful when capturing subjects with complex lighting, or when you intend to combine multiple exposures into one high dynamic range (HDR) image. Auto-exposure bracketing (AEB) automates the process within a range defined by the user. When AEB is activated, the camera’s exposure meter will show multiple exposure level indicators specifying the AEB range of the consecutive shots.
Standard metering modes
Reflected-light metering is how cameras measure subject brightness to determine optimal exposure. Most digital cameras offer several metering modes that feature varying degrees of sophistication. Regardless of the metering mode, it’s important to remember that every reflected-light meter, including your camera’s, is calibrated to provide an exposure reading that renders the subject as a middle grey tone. The most significant difference between the metering modes is in their method of interpreting the distribution and variation of the scene’s tonal range.
Evaluative/Matrix/Multi metering. This is the default and most sophisticated metering mode on most DSLRs and mirrorless cameras because it works in a variety of situations and provides accurate results most of the time. It works by dividing the frame into multiple metering segments and analyzes their brightness and, sometimes, colour. The resulting matrix of multiple segments is evaluated based on composition, colour, and distribution of tones. Nikon cameras that feature 3D matrix metering also factor the distance information provided by the autofocus modules. Some mirrorless cameras evaluate the scene for the presence of prominent faces.
Centre-weighted metering. This metering mode is designed primarily for portrait photography. It considers the average brightness of the entire frame but gives the greatest prominence to the tones in the central region. Centre-weighted metering isn’t recommended for scenarios in which multiple shots of various subjects are anticipated, as the exposures will fluctuate.
Average metering. This mode measures the average brightness of the entire frame and sets exposure to yield a middle grey tone rendering of that average. This mode produces particularly accurate results for landscapes with the sun outside the frame. It provides relatively consistent exposures across multiple shots of different subjects under the same lighting.
Spot metering. The spot meters found in cameras are similar to those in handheld light meters. They read light from a small central area that typically comprises one to five percent of the frame. On Nikon and most mirrorless cameras, the spot meter overlaps with the active focus point. On Canon cameras, the spot meter is always found in the centre of the frame. Spot metering is both incredibly empowering and finicky. When using any of the auto-exposure modes with spot metering, the camera will set its exposure values to produce a middle grey rendition of whatever subject the spot is pointed towards, and all other tones present in the scene will align according to their relative brightness. For this reason, using spot metering in any of the auto-exposure modes will produce wildly inconsistent exposures of diverse subjects under the same lighting. For example, when using auto-exposure, spot metering a black dog will produce an exposure where the dog appears grey and the scene is overexposed; spot metering a white dog will produce an exposure where the dog appears grey, and the picture is underexposed; and, spot metering a grey dog will yield an exposure where the dog appears grey, and the scene is correctly exposed. Ponder this point.