What’s a light meter and how does it work?
Light meters work by measuring the ambient and reflected light levels within the area you plan on photographing. Using medium gray, or 18% gray as a reference point, the meter takes this information and converts it to a combination of shutter speeds and lens aperture openings (f/stops) that can capture the highlight, shadow and midrange tones of most average scenes.
The thinking part comes into play when taking high- and low-key photographs or other scenes dominated by overly bright (sea and snowscapes) or darker (night clubs and coal mines) levels of ambient light. In such cases, the exposure has to be adjusted to allow more or less light to strike the camera’s imaging sensor or film, depending on whether you want to lighten or darken the scene.
With the exception of a few entry level and specialty light meters, most currently manufactured light meters have digital displays and solid-state innards.
Light readings can be taken two ways: by reading the light falling on the subject (incident) or the light reflected from the subject (reflected). Regardless of which method you choose, almost all handheld light meters can take both types of readings.
Reading the light falling on the subject
Without resorting to add-on accessories, your camera is designed to take reflected readings only. If you are photographing scenes containing average reflective qualities, which include most outdoor scenes, landscapes, evenly-lit interiors and copy work, you are good to go, especially if you choose a scene mode and white balance setting that’s close to the realities of whatever you’re photographing. If however, you are shooting overly bright or overly dark scenes, you’re most likely going to have to adjust your exposure in order to record the tonal values of the scene in their proper context.
Reading the light reflected from the subject
When taking reflected light readings of scenes containing mixed light values, take your reading from an 18% gray card or similar smooth, medium-gray surface. Reflected readings taken from gray cards give you readings that are the equivalent of an incident reading.
The beauty of incident readings is that because you’re reading the light falling upon your subject, it doesn’t matter how light or how dark your subject is. Because incident readings are based on medium gray and the light itself that is falling on your subject and not the light being reflected by your subject, all of your subject’s tonal values will fall into place with no additional calculations required on your part.
Reflective meters do offer benefits, too. These include smaller, specific readings and the ability to gain a better understanding between the exposure value (EV) of different subjects in the same scene (i.e. to determine how many stops different a black shirt is versus a white shirt in the same lighting).
If you are shooting in an evenly lit area, you’re better off taking an incident light reading, which involves sliding (or attaching)
Spot meters are reflective meters that read a narrow slice of the image area, usually no wider than 1- to 3° of the total scene, although a few companies offer “semi” spot meters with wider, 5- to 10° fields of view. Spot meters are invaluable for taking readings of backlit subjects and stage performances where the subject is often brightly lit but surrounded by large swaths of darkness that would confuse meters that cannot take pinpoint light readings.
In addition to handheld spot meters, many hand meters can be outfitted with optional add-on accessories that allow you to take spot readings with a “normal” flash or ambient light meter. Most cameras also allow for spot metering with special settings.
The meter in your camera cannot read flash, but almost all digital (non-analog) handheld light meters can read both ambient and flash exposures with equal ease and accuracy.
Handheld ambient/flash meters can trigger electronic flash packs using a tethered flash sync cord (all flashmeters feature a universal PC-type connector) or via wireless radio triggers. Many flash meters can also be placed on an untethered stand-by mode and capture a reading when the flash is triggered either manually or by the camera.
Other types of light meters include color meters, which read the color temperature of the light. Color meters are important if you are shooting under lighting conditions other than daylight or tungsten (fluorescent, sodium vapor, metal halide, etc.), where it’s difficult to visually determine the correct filtration to record the scene correctly. Color meters can be programmed to display readings in terms of Kelvin ratings (5500K is equivalent to daylight) or CC (color correction) filtration. In the days of film, color meters were more popular, but with the advent of digital cameras, which can be color balanced relatively easily without the need for filters, adapter rings and holders, color meters are not as critical for photographic applications.
Color temperature can shift (cooler, left; warmer, right) depending on the
light source and ambient conditions. Color meters help remedy this issue by reading color temperature more accurately.
Although many hand meters display frames-per-second (fps) readings for cinematographers, there are also cine-specific hand meters which, in addition to frames per second also display Lux, foot candles and shutter angles, which is information geared mostly for motion-picture production.
That’s both a fair and excellent question. Almost every camera contains a dual or triple mode light-metering system that, depending on the mode, can read the light globally (Average metering), a small portion of the total image area (Spot metering) or central or locally-weighted light readings of specific portions of the total image area (Segmented or Matrix metering). And with few, if any exceptions, the meter in your camera is quite accurate.
The limitations of built-in light meters include the inability to take electronic flash or incident light readings, though incident light readings can be taken with your camera if you resort to any of the optional screw-on or clip-on lens accessories that enable you to take TTL incident light readings. Built-in light meters are also awkward to use when “walking” a set or taking light readings in complex interior lighting scenarios in order to measure the light-level differences between one part of the set and another.
Without getting into the details of particular makes and models, it’s more than fair to say that although they function somewhat differently, the accuracy levels of camera meters and handheld meters are all fairly equal, and in the right hands either can calculate both ambient (in the case of hand meters) and flash exposures to within 1/10-stop accuracy.