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If you already read my articles on Aperture, Shutter Speed, and ISO, you know how to utilize the three variables that control exposure in a camera. But how does the camera measure the brightness of the scene to set shutter speed or aperture automatically or tell you, as the photographer, how much to adjust your exposure settings to achieve a specific result for the final image? It achieves this with the in-camera light meter.
Not all cameras have built-in light meters (almost all new ones do) and not all photographers use light meters. The famed photographer, Henri Cartier-Bresson, in a 1971 interview for Scholastic, with Sheila Turner, said, "And photo-electric cells in a camera [a light meter]—I don’t see why it is done. It is a laziness. During the day, I don’t need a light meter. It is only when light changes very quickly at dusk or when I’m in another country, in the desert or in the snow. But I guess first, and then I check. It is good training."
Thirty-six years later, in a 2007 interview for The New Yorker, Cartier-Bresson's wife, Martine Franck, also a photographer, told us her thoughts on not using a light meter when she said, "I think I know my light by now."
Incident versus Reflected Light
An army of photographers came of age before the light meter was an integral part of the camera. Before your camera measured exposure, you had two options: 1) carrying a handheld light meter or 2) guessing the exposure. I remember, as a child, holding my father's light meter in front of me and sharing the readings with him as he readied his trusty Leica rangefinder. Back in the days of film, missing an exposure could be a costly mistake, as there was no instant digital review to ensure you captured the moment in a way you had intended.
Let us digress for a moment and talk about light and the handheld light meter and the difference between it and the in-camera light meter. In any scene, except in a dark closet inside a dark room or in the bow-thruster void of a merchant ship (ask me how I know this), there is a light source that is either artificial (light bulb, candle, strobe, the glow of a computer screen, etc.), or a natural light source (the sun, the sun reflecting from the moon, or the light of the stars). The handheld light meter reads what we call the "incident" light of a scene—the light falling on the scene from the given light source(s). When I was outdoors, I would hold Dad's handheld analog light meter in front of me and point it back toward him. I was measuring the amount of light falling onto my position from the sun.
There are exceptions, but most in-camera light meters read "reflected" light in a scene. The light source illuminates an object and that light is reflected toward the camera and through the lens. Speaking of "through the lens," you may have seen the acronym "TTL" in your camera's manual. Now you know what it means. TTL metering is the industry’s way of telling you that your camera's light meter is reading the brightness of a scene through the lens of the camera—not through a separately mounted meter. Some cameras do have a non-TTL metering system. We will return to the “incident” versus “reflected” light topic a bit later.
OK, so your camera's built-in light meter tells the camera how bright the light is that is reflected from the objects in the scene. In order to assign a value to "bright" or "dark," we need to have a baseline so that we can determine "brighter" or "darker." For most cameras, the meter evaluates a monochrome (black-and-white) version of the scene and uses a mid-tone middle gray as the baseline. Darker grays approaching black are read opposite of lighter shades of gray that approach white.
Depending on the camera, you may get a graphical representation of the light meter's reading. Many cameras show a scale in the viewfinder and/or on the LCD top or rear screens that shows if the current aperture and shutter speed settings will over- or underexpose the image. You can use this information to make an informed decision to alter the exposure of the image from the baseline meter reading. The scale generally has hash marks that indicate stops of exposure.
So we know the camera's meter is measuring the amount of light reflecting from objects in the scene framed by the lens, and we know that it is usually evaluating a monochrome image of that scene. Now we have to ask, "Where in the scene is the camera measuring this reflected light?"
The answer leads us to investigate your camera's metering modes.
Over the years, the camera's light meter has gotten more precise and sophisticated, but, luckily for us photographers, the operation of that meter has remained fairly straightforward. The basic camera light meter modes are multi-segment, center-weighted, and spot. Multi-segment or multi-zone usually gets referred to by different names depending on the manufacturer. Nikon uses "matrix" metering. Canon calls it "evaluative." Olympus has "ESP/Digital ESP." Leica calls it “Classic.” There are others.
Also, your camera may have additional metering modes for specific situations. For instance, Nikon’s D810 camera features a “Highlight-Weighted Metering” mode. For the purposes of this article, we will focus on the three standard modes common to most camera systems.
This metering mode takes reflectivity information from the entire scene to try to achieve a balanced exposure for the entire scene. Meters are all different, but many multi-segment meters read information from five different areas that cover the majority of the scene and then produce an average reading of the segments. Some multi-segment meters evaluate the five segments, but alter the average by giving information of the center of the scene more weight—usually 60% in the center and the remaining 40% for the other regions.
Many of today’s digital meters measure thousands of zones (pixels) in the frame, not only for monochrome reflectivity, but also for color and even for the relative distance of the subjects inside the frame to give the photographer a more precise measurement of the light.
This mode evaluates only the reflectivity information at the center of the scene as framed through the lens. Many cameras have markings on the viewfinder that show a large circle around the center of the viewfinder's center. Depending on the camera, this larger center circle may correspond to the area being evaluated in the center-weighted metering mode. Your camera’s manual will tell you if the markings in the viewfinder are related to the metering.
Reduce that center circle even more and you will be using the camera's spot-metering mode. This allows the photographer to meter reflectivity from a very small portion of the scene. In the era before digital, the "spot" was relegated to a tiny region of the center of the frame. Many of today's digital meters allow you to move the spot to another area of the frame, usually co-located with a focus point.
On cameras where you cannot move the spot region, you can aim your lens at the subject you wish to meter, depress the shutter half-way to take a meter reading, and then use an "auto-exposure lock" button or switch to lock-in that reading while you re-compose. Again, your owner’s manual should walk you through this camera-specific process.
Choosing Your Mode
Have you noticed that I mentioned your camera’s manual a few times in this article already? Is it now in your hand or next to your computer? Good! Definitely read up on the particulars of your camera’s metering modes—what segments are active, how to select different modes, and if the viewfinder indicates where you are metering.
The rubber meets the road when you, as the photographer, choose a metering mode to help you best achieve the image that you want to produce. There is no right or wrong metering mode. There are, however, times when you might have a certain vision of what you want your image to look like and the metering mode you select prevents you from getting that result.
For example, you might be sitting in a restaurant and taking a portrait of the person sitting opposite you. The lamp over the table illuminates their face against a dark backdrop of a dimly lit restaurant space and you envision an image where the person's face is virtually the only region of the image illuminated. If you take the portrait using multi-segment metering, you might end up getting an image with the subject's face overexposed as the camera tries to balance the exposure between the dark background and the highlighted face. Depending on your framing and camera's capabilities, center-weighted or spot metering might help best get you the intended results.
The advantage of digital imaging is that you can review your image as soon as you take it, and make adjustments to your metering mode or exposure as needed to alter the image to better match your artistic vision. In the days of film, your "misses" were only revealed after the film was processed.
We discussed earlier the difference between reflected and incident light and how camera meters usually measure reflected light while most handheld meters measure incident light. Today's in-camera meters are known for their incredible accuracy, but there are reflected-light scenarios that can fool your camera's built-in light meter.
This trickery can be found in scenes where there are extremes of tonality. The in-camera reflective light meter is happiest when the scene lives within the mid-tones of an image. Snow scenes, expansive black or white backdrops, and sunset landscapes can all fool the in-camera meter and give you gray snow or a no-longer-white backdrop, a less-than-black backdrop, or a dark world surrounding the setting sun.
To overcome this, the photographer will have to manually adjust the camera's exposure to correct for the error.
And, just so you don’t think the handheld incident meter is infallible, there are also scenarios where the in-camera reflective light meter works better. Back-lit scenes are a prime example of this.
If you are just getting started in photography, I highly encourage you to experiment with your camera's metering modes. Also, start paying closer attention to the scenes you are photographing, in terms of light. Introduce "metering" into your evaluation of a scene. What is your subject? Is it the brightest or darkest part of the scene you have framed, or is it about the same as everything else? What do you want the camera to expose for? If you expose for your subject, what happens to the remainder of the image? What do you hope to lose in light or darkness? Think of your composition and the final result you want. Then, use that pre-visualization to help you choose the best metering mode for your vision.
With digital photography, experimentation is free. Change modes. Alter your exposure. Tinker, tinker, tinker until you get the results you want. Understanding how your camera meters the light, while you become cognizant of the light of your scene, will greatly benefit your photography.
Or, if you already know your light, set your camera to Manual, ignore the light meter, set your aperture and shutter speed, and take a picture!