As Andy Rooney might famously say, “Have you ever wondered why sometimes your photos will come out just great, and other times they will either be way too dark, or bleached out to the point that they just look awful?”
I think most photographers understand that modern cameras have a light meter built into them and the camera’s internal computer uses the meter to determine how to “properly expose” the photo. But how, exactly, does the light meter work?
You might conclude that this meter and in-camera computer system is pretty “smart.” The reality is that a meter is actually “dumb” (in fairness, it really doesn’t make sense to refer to an inanimate object as “dumb” or “smart”). In reality, the internal exposure system in modern cameras is very sophisticated.
But a light meter doesn’t think–it measures!
For many years, photography was all about acetate-based film. Today, photographers mainly use digital sensors to record images. Both of these systems are based on the same principal ingredient: Light. The technical capture of images is based on quantity of exposure to light. As we covered in the tutorial on “F-stops,” the amount of light which strikes the sensor is based on time of exposure, size of lens opening and the sensitivity setting (ISO) of the sensor.
My Philosophy. So how do we use these variables to obtain the proper exposure? Before we go into mechanics, lets make a couple of important points. First,
Letting the camera (sophisticated as its system may be) choose these variables for us is the worst and least dependable way to obtain correct exposure
Second, “correct” exposure will always be, to some degree, a subjective judgment. Sure, there is a point at which all would agree that an image is underexposed (too dark) or overexposed (too light). But there is also a range of acceptable exposure, and very small differences may give nuanced results in color saturation and brightness of an image. Slight “overexposure” will tend to give a more “pastel” color result, with slightly brighter image, while slight “underexposure” will tend to give slightly more saturated colors, but a darker image. This is a part of the reason why modern cameras often allow exposure adjustment in fractional stops. Understanding correct “base” exposure will help us use these “nuances” to our advantage.
The “Science” of the Light Meter. The primary measurement tool is a photo-sensitive “light meter.” It is actually possible to get a reasonably accurate exposure without a light meter by knowing the reciprocity of a particular ISO, based on a standard set of conditions (the traditional one is the “sunny F16 rule,” which says that proper exposure for a particular ISO in bright, sunny conditions is 1/ISO, when the lens aperture is set at F16). Using this standard, you can interpolate between other stops, and guess pretty accurately about other light conditions. But it is still a guess, not a measurement.
Reflected Light Meters. How does the light meter measure light? There are two different ways light meters measure light. The most common method, which is used by light meters built into cameras, measures the light which is reflected from an object. This is a very important concept to understand. Photographic images often have multiple objects. This makes it paramount that the photographer understand which of those objects are the most important to be properly exposed! Often this means that the photographer will have to make choices. This type of meter is referred to as a “reflected light meter.”
Ambient Light Meters. The second type of photo-sensitive meter reads the “ambient” light. This type of meter is normally a hand-held meter which can measure the intensity of light falling on a subject rather than being reflected from the subject. This method requires the photographer to point the light meter toward the camera. One of the principal disadvantages of ambient light meters is that often the subject is far enough away or sufficiently transient (e.g., wildlife) that it is difficult or impossible to truly measure the light that is actually falling on the subject. Since the meter must be pointed at the camera, it should be obvious that such a metering system cannot be part of a camera’s built-in metering. Nor is it always possible to position an ambient meter in the scene near the light we are tying to measure.
Limitations of the Sensor. It is not uncommon for a scene to simply have more variation from light to dark, than can be captured. In this case, the photographer must make the important choice about which parts of the image must be properly exposed. Understanding how the camera’s meter works is the key to getting this right.
Metering Methods. Modern mid to high-end DSLR meters typically have 3 choices for measuring light: Balanced Matrix Metering, Center-weighted and Spot. Traditionally, SLR cameras had “Center-weighted metering (some pro models added Spot metering).
Center-weighted metering assumed that the most important part of the photograph was always in the center of the photo (that area in the square brackets in the center of the viewfinder), and thus weighted its measurement, typically about 80% in the center and 20% in the rest of the image.. The problem with this approach was that most good photographs do not place the subject “bulls-eye” in the center of the image. When available most experienced photographers did not rely on this method.
Spot-metering allows the camera to meter a narrow area in the viewfinder. Advanced photographers use this method most often, metering those specific areas they want properly exposed, letting the other areas fall wherever they fall.
Balanced Matrix Metering has gained great popularity in current day cameras (note: this is what Nikon calls it, Other manufacturers may have different names, but the concept is the same). The camera’s system has internally memorized thousands of lighting situations. The meter samples several areas of the image in the viewfinder and inputs this data into the in-camera computer. The computer compares it against its internal list of situations and suggests an exposure base on its comparison. My experience is that this suggestion is dead-on about 90% of the time. The other 10 percent, the meter can be completely fooled. If you are intent on getting the image, and may never have the chance to repeat it, this may not be the best system to rely on. Seasoned photographers more often than not use their spot-metering system to obtain the correct exposure (if you shoot digitally, and shoot the camera’s native “RAW” format, there is an even better way. We’ll cover that in a future topic).
What the Eye Can See. The human eye is a rather amazing optical instrument. The eye can see many “stops” (see “How F Stops Work”). The digital sensor, however, is not the human eye (yet–technology continually amazes me. I have read that there are sensors in development that may approach the technological sophistication of the human eye). The digital sensor can “see” (record) a range of about 5 photographic stops. The human eye, in contrast can see thousands off stops. So, often what we “see” simply cannot be captured and displayed with current technology.
It is critical for the photographer to understand this fundamental limitation of the digital sensor (or film). When using the light meter to measure the light in a scene, the photographer must make critical choices about which objects must be in proper exposure, and which will simply fall where they fall. Obviously, the subject must be in proper exposure. The range of exposure that the sensor is capable of is known as “exposure lattitude.”
Getting Exposure Scientifically “Correct.” In order for a light meter to measure light correctly, it must be calibrated to a “standard.” This standard goes back to black and white film days, and is known as “neutral gray,” halfway between pure black and pure white. Convention has this as 18% grey (technically, ANSI standards calibrate the light meter at 12%, but that is a story for another day — for our purposes, 18% grey is the standard reference). Theoretically, if you properly expose an object that is 18% grey in a scene, the entire scene will be well exposed, within the limits of the sensor’s exposure latitude.
Here is the critical point: The Light Meter is calibrated to “See” anything it is pointed at as 18% grey, and thus Directs you to Adjust your settings to get that result.
So, if you meter something that should be very light, such as snow, and follow the suggestion of the light meter, your result will be grey (actually, in color, blue/grey) snow! Likewise, if you are shooting something that should be black or very dark, like a black bird, or a bear, if you blindly follow the suggestion of your camera’s meter, you will end up with a grey bird or bear.
In a photographic scene, there may be many different elements. If you point your spot meter at a light or dark element, you will have the exposure variances noted above. What about if you use one of the “smart” metering methods, like center-weighted or matrix? These are the circumstances responsible for those “Andy Rooney” shots we started out mentioning. What happens is the bright or dark influences in the scene “fool” the light meter. Snow is a great example. Another example is shooting into the sun (known as back lighting). The meter is unduly influenced by the snow, and you end up with an underexposed image. Or, in the case of backlighting, the meter sees the bright sunlight only, and you end up with a silhouette.
Finding a “Neutral” Point to Meter. If you point the spot meter at a neutral object in the scene, you will theoretically get properly exposed snow and black objects. Unfortunately, many scenes do not have objects in them that are 18% grey. You can purchase a cardboard, or plastic card that is 18% grey, which can be carried in your camera bag. If possible, the card can be placed in the scene and metered. Again, this is not always a practical solution.
In real life, certain objects and colors will work as a good substitute. One of the things a photographer learns to do is look for something “neutral” in the scene to meter off of. Light grey tree trunks and rocks, blue sky on a sunny day, and grass are all good approximate 18% grey substitutes.
Learning to “Adjust.” There are other references that can be used with an “adjustment.” Caucasian skin, for example, is one stop brighter than 18% grey, so in a pinch, you can meter off your hand and open up one full stop from the setting, to arrive at the scientifically correct exposure. Snow, is 1 1/2 to 2 stops brighter, so in a winter scene, you can meter off the snow and open up to compensate. For this method to work, you must use your camera’s spot meter, and the longest lens (or lens setting in the case of a zoom lens) to “focus” the meter on the area you are trying to meter. Using the matrix or center-weighted metering will defeat this, by trying to “average” the entire scene. The best method is to learn how the meter measures the light being reflected from the object and learn to adjust accordingly.
A final point about reflected light metering: The meter must be pointed at the light you are metering. Be careful not to meter light in a different area that the light actually falling on the scene.
Getting the Exposure Esthetically Correct. As I mentioned in the beginning, true correct exposure will always be a matter of subjective judgement. When I shot slide film, I would routinely “bracket” (the practice of shooting several exposures, one or more “over” the correct exposure and one or more “under” the correct exposure). There were those who did that routinely as “insurance” that they got the technically correct exposure. That is not why I did it. I always preferred to measure the subject properly and rely on proper techniques to get the exposure right.
I bracketed because I would often find that I would find the “look” of slightly over or underexposure more esthetically pleasing. Higher end DSLR cameras allow adjustments of f-stops and shutter speeds by 1/2 or 1/3 stop. I usually would bracket 1/3 over and under for this purpose. With digital cameras (especially if you shoot in the camera manufacturers’ RAW formats), you can simply adjust for these exposures in post processing. With digital exposure, it is more important to expose properly for the maximum capture of data highlights (again, a topic for another day).
Understanding the “science” of the light meter will be the most important thing you can do to make your exposures consistently good. And, one you have internalized these concepts, you can begin to use them creatively to “make” photographs the way, as an artist, you want them to appear. For more in depth coverage of exposure issues, I highly recommend, Bryan Peterson’s “Understanding Exposure,” which is listed on the Blog here under Recommended Reading.