Engadget Primed: Camera metering explained

Primed goes in-depth on the technobabble you hear on Engadget every day -- we dig deep into each topic's history and how it benefits our lives. You can follow the series here. Looking to suggest a piece of technology for us to break down? Drop us a line at primed *at* engadget *dawt* com.

Is learning how to meter with your camera really necessary? With all the intelligent models out today, who needs it, right? Well, you may, depending on the type of killer photography you hope to produce. When it comes to the person behind the camera, there are a few types of photographers. First, there's the photog who just wants to snap away, not terribly concerned about how their photos turn out -- or, at the very least, not interested in knowing how to alter the camera. If this describes you, that's fine -- the latest-and-greatest compact cameras may be your cup of tea. But then there's the amateur shooter who desires a better understanding of how their cameras determine exposure, and if you fit into this category, this piece should be right up your alley.

Metering is not a subject you can easily master, let alone explain entirely in an article of this length, but we can give you an idea of how it all works. I consistently see the faces of my workshop students glaze over as I wax rhapsodic about the wonders of metering, but I also notice wry smiles from time to time, which shows me the wheels are turning -- they begin to realize all the things they can do if only they can conquer this aspect of photography. However, comprehension and applied mastery are two separate balls of wax. Putting what you learn to practice is the start, and you can improve over a number of months, but true metering control in any situation requires years of practice.

So for our 2012 pre-apocalyptic installment of Primed, we'll break down the world of camera metering, giving you a bit of history, dissecting the main components, describing what your camera wants to do and telling you what the future may hold. By the end, you'll have a better understanding of this vital photographic topic... either that or you'll be in a nice deep sleep.

Note: Check out our recent Primed articles on aperture and image sensors to add to your metering knowledge.

Table of Contents

Since its inception as early as the tenth century A.D. by Ibn al-Haytham, a Muslim Persian scientist born in southern Iraq in 965, the primary function of a camera was, and is, to collect and record light. Yet not until the early nineteenth century was a process created to make a photograph permanent. By the mid 1820s, a French man named Nicéphore Niépce (don't ask me how to pronounce his name) captured an eight-hour exposure -- View from the Window at Le Gras -- using a rudimentary yet remarkably innovative setup resembling a pinhole camera, with an asphalt-like substance coated on pewter (hardening when exposed to light), to process the image permanently. The world was visually changed from that moment on. (It should be noted there has been a recent dispute with a claim Thomas Wedgwood, an inventor who created a method to copy visible images chemically to permanent media, captured the first photo in the 1790s, while another states Niépce produced a photograph in 1822, an engraving of Pope Pius VII, using the heliographic process -- perhaps there can be a dance-off to settle the score.)

In the years to come, a number of chemical processes were invented and refined to create a permanent photograph through glass plates, film and photographic paper. From the 1820s up into the 1980s, metering really hadn't changed much. Even as the photographic medium progressed from the chemical process using silver halide (film), to the digital process with a charged-couple device (CCD), the sensitivity of the material used was still measured using the ISO rating. However, the way you metered changed, from photographic charts to handheld incident light meters, on to reflective meters built into the camera. (We'll address incident versus reflective meters later in this article.) Nevertheless, the basic equation of light + medium sensitivity = exposure, pretty much stuck from photography's inception. How much light you have (measured in exposure value, or EV) combined with the material's sensitivity to light (measured in the past using ASA or ISO rating, ISO being the main one used today), gives you an exposure (the total amount of light permitted to enter a camera and be received by the image sensor or film).

Back in the 1800s, when photographers coated their own plates -- in essence making their own film and prints in portable darkrooms -- exposures were too long to capture portraits, so the first subjects documented were still lifes, buildings and street scenes. Essentially working in low ISO numbers, processes such as daguerreotypes and calotypes were less sensitive to light, thus slow. The standard outdoor lens was the Achromat Landscape, featuring an f/16 non-adjustable aperture. With small apertures came longer capture times -- twenty to thirty minute outdoor exposures -- resulting in ghost images of people in various cityscapes (imagine the documentation of the 1906 San Francisco earthquake). As portraits became possible and popular, deadpan expressions and slightly blurred facial features were also evidence of these long exposures.

Then the wet collodion process broke through in 1851, and the increased sensitivity of the medium meant dramatically shortened exposure times. Soon after came lenses with adjustable f-stops, followed by more aperture control when it was discovered that aperture size affected depth of field. The invention of film roughly hit at the same time as the collodion process, in the form of glass plates coated with photographic emulsion. Then, in 1885, the first flexible photographic roll film was released by Eastman Kodak, offering increased light sensitivity which enabled shorter exposures. As the twentieth century took hold, the Zone System, a new systematic method of metering and exposing to maximize proper tones in any given scene, was perfected by Ansel Adams and Fred Archer, stretching the capabilities of photography. From the time Kodak sold its first roll in the 1880s, to the early 1990s, film quality continued to improve -- grain became tighter in higher ASAs or ISOs, color more accurate or vivid (depending on the brand and type of film chosen), and latitude covering higher contrast increased some -- yet the process of metering remained somewhat consistent, even if the models in the Kodak commercials made life look pretty darn special.

Creating high-quality images is a marriage of three facets: using photographic techniques, knowing your gear and having the artistic ability to react at a moment's notice – metering is a huge part of this amalgamation. It is one of the most important aspects of photography, yet most photographers struggled with. Exposure is the foundation of all great images, and when you learn how to control your in-camera meter, the door towards photographic creativity opens, empowering you to make more effective decisions. It's a technical subject that builds you a solid base on the path to capturing extraordinary photographs. You can create mood, enhance color, emphasize light and add subtly to detail when you capture a proper exposure, whether you are photographing a portrait of a friend, a grand landscape, a vacation moment or a hamburger for an ad.

Another reason to learn metering is that you can apply the concept to almost any art or craft, knowing the limitations of your tools -- in this case, your camera and image sensor. Being well-versed in what's possible, and figuring out ways around potential disadvantages, helps to create better final images. Through lighting, contrast and exposure, pros use this knowledge to their advantage on every shoot.

From camera to computer

In this day and age of the digital darkroom, where editing programs are so robust, the phrase "fix it in Photoshop" seems to be here to stay -- and because of this, many question the need for metering knowledge. I explain this doubt with the analogy of the camera's exposure being the recipe or items you carefully select and mix together, and Photoshop being the oven you cook it all in. You need both to generate a high-quality final creation. Capture as much possible information in-camera and the easier post-capture editing becomes. Less adjustments are needed, and because your RAW image file stores the appropriate detail, the better the final result; tones and colors match more accurately, and less highlight and shadow detail is lost. Image editing programs are a wonderful expansion of photography, but if you rely on adjusting every aspect of your photo using software, your images with lack impact and consistency, quality will level off, and computer time will exponentially rise. Finally, the knowledge of metering also prepares you for processing your digital files in computer -- the added knowledge of tonal range, contrast and light helps to create the most realistic renditions.

Bracketing

Photographers who lack the understanding of how to meter use a technique called bracketing on a regular basis, capturing a series of exposures of one scene hoping to nail one solid frame. However this adds post-capture time, forcing you to review a higher number of images, while also filling up more storage space on your memory cards. In many cases, bracketing may not be an option due to a moving subject or a single precise moment that can't be captured in a series of shots.

There are some advantages to bracketing, the biggest being the higher percentage of chance you capture the exposure you desire, and with extremely contrasty scenes sometimes there are a mix of exposures that work, therefore the decision becomes which type of mood or detail you desire instead of which exposure. Bracketing can also assist in the education process of metering, showing you how your camera reacts to colors, tones, over and underexposure. This is best applied when combined with the knowledge of metering.

Reiterating the aforementioned basic equation, exposure is the total amount of light allowed to enter a camera and received by the image sensor or film through the course of capturing an image. This is created through three main components: aperture, shutter speed and ISO.

Aperture is defined as an adjustable iris (circle, hole or opening) inside your lens that changes depending to your setting (called the f-stop). It controls two aspects of exposure. First, the amount of light entering the camera and lens, through the f-stop -- a measurement used for the size of the lens opening; the larger the aperture, the more light, the smaller the aperture, the less light. Secondly, the f-stop adjusts the depth of field in a scene. Measured in inches, feet or meters, this is the range of distance over which the image is not unacceptably less sharp than the sharpest part of the image. For more on apertures, see our recent Primed article: What is aperture, and how does it affect my photos?

Shutter speed is defined as the rate at which the shutter, a mechanism inside the camera, opens and closes to expose a scene -- whether on film or onto a digital image sensor. It too controls two aspects of exposure: the amount of light entering the camera and lens, and the speed at which the scene is captured, measured in time (fraction of seconds, seconds, minutes or hours).

Digital ISO (known as ISO or ASA with film) is the third factor of metering, and one that gives you more choices with your exposure settings. Since you can control shutter speed and aperture through ISO, the number you choose can affect the final outcome greatly. ISO determines the sensitivity of your image sensor or film to light. The higher the number, the more sensitive; the lower the number the less sensitive. As with any aspect in exposure, there are trade-offs. The "faster" or higher number ISO (i.e. 400, 800, 1600), the less light is required (a shorter time is needed to expose the scene onto the image sensor). The lower the number, the more light is needed (a longer exposure). For more information, see our recent Primed article and review the section on ISO: Why your camera's sensor size matters.

These metering components can be altered in 1/3, 1/2 or 1-stop increments, and the combination of all three when balanced just right creates a solid exposure. If you are wondering what a stop is, it's a unit used to quantify ratios of light, one stop either halving or doubling the amount of light entering the camera.

As the foundation for all great images, without the knowledge of exposure, your creativity lacks, you miss moments, lose subtle colors and spend more time in post-capture editing. Unless you comprehend how your meter works, capturing a good shot becomes hit or miss. The resulting image may not be what you expected, and if it is, you may not understand how to repeat it.

Your camera's internal reflective meter is the best tool for calculating exposures, if you understand how it operates, and how it can be fooled under a variety of circumstances. It measures light reflected off your subject and is influenced by the amount of light, the tones, color and contrast, then converts this reading into a recommended exposure setting. There are handheld versions called incident meters, which measure the amount of light that falls onto your subject, held next to your subject, then pointed toward the light to measure the source. Incident meters aren't as easily fooled as their reflective counterparts, however the option to place the meter next to your subject or in the same light is not always practical during a shoot -- and it's yet another piece of equipment to carry. So, one detects the light after it hits your subject (reflective), while the other reads it before it strikes the subject (incident). Which is more accurate? Neither. They both read light differently. For this article, we'll address in-camera reflective meters, since 99 percent of photographers use these to read light and expose.

Your meter has a one-track mind

Point your camera at any scene and its main objective is to make whatever it's metering middle grey. Say what? I'll clarify. Referred to as Zone V by Ansel Adams, middle-toned or 18-percent grey by others, this grey is not a color, yet a tone halfway between white (highlights) and black (shadows). Your meter is telling you to shoot at a certain setting, but what it's really saying is 'take the photo at this exposure if you want to make this area or subject middle grey in tone' -- whether a tan face, the bright white moon, a landscape with a mix of colors or a dark burnt log. If your subject is middle grey in tone your exposure will be accurate, but if it's not, then you have some adjusting to do. Of course not everything we shoot is middle grey, so I use the term "meter's recommendation" when it comes to the exposure settings it offers, since your meter is not necessarily giving you the correct exposure. How do you distinguish what's middle grey, how much brighter or darker the tonal values in the scene may be, and how much to alter your meter to fix a tone from what is recommended? This is the part taking time, experience, miscalculations and trial and error -- a massive learning curve requiring years to perfect.

Your Metering Needle

Found in your camera's viewfinder or on the top LCD screen, your metering needle is another important tool, coordinating shutter speed, aperture and ISO to create a well-exposed photograph. Normally a linear area with notches, and typically denoted by a "0" in the middle, with "+" and "-" on each side, most metering needles (or levels) show a four-to-six stop range, depending on your camera model. Zero is the default setting whenever your camera is in an auto exposure mode. These modes include aperture (Av) priority, shutter priority (Tv), or program (P), telling the camera to determine an exposure based on the meter's middle grey-toned goal. Switch to manual exposure mode (M), and once you set your needle to "0", you can either overexpose or underexpose, overriding this recommendation to match your subject's true tone or create a mood; overexpose if you want to make your subject lighter than middle grey, underexpose if you want to make it darker than middle grey. You can also make this adjustment in an auto-exposure mode using exposure compensation (EC). Often seen as a "+/-" symbol on one of your camera buttons, EC offers the same method of moving to the plus side to brighten an exposure or match a brighter tone, or the minus side to darken a tone or scene.

A good initial question to ask yourself during this metering process is "what is my subject?," then meter attempting to match your subject's tone as accurately as possible, and let the rest of the scene fall where it may -- this method won't insure a perfect exposure every time, but it's a good starting point toward learning how to meter. It should be noted there are a myriad of exposure modes in cameras today, including pre-programmed scene modes, but these are considered auto exposure modes since they determine exposure in a way similar to aperture priority, shutter priority and program.

The Latitude from Highlight to Shadow

One big misconception of exposure is that you have to maintain detail in every area of your scene, all the time --- this is simply impossible, due to film or an image sensor's limitation in handling contrast (measured in stops of light). Film is capable of a five to six stop range, and an image sensor covers roughly six to nine stops. Human vision can pick up roughly 14-20 stops of light in one scene (without adjusting the pupil), so the assumption of trying to maintain all detail in all of our images is an exposure myth. If the contrast ratio from light to dark stretches more than six to seven stops, in most cases you cannot obtain every bit of detail in a single frame. Another strong belief is that by using your LCD screen, histogram or Photoshop, you can fix exposures, but unfortunately this can lead you astray -- your creativity lacks, you miss moments, lose subtle colors and detail and spend an inordinate amount of time in post-capture editing. Technology has come a long way and these new digital tools provide an advantage in the metering department, but they cannot replace a good understanding of exposure -- more on this later.

Learning how to judge tonal values is the big exposure challenge, so having a reference point always helps. One starting point to recognize is that two stops overexposed from "0," middle grey, gives you white with detail, and two stops under produces black with detail -- this depends on light and contrast, and is not exact, yet it's a rough reference point for you to begin to understand tones and metering. Go a half-stop further on each side, and you begin to lose most if not all detail in the bright highlight and darkest shadow areas.

Once you have some knowledge of how your camera meters a scene, metering modes are the next function to grasp, learning how the differences between each can drastically affect the recommended exposure. The mode you choose can and will change your meter's recommendation for any particular shot.

Spot or Partial metering

The toughest mode to master, spot metering reads the light from a small section, defaulted in the center of the frame (usually between 1-5 percent), whereas Partial meters a slightly larger area (5-9 percent). Using this mode allows you to select a small part of the scene, meter it directly without the influence of any other area, and judge the tone. In studying exposure for years this became my favorite mode, utilizing it more as an advanced method of reading various parts of a scene, similar to the way Ansel Adams used the Zone System. A bit of math on the fly is required with this method but it has its advantages.

Center-weighted (CW) or Center-weighted Averaging

Another main type of metering mode used in cameras going back many years, Center-weighted meters roughly 13-15 percent of the center area. If you choose the Center-weighted Averaging option, the entire composition is included in the reading, with the meter concentrating 60 to 80 percent of the sensitivity towards the central part of the viewfinder. Center-weighted metering is good for a variety of uses, especially where one tone is relatively large in the frame, such as a similarly toned area of sky, lake or grassy hillside.

Using certain metering modes like Spot, Partial or CW, can show you exactly where you are metering. This becomes a tool for learning since you can determine if you guessed a tone correctly once you review the image in computer. As you make mistakes or nail a tone, you learn, and your accuracy improves over time.

Pattern metering

Also known as Evaluative (for Canon users), Matrix (for Nikon users), Average, Multi-metering or Multi-pattern, this metering mode reads the light intensity in several points around the frame, averaging them for the final suggested exposure setting, favoring no major portion of your composition. Many feel it's the most accurate of all metering modes, but it depends on the subject, as well as the balance of light and tones. Metering using Pattern when in manual exposure mode is tough since you are really in a black box, not having a clear idea of how the camera determines the exposure -- how it weighs highlights versus shadows at various points -- thus any manual adjustments are hit or miss, unless you meter one continuous tone throughout the frame, such as the sky, a like or snow. It is best for auto exposure modes when you have little choice or time to meter, such as when capturing a fast-moving subject coming in and out of shadows. The comparison below gives you an idea of how metering modes react to a specific scene depending on the tone, light and subject matter.

There are three others features of digital that add to this metering conversation: autofocus, your LCD screen and the histogram. The autofocus (AF) point can play a part in the meter's recommended exposure, depending on the camera you own and the setting you use. Some systems allow you to link or unlink autofocus to metering with the option of locking an exposure once AF confirmation is achieved (through auto-exposure lock). It's hard to determine how much weigh is placed on the autofocus spot, as compared to the metering mode used, adding some confusion to the recommended setting and requiring a bit more experience.

Some feel they can work around exposure by reviewing the LCD screen to see if they correctly metered the scene; then a miscalculated exposure could be fixed. The main problem with this method is, under a variety of circumstances, reviewing your image for exposure on your LCD screen is not preferred -- whether due to bright outdoor conditions reflecting on the screen, the size of it as compared to a large computer monitor in controlled conditions, or simply because it doesn't have the quality to judge exposure. Imagine watching your high-definition television in the backyard on a bright sunny day, then again at night in a darkened room -- although you hadn't adjusted the television brightness, contrast or color, there is a drastic change in the way the screen looks to your eye; the analogy of how your LCD screen can fool you on exposure. You can get a ballpark figure of exposure from your screen, but not true accuracy.

It's hard to get away from this habit, but it's good to know why your images may look different in your computer than on your LCD screen. Deciphering a histogram is critical to unraveling its real purpose, and similar to in-camera meters, it is often misunderstood and misused. The statement I routinely hear in my photo workshops is I don't know why my image wasn't well exposed -- I used my histogram. The fallacy of being your exposure solution has tricked most photographers into believing that digital solved the difficulties of exposure. Working in tandem with the LCD screen, many use the histogram to judge exposure, and although it is a wonderful digital tool, it is more sophisticated than most realize.

Histograms may help you determine certain aspects of exposure, such as pixel value, but most seasoned pros rarely review them in the field. First off, there is no such thing as a correct, or proper histogram. Having a perfect bell curve to your histogram does not create a perfect exposure -- this only tells you there is an abundance of middle tones in your exposure. This would be fine if you are photographing a grey rock, but not if you were capturing snow. Well-exposed photos can have awful looking histograms, where the graph is all the way to the left or to the right, maybe even cutting off highlights or shadows. This goes back to the fact that capturing every detail in every shot is impossible due to the limitations of an image sensor.

Other photographers rely on 'blinkies' (blinking areas on your LCD indicating overexposed highlights) to determine exposure, but once again, your image sensor's dynamic range cannot cover certain areas within many contrasty scenes, so this too can throw you off. If you judge your histogram, allowing it to control your exposure, or darken your exposure until the blinkies go away, you might underexpose the most important part of the scene: your main subject. The histogram is a better tool to utilize in post-capture processing -- to help study the range of our image sensors and ensure that you retain specific highlight or shadow detail. Would you ever use the histogram in the field? Sure, but this should be when you first comprehend exposure and how to meter light, then it might add to the metering decision. For instance, if you want to keep detail in a subject whose tone is close to white or black, you would meter your subject, expose the scene and check the histogram to guarantee no loss of detail in the critical areas.

Frédéric Guichard, chief scientist at DxO Labs, sees metering innovations coming through application of high dynamic range (HDR) technology -- we agree. HDR has altered and stretched the bounds of metering, and new systems including cell phone cameras have begun incorporating these options with relative ease and decent success. "Other metering opportunities include a trend towards mirrorless cameras, pushing manufacturers to innovate and find solutions for better autofocus speeds in these models, possibly through ways of measuring autofocus directly on a mirrorless camera's sensor," says Guichard. "Innovations regarding phase detection, a common type of autofocus, being integrated to the pixels is yet another potential metering advancement; Fuji's discontinued FinePix F300EXR model contains this feature."

Metering systems today have advanced and improved in many ways, yet the general concepts remain the same - get as much in-camera when it comes to detail, light, and exposure, and you images will shine. I compare metering a scene with a racecar driver – anyone can press on the gas, but it takes someone with experience and knowledge to drive that car well. So goes exposure –metering systems will continue to become more automated and intuitive, but learning how to operate these cameras and knowing how to choose the best mode for the scene will still require a bit of know-how.

There are no easy ways to capture consistent solid exposures outside of learning the fundamentals of metering, understanding how your camera's meter works, and recognizing the parameters of your image sensor. Then by following a step-by-step method you practice, play, and practice some more. Learning exposure can take years, but if you do so this newfound knowledge can give you full access toward creating the images you have in mind, or replicating what you see in a powerful way.

Sean is a commercial photographer, author of The Complete Guide to Nature Photography, photo expert, and all around nice dude.