Film photographers have known for years the importance of correct exposure. If you overexpose a slide, the highlights are gone irretrievably. If you underexpose a slide, the image will be murky, with no true black tone in the darkest areas. Negative films have a little more leeway, in that you can make some adjustments when printing the negative, but again, the image quality won’t be great if the image is over- or underexposed.
An all-too-common attitude among digital photographers is, "Oh, I’ll fix it in Photoshop."
Hey, I’m guilty of it, too. During my first few years in digital after 40 years of shooting film, I slipped into sloppiness with my exposures. But I’ve learned that while you can do some amazing things with Photoshop and other image-editing software—things that film photographers can only dream about—you can’t make an under- or overexposed image look as good as a properly exposed one.
So, correct exposure is important whether you shoot film or digital images. Fortunately, there are a number of tools to help you get it.
SLR cameras, film and digital, generally provide several metering modes, including a multi-segment one, the old standby, center-weighted average, and sometimes spot metering.
Multi-segment metering produces good exposures in a wide range of exposure situations with no hassle, so it’s a good "default" setting for quick shooting. While some photographers who grew up with center-weighted metering still use it (that was the only metering mode my film cameras had), I find that multi-segment metering gives me better results more of the time.
Spot metering, done properly, can produce the most accurate exposure of all, but doing it properly takes time and thus rules it out for action shots and fast-breaking events. And the spot meters built into SLR cameras generally read too large an area for precise control, especially when you’re using a wide-angle lens.
Before cameras had built-in TTL meters, photographers used handheld exposure meters, and these can still be useful accessories for today’s photographers. The most useful handheld meters for outdoor photographers are spot meters. These read a small (generally one-degree) area, so they can measure the brightness of subjects or portions of subjects precisely, such as that moose in yonder meadow. Ansel Adams used a spot meter, as do his legion of followers. (See the Exposure Meter sidebar for more details.)
Bracketing exposures means making one shot at the exposure you think is right, then additional shots giving more and less exposure. The idea is to make sure you have one good exposure in tricky situations. Bracketing is difficult with moving subjects and fast-breaking situations, but many cameras offer automatic exposure bracketing (AEB), in which the camera shoots the bracketed exposure series in rapid succession with one push of the shutter button when the camera is set to continuous-drive mode. In single-shot drive mode, you have to press the shutter button to make each bracketed shot. You generally can set the bracketing amount in ½- or 1/3-stop increments up to +/- two or three stops; I usually use ½- or one-stop brackets.
You can override the metered exposure with the camera’s exposure-compensation control. Generally, this is done by pressing the +/- button and rotating the control dial until the desired degree of compensation is set (most cameras offer +/- two to three stops of compensation, settable in 1/3- or ½- stop increments). Exposure compensation is primarily useful when you encounter an exposure situation you’ve previously found to be problematic for your metering system. For example, when I’m photographing a bird against greenery, I dial in some minus compensation because my meter (like most meters built into SLRs) tends to overexpose green tones. I also dial in plus-exposure compensation when photographing birds against bright skies and minus-exposure comp when photographing white birds against deep blue skies.
AE lock lets you lock in an exposure setting, then recompose a scene without affecting the exposure. If you have a subject or scene that you expect will present problems to your metering system, you can point the camera at a middle-toned subject, lock in that exposure by activating the camera’s automatic-exposure lock (AE-L), then compose your scene and shoot. Some cameras lock focus with exposure; in this case, you must aim the camera at a medium-toned subject that’s the same distance away as your real subject or focus manually.
|In the early days of the digital age, many photographers became lulled into a false sense of security, believing that they would be able to fix any exposure problems in Photoshop. Not true! There’s simply no substitute for getting the exposure right when you shoot. When faced with tricky scenes like those on these pages, you might find that your camera’s auto setting won’t yield the best results. Instead, use tools like spot metering and bracketing to ensure that you get your exposures right.|
Digital photographers have a tremendous exposure aid in the camera’s LCD monitor. It allows you to check the image right after shooting it to make sure it’s what you expected. Every D-SLR I’ve used has occasionally completely blown out an image for no apparent reason. I’d much rather discover this while I can reshoot than not find out until I get home and it’s too late.
With some cameras, the image will look too light or too dark on the LCD monitor when it’s properly exposed (i.e., when it looks just right on your computer monitor). Check out your camera to see how well what you see on its monitor agrees with what you see on the monitor of your image-editing computer. And be sure to judge the image on the LCD monitor square-on, not at an angle.
Most D-SLRs (and some compact digital cameras) can display histograms on their LCD monitors. A histogram is a graph of the tones in the image, with dark tones on the left and bright tones on the right. The exact shape of the histogram depends on the scene—a mostly dark scene will result in a histogram lumped to the left; a bright scene, a histogram lumped to the right. Ideally, the histogram should go all the way from the left to the right.
If the histogram doesn’t go all the way to the left, the image is overexposed and contains no true black or very dark tones. If the histogram doesn’t go all the way to the right, the image is underexposed and contains no white or very bright tones. A well-exposed image will have a histogram that goes from one edge to the other, even if most of the tones are to one side or the other (unless, of course, you’re shooting a very low-contrast scene, such as a foggy pier shot that contains no dark or light tones).
With most D-SLRs, you can activate highlight and shadow warnings, which cause the highlights to flash on the LCD monitor when overexposed or the shadows to flash when underexposed. These warnings aren’t 100-percent accurate, but are one more tool to help you get good exposures.
Many photographers use a technique that double-processes the RAW files to get a perfect image. You process the file once for the highlights and once for the shadows.
Manual Exposure Mode & BDE
Sometimes I don’t use an exposure meter. I frequently photograph birds flying (or swimming) across a lake that has bright areas reflecting the sky and dark areas reflecting shaded foliage. In AE modes, the meter will underexpose the bird as it moves across the bright areas and overexpose it as it moves across the dark areas. There’s no way I can keep adjusting exposure comp quickly enough to compensate. So I just switch to manual mode, set the Basic Daylight Exposure (BDE) (an exposure of 1/ISO at ƒ/16, or equivalent) and fire away. The bird is always correctly exposed, no matter what the background brightness. Because the subjects are moving, I shoot with the lens wide open to get the fastest possible shutter speed: At ISO 200, the BDE for bright sun is 1/200 sec. at ƒ/16, or 1/400 sec. at ƒ/11, 1/800 sec. at ƒ/8, 1/1600 sec. at ƒ/5.6 and 1/3200 sec. at ƒ/4 (which is what I use with my 300mm ƒ/4 lens).
BDE is useful on clear sunny days, but not as useful on partly cloudy days, when the sun comes out, ducks behind a cloud and comes out again. When the light is rapidly changing, I use multi-segment metering and aperture-priority AE (and sometimes AEB with continuous drive).
Another RAW advantage is that nothing is done to the pixels until you save the image as a JPEG or TIFF after processing it. When you shoot a JPEG image, the camera processes it using default or user-selected parameters (white balance, sharpening, contrast, saturation, tone curve and more) and saves the image as an 8-bit lossy-compressed file. When you then open this image in your editing software to work on it, the missing pixels must be reconstructed, then pixels are again thrown out when the image is recompressed when you resave it. Thus, there’s a loss of image quality every time you resave a JPEG file.
With a RAW file, nothing is done to the pixels until you save the image as a TIFF or JPEG file after applying all your changes—and even then, your original RAW image remains safely unaltered.
Double-Processing RAW Images
Because RAW images contain more and better data than JPEGs, you can double-process RAW images to improve shots of contrasty scenes. Process the RAW image so the dark areas look right and save it; process the same image so the bright areas look right and save that. Then, simply combine the two perfectly registered images in your image-editing program to get the best of both in a single shot. Photoshop’s layer mask feature is a big help here.
One of the best tools at your disposal is the camera's histogram. When you're in the field, you can quickly check the histogram and instantly see whether your exposure is go
Exposure meters measure the amount of light that strikes them, then provide you (or your camera, with built-in meters) with a series of shutter-speed/ƒ-stop combinations that will result in a proper exposure for the ISO speed in use. Simple enough, right?
Well, that depends on at what you point the meter. Exposure meters are calibrated to recommend exposures that will properly reproduce a medium-toned subject. How well brighter and darker subjects reproduce depends on how much brighter and darker they are than a medium-toned subject, and the dynamic-range capabilities of the film or digital camera’s image sensor, A/D converter and imaging engine.
There are two basic types of exposure meters: reflected light and incident light. Reflected light meters measure the light reflected from (or emanating from, in the case of light sources) whatever you point them at; incident light meters measure the light incident (falling) on the subject. Both types are calibrated to reproduce a medium tone as a medium tone in the resulting photograph.
If you expose according to a reflected-light reading, whatever you take the reading from will reproduce as a medium tone in the resulting photograph. The problem with reflected light meters (and the meters built into SLR cameras are reflected meters) is that not everything you read with them is a medium tone. Problems occur if you meter a subject that’s brighter or darker than a middle tone.
Left: Sekonic L-758DR DigitalMaster; Right: Adorama Digital Spot Meter
If you read a middle tone, a certain amount of light strikes the meter cell, and the meter calls for a certain exposure. If you read a brighter object, more light strikes the meter cell, so the meter calls for less exposure, and the bright object is rendered as a medium tone in the resulting underexposed photograph. If you read a darker object, less light reaches the meter, so it calls for more exposure, and the dark object is rendered as a medium tone in the resulting overexposed photograph.
More problems can occur when you meter a scene containing a bright object against a dark background. The meter can be influenced by the dark background, calling for too much exposure and blowing out the bright subject. Conversely, when you meter a scene containing a dark object against a bright background (as when photographing a backlit subject), the meter can be influenced by the bright background, calling for too little exposure and rendering the dark subject as a silhouette with no detail.
In order to deal with these problems, camera manufacturers have come up with multi-segment metering systems that break up the image area into a number of parts, then take into consideration such things as the overall brightness, brightness distribution, brightness of the subject (the AF system provides the metering system data on the subject’s position and distance), contrast and, in some cases, even color. The camera’s onboard computer then uses complex algorithms (and with some cameras, an extensive onboard library of actual scenes) to come up with a good exposure in an amazingly wide range of exposure situations. I use my cameras’ multi-segment metering most of the time, overriding it (via exposure compensation) when experience has shown that I must.
Another way to overcome the problems of reflected meters being "fooled" is to use a handheld spot meter (or the camera’s spot-metering mode, if it has one) to read specific areas of the scene. Read the main subject, and if it’s of medium brightness, expose according to the meter’s recommendation. If the subject is particularly bright and you want it to appear that way in the image, give a stop or two more exposure than the spot reading suggests. If the subject is particularly dark and you want it to appear that way in the photo, give a stop or two less exposure than the spot reading suggests. You can also use the spot meter to check the brightness range of the scene, reading the brightest and darkest important areas to see how much they differ. With a little experience, you’ll learn how much brightness range your film or digital camera can handle, and how to expose the subjects you regularly shoot to get them to appear the way you want them to in your photos.
Probably the most accurate way to expose with a spot meter is Ansel Adams’ famous Zone System. We don’t have room to go into that here, but Adams’ book The Negative lays it all out beautifully.
Another way to avoid problems that can fool a reflected light meter is to use an incident light meter. Incident meters read the light incident (falling) upon the subject, rather than the light reflected from it. Thus, they can’t be "fooled" by particularly bright or dark subjects or backgrounds. But incident meters must be positioned so the light striking them is the same as the light striking the subject (so you can’t use one here in the shade to determine the exposure for that glorious sunlit Kodiak bear just across the river). They can’t determine the correct exposure for light sources, such as fires and sunsets, and don’t tell you anything about the relative brightness of the subjects in your scene.