Creating a proper exposure is what you're after when you take a photograph. Understanding how to get it can be a bit harder. Part of the problem is, this is a widely variable thing depending on camera settings, conditions, and your subject, so there is no such thing as a quick overview. This is a long page, but it's a complicated subject.
First off, our eyes see a much greater range of light than film or a digital sensor can — at least, at any one time. Our eyes can adjust very quickly to light, so we're often not aware that, when our eyes are adapted to bright light, they cannot see things in darker light anymore. But you know what happens when you've been in the dark for a while and someone turns on a bright light without warning, and you might have noticed that, after being outside in the sun for a bit, you can't see anything indoors for a few seconds. The same happens for film and digital, but it's an even narrower range, something we'll call latitude, or contrast range. Basically, if you're looking at a scene in front of you, you may notice that you have a broad range of light conditions between highlights (light-colored things, or things in direct light) and shadows (dark-colored, or things in shadow.) If the difference between these is too high, too much contrast, they can fall outside of the latitude of our film/sensor, and we'll lose detail. Highlights may get "blown out" to pure white, shadows might become pure black, in the resulting photograph.
Now, we have a meter within our cameras (almost always, anyway) for measuring light, or exposure. A "proper" exposure is one that gives you the results you want from the scene in front of you — allowing for the limitations of latitude. Face it: you may not always be able to get the exposure you want from the conditions that exist. But we'll try hard, and understanding what the camera does is important.
If we were to take a large collection of photographs and average them out for the light coming off of (reflected from) the subjects within, we'd be wasting our time, because camera manufacturers have already done that. But we'd get a medium grey tone, often referred to as 18% grey. Since this represents an average tone, or middle range of the reflectivity of subjects we're likely to photograph, this is what the meters within cameras are programmed to seek.
[Quick side note: Why 18% reflectance? Shouldn't it be 50%? No, because most of what we see around us is low reflectance subjects. Consider a piece of white paper outside in bright sunlight — it can hurt our eyes with brightness. But that's not even close to a mirror reflecting sunlight into our eyes, and even mirrors are not 100% reflective. For most of what we see, it averages out to 18%.]
The little block to the left represents 18% grey, more or less. And the reason I say that is because your monitor may not be adjusted to a precise point to represent this to you properly, and it's lit from behind. But essentially, if you had a piece of cardboard this color (and you can buy them at well-stocked photo stores, for far more money than is necessary,) you'd have that average tone that camera meters are seeking.
Camera meters, for the most part, are color blind. What we're after is a middle tone, not a particular neutral color like this, so this could actually be 18% green, blue, or red, and it would make no difference. All we're measuring is the light intensity. If it helps, grass and foliage often falls very close to 18% reflectance, believe it or not. Also asphalt on typical roads, and the roofing tiles of many houses. The important thing to remember is, it's not what your subject looks like, but how you want it to appear in the image. This is often the tricky part.
In the image to the right, there are two sample photographs taken in wildly different lighting conditions, and a graduated bar representing what all light levels are. The brackets on either side of the arrows represent the exposure latitude, or how much difference between highlights and shadows we can capture (for a particular film or digital setting.) And the arrow points to where 18% grey would actually fall.
You'll notice that's not in the middle of the graduated bar (though it is in the middle of the latitudes.) But remember, that bar represents the range of light we can see within. The top photo with the cat was taken in bright sunlight, while the bottom photo of the lighthouse was taken by moonlight. It's what the meter is seeing, gaining from the subject you're pointing at, that guides it towards exposure.
I want you to notice something. In the image of the cat, the highlights (the white fur) have gone pure white, exceeding the latitude of the film/sensor, while the shadows have gone pure black, also exceeding it on the opposite side. What this means is, those portions did not actually fall within the bracket representing the latitude, and since they were outside of this, they overexposed or underexposed and became extreme. It was actually very easy for me to see in the shadow of the cat, and in fact this was moderately bright (note the position of the lower part of that bracket on the light bar,) but the very contrasty conditions of the scene went outside of what the camera could capture. You also may have noticed that the latitude for the cat is narrower than for the lighthouse, and this was on purpose — I wanted a more contrasted image for the cat.
The lighthouse, on the other hand, was taken in dark conditions under a full moon, but with much lower contrast film, so it has a wider latitude. The middle point is much, much dimmer than for the cat, but that doesn't matter — what we want to accomplish is knowing that the camera chooses the right middle tone. In moonlight, the direct light is not very bright, but it bounces around a lot, throwing ambient light into the shadows, so the difference in highlights and shadows actually becomes much lower. The same happens for hazy or overcast skies, morning or evening hours when the sun is low, and most interior conditions. Direct sunlight, however, creates a much larger difference between direct light (light straight from the sun) and ambient light (light that comes from the sky or nearby reflection.)
You'll notice that the white lines on the lighthouse are not truly white, much dimmer than the light coming form the top. And even the black lines get pretty bright where they reflect the moonlight directly. We tell ourselves these are "white" and "black," but the camera does not see this — it sees only light levels, better than what we think we see. So we need to think in terms of what is there, not how we interpret it.
Now that we know what we're after, how do we get it?
Cameras nowadays have a wide variety of ways to judge exposure, often called metering options. This is what makes my task (teaching this) so difficult, and most times, your task (getting a good exposure) easier. Most times.
The fancy options, often called "Evaluative" or "Matrix" or something techy-sounding and proprietary, try to judge difficult scenes for you and adjust camera settings accordingly. They are often linked to your focus point so they measure light from your subject initially, but will read the surrounding conditions as well and try to judge if you're shooting in heavily backlit conditions, low light, and so on. Many times, they're not too shabby at this, but they can still be fooled, and it's impossible for me to predict exactly how.
So let's stick to basic conditions. We know the camera goes for the middle tone. But this is only useful if your subject, what you're capturing in the frame, averages out to this tone in the first place. If it doesn't, like a snowy field or a night sky, the camera doesn't know this at all. It's just measuring light. So it produces settings for your camera to make these that middle tone, and messes up your exposure. The snow field might become very dim, a dingy grey, while the small moon in the night sky you were after gets way overexposed (and consequently, blurred terribly because the shutter was open too long to try and light up that sky towards grey.)
This is where you come in. Even though you have this wonderful piece of technology in your hands, you have to be the person in charge. Look at the scene in your viewfinder. If it's not a nice mix of lights and darks, but instead favors brightness too much for instance, you're probably not going to get a good exposure. What do you do?
First, see if there's something nearby that will serve as that middle tone. It should be in the same lighting conditions as the framing you've chosen — don't aim into shadow for a middle tone when your subject is out in sunlight. Get an exposure reading off of that, lock it, and aim back towards your intended subject to get the shot.
Display from top right of
camera body; others similar.
Lock it? You'll have to check your camera as to how to do this, because they all vary. In many cases, you get an initial exposure reading by half-pressing the shutter release — you'll see the shutter and aperture settings light up in your viewfinder. If you continue to hold down the shutter release, just at half-pressure, you can hold these settings when you re-aim the camera. Try it and see! Aim into shadow, for instance, and half-press to get the exposure settings. Keep holding it down gently, and aim into sunlight — not into the sun, just at a brighter view. If the settings did not change as the camera view got a lot brighter, you've locked exposure. Be careful not to accidentally release the shutter button as you do this, or you'll get a new exposure reading when you try to get the picture.
Many cameras also have an AE Lock button (Auto-Exposure Lock,) usually denoted with an asterisk * — see image at left. Pressing that will lock exposure and allow you to re-aim — sometimes this is only for six seconds or so, so don't tally. If neither of these seems to work, check both your camera manual or your custom camera functions — sometimes the ability to lock exposure is assigned to a different button.
How else? Many cameras have the ability to "dial in" exposure compensation. Typically, you may see a graphic display similar to what I've shown here, with a middle setting between + and - signs. These naturally mean over- and under-exposure respectively. To activate this function, you often need to hold down a button marked +/- and either turn a control dial/wheel or use your rocker buttons on the back. You can adjust the exposure this way, but be warned: this is not a one-shot deal, but remains in place. You have to set it back to go back to normal exposures.
When you're doing a lot of shots in tough conditions, like birds against the sky, or a snowy day, or a day at the beach (sunlight off water and sand are much brighter than average,) this method works best — you set it once and don't have to think too hard about it. Single shots, however, work better with the first method. Remember to check your camera settings when you put it away, and again when you get it out. It's really easy to forget and leave it set for compensation and blow a lot of shots you didn't need to.
How much to compensate? This is tricky, because it depends on what you want. For beach scenes, adding in between 1/2 stop to 1-1/2 stops usually works. For snow, a tad more, 2/3 to 2 stops. Birds against the sky? Same as beach, 1/2 to 1-1/2 stops. Remember: if it's bright, make it brighter, because the camera wants to darken it to middle toned, so you're bringing it back to where it belongs. Do the opposite for dark subjects.
With digital, of course, you can always check to see if you're getting what you want, to a certain degree. The limitations of the LCD screens on most cameras means you can't tell for sure until you download the images to your computer. But there are a couple of shortcuts, one of which works for film cameras too.
There's a technique called, "bracketing" that requires multiple frames, but generally ensures that you get what you wanted, if it's possible. It also works well for tricky situations, where you have a very narrow range between the exposure you want and one that goes too far, such as getting enough detail from snow or white feathers while still making them appear white. To bracket, you take several frames, compensating for various degrees on each, until you know you've captured what you wanted. This is done the same as exposure compensation above, but vary the settings for each frame. To be sure, you take a guess at what settings are likely to work, then you use settings on either side of that, both over- and under-exposing around the guessed exposure — typically changed by 1/2 stop each, or 1/3 stop if the situation is very tricky or "delicate." Some cameras have automatic bracketing already built in, and will take three or five frames right after one another without you having to change anything, which is nice in that you end up with the settings returned to normal. When you saw this in your camera manual, now you know what it's for.
The other method of checking is for digital only, until they get really fancy with film cameras (don't hold your breath.) It requires calling up the histogram when reviewing the image you just took, and knowing how to read that. The histogram is a bar-graph showing the amount of light gathered at all light levels within the image, ranging from pure black at the left side to pure white at the right — notice the graduated bar just below the bar-graph telling us which end is which. The image at right shows the histogram for the opossum picture at the top of the page — what we're looking for in this case are bars on the right (bright) end that appear to run off the edge, as opposed to dropping back down to "sea level." Or the opposite, if your image is favored towards shadow and you don't want it so dark that you lose detail into blackness.
But what we see here is actually pretty good — the weight of the image is right in the midtones, just slightly higher than where 18% grey would fall. That small but sharp peak at the right side is where the white of the opossum's face comes in, while the multiple peaks at left is where all the shadows in that image lie. The fact that both drop down to near zero means that the pure whites and pure blacks are not significant in the image, and thus we probably didn't cut off either the highlights or shadows, though we came close on the shadow part. These histograms were taken from Photoshop, by the way, but the ones produced by your camera should be very similar.
Alternately, here's a histogram laid in over the B&W cat photo we saw above. Radically different, it shows that both the highlights and the shadows appear to run right off the ends of the frame, meaning (especially in the shadows) that we've cut off detail. Anytime your histogram runs too heavy at either end, you're probably either underexposing (left side) or overexposing (right side.) In this case, somehow I did both, but there's two reasons for this. The first is that the image was too contrasty, and exceeded the latitude of the camera settings. The second is that I actually wanted it this way, and intentionally clipped both the highlights and shadows to drive the entire image towards extremes to achieve this affect. It's okay when it's intentional ;-). Notice the faint hint of detail in the cat's eye on the right? That's where I wanted it.
But you may not want your shadows this dark, especially when photographing people, for instance. And the bright sunlight isn't letting you get the range you want — you are exceeding your camera's latitude. Forget about it? Hah! We're not done yet! One simple way to keep detail in the shadows is by using fill-flash — firing off your camera's flash/strobe unit to throw some light into those shadows and make them brighter, so they fall within the range that the camera can capture, or even just soften them a bit. Don't worry, you're not going to overexpose your highlights by doing this, since sunlight is so much more powerful than your camera's light that the difference to the highlights will barely be noticeable. People may think you're weird for using a flash in bright daylight, but you know what you're doing with it, and getting better results than they are ;-)
Or, you can bounce some light into the shadows with reflectors — I do this regularly with macro and close-up subjects. If you've ever seen a model shoot and wondered what the guys running around with big white or silver sheets are doing, now you know. Outside of the frame where the camera cannot see, you can bounce some light, as if from a mirror, into the shadows of your subject. For small subjects, a sheet of white paper or cardboard works fine, while for larger subjects, you might want to use white foamcore (the paper-backed Styrofoam sheets you can obtain at office-supply stores for presentations.) I've also used a collapsible sunshade for a car windshield, which either fold up or twist into a much smaller shape for storage. These often require an assistant to wield, but it's not hard to recruit someone for this, and when you explain why you're doing it, they're usually psyched to help out. Lacking such a helpful person, you can occasionally use the white side of a building, or position your subjects right where a reflection from a window does the most good — just try not to make them squint.
So what do these other camera metering options mean? Okay, there are several options, so let's tell you what they mean, and you decide if you can use them as you work.
Spot Meter: This means the exposure reading is only obtained for a small spot in the frame, and nothing else. It can be anywhere from 1 to 5% of the frame, and is often active only in the center (sometimes you'll have a small shaded area denoting the spot-meter area in the viewfinder,) but on some advanced cameras it's linked to the focus point. Check your manual to be sure. You can use spot-metering to get a precise reading from a small area and use that for your overall exposure setting. Naturally, you're aiming for a middle tone.
Multi-spot Meter: This allows you to take several spot meter readings, and the camera averages them as you go. So if you don't have a nice middle tone (you might be shooting a black dog in bright snow,) you can take multiple readings of highlights and shadows and split the difference.
Partial: This is kind of a broad spot meter, may range from 5 to 10% of the viewfinder frame. Not as useful as a spot, because it often overlaps your subject area and you have to start guessing, but still better than nothing at all, or even the choice below. Again, often denoted in the viewfinder by a specially shaded area.
Center-weighted: This was the old standard and can still be found on many semi-manual cameras (my Mamiya medium format uses this.) About like a partial meter, it takes a reading from 5-10% of the center of the frame, but also takes an overall reading from the entire frame. It compares these two together, giving 70% authority to the center of the image. In this way, it compensates for situations like backlighting, to a degree. The problem is, it takes experience to know how far the outer edges can deviate from the center before you have to compensate. Not recommended if you can avoid it, but still better than none at all.
Field meter: If your camera is pretty old, or provides exposure reading but doesn't tell you it's center-weighted (you may have to look it up online,) it may have this. It's hard to find anymore. All it does is read the entire scene, so it's averaging everything you see in the viewfinder. Very frequently, you have to second-guess such a meter, so thankfully they're mostly gone now.
In the illustration at right, we have either a partial or center-weighted meter system, represented by the blue circle in the center, and the classic hard-to-handle situation, "black dog in white snow" (only in this case it's a stream.) If we let the camera have its way, the large amount of white snow in the metered portion of our image will certainly make the image too dark. So, we twitch the camera down to where the center of the image is represented by the yellow circle, getting a decent portion of the dark subject in the frame — you can see it's split pretty well between dark and light. Lock that exposure and hold it as you aim back to where you want, and trip the shutter. Voila, a good exposure. Other metering options may behave much the same way in this situation, except for a spot meter. Where would you read for a spot meter? Maybe off of a tree trunk, or the cluster of underbrush at the upper edge of the frame.
For most shooting situations, your camera meter works fairly well, but if you want good control, you need to remember how exposure works and what you want from your image. Concentrate most on your subject, which is the focal point of your image. The background can go to extremes without usually creating a bad affect, but losing detail from your subject can lessen the impact on the viewer.
Another thing to bear in mind is whether the scene benefits from purposefully over- or under-exposing the frame. Again, remember that we're not really messing with proper exposure, but what the camera suggests as a proper exposure — we know better now. In the example seen here, I accepted the camera's reading from the sky and used it, even though the sky was much brighter in real life than seen in the image. The reason is, it brought out much richer colors than I was actually seeing while there. When you think about it, the closer a particular color gets towards white, the more washed out it appears, like bleaching your brightly-colored clothes. You can also create intentional silhouettes from situations that are extreme, allowing your subject to drop entirely into shadow while keeping the background at a manageable level. And you might even want to create a moodier image, even when it's easy to manage the exposure in the situation, and opt for underexposing the image to create twilight, for example.
You should know that the sky is often very much brighter than the foreground, and this most especially holds true when shooting into the light. You will, for instance, not get a sunset photo with rich sky colors and foreground detail — the light levels are too far apart. The sky gets to be the richest, deepest color when facing opposite the sun (sun at your back, or over your shoulder,) since this is how humidity in the air works. Not to mention that you have the most sunlight falling onto your subject, making it the brightest it can be — this means the light levels come closest together. So when shooting sunrises and sunsets, consider your foreground only from the aspect of its silhouette capabilities, or keep it small enough and close enough to illuminate with a flash. Recognize that, when shooting into the light, to expose for the foreground you're probably going to wash out the background sky to pure white. Sure, there are tricks like graduated filters (shaded along the top to drop the sky above the horizon down in intensity,) and High Dynamic Range editing (combining two or more images exposed correctly for foreground and sky separately,) but you should know these are easy to spot by anyone with experience — I've always considered them mere novelties, myself.
Despite the length of this page, it's only a start — most of your knowledge of exposure is going to come from experience. Much of photography is actually a compromise, in that you often have to choose a few preferred aspects over others, but cannot have it all. As the old saying goes, good, cheap, fast; pick any two. The good photographer knows how to select image situations that emphasize what can be done, and avoid what cannot.