How to Take Extraordinary Photographs, Part 1: Exposure

As photographers we’re limited by what light can do and what light we have. When taking a photo, we set out to capture the right amount of light to portray our subject the way we want it to be seen.

The amount of light captured by your camera is called the exposure. The concept is simple, but in practice it can seem complicated. There are three settings on your camera that affect the amount of light it will capture, and each will affect the look of your photo in a different way. The camera can choose those things for you, but it cannot know what the scene it is pointed at should look like. When you understand those controls you can look at a scene, decide how the photo should look, and set your camera accordingly.

The three factors that control the amount of light your camera captures are:

The three elements are inextricably linked, as indicated in Figure 1, “The exposure triangle”. Change one without balancing it with the others and the exposure will be affected.

Figure 1. The exposure triangle

Changing any of those settings changes the amount of light your camera captures. The trick to achieving the right exposure is to balance all three. If you choose to increase one of the settings, one or both of the others will have to decrease to compensate.

It sounds easy—lower one setting and raise one or both of the others to counteract the shift; raise one and lower another. There’s a light meter built into your camera that will show you when they’re in balance. So it’s quite simple, as long as you consider the secondary effects of the settings. Secondary effects, you say? That’s right. They all have secondary effects and they need to be taken into consideration when you plan your shot.

Before we look closely at the three factors that control exposure and their secondary effects, let’s consider how we measure exposure and how we choose which exposure is right.

Your eyes automatically adjust to variations in lighting. An indoor light that appeared too dim to see by when you first entered from outside on a sunny day is bright enough to read by a few minutes later. There’s no change to the lighting; your eyes have adjusted to the new light level.

The sensor in your camera merely records the light that falls on it. It’s unable to become accustomed to brightness or more sensitive to darkness unless you tell it to. But if your eyes are constantly adjusting to the light around them, how can you know how bright (or dark) it really is?

Well, help is at hand in the form of your camera. As well as recording the light, it can measure it. In each camera there’s a light meter. If you have an SLR camera you should see the meter in your viewfinder, while compact cameras usually display their meter on the LCD display. Most cameras will only show the exposure meter when set to a mode that allows the manual adjustment of exposure, so if you don’t see a meter, make sure you’re out of the automatic or preset modes (consult your manual if you need to find out how to turn on the display of the exposure meter).

The exact look of the meter display varies from camera to camera, but the basic idea is represented in Figure 2, “A typical in-camera exposure meter”. The indicator below the scale moves according to the current exposure settings and the amount of light falling on the sensor.

Figure 2. A typical in-camera exposure meter

The meter measures the light reflected into the camera, compares it with the amount of light the camera’s exposure settings capture, and indicates if the scene is too bright or too dark. The central point on the meter represents the amount of light that will give your image a tone midway between dark and bright.

The numbers on the scale of your camera’s exposure meter do not represent a linear scale of brightness. The +1 point represents twice the amount of light. The +2 mark is double that—four times as much light as the midpoint. In the other direction, -1 and -2 represent a half and a quarter (respectively) the quantity of light.

There are several ways for a camera to measure the light in a scene and these are called metering modes. Put simply, your camera can average out the amount of light in a scene, or be more focused—taking a reading from only one spot in a scene. Of course, it’s slightly more complex than that and more advanced cameras have a range of metering modes. A detailed explanation of all the possible modes your camera may have is beyond the scope of this book, and your camera’s manual should provide all the details you need. Much more detail on metering can be found in the tutorials at Digital Photography School.

Most scenes have a range of shades in them and aiming for a mid-tonal point tends to produce a well-exposed image for a lot of situations, but not all. And some scenes don’t need to be of average brightness. If you point your camera at a well-lit, pure white wall and adjust your exposure so that the meter is centered, the wall will look off-white in the photo. It has a similar effect when shooting black, too; blacks will tend to become grays.

It’s dangerous to rely on the meter alone for your shots to look right. You need to look at the scene and judge for yourself if you want it darker or lighter, then adjust your exposure accordingly. Judging the best metering mode for a scene, so that you achieve the desired result, is a skill that takes practice.

White Balance

After having not enough light, the biggest problem with light in photography is when it’s the wrong color. Sometimes it’s obvious that your light is colored and you might even want it that way. The problems arise when the coloring is more subtle and unwanted, and this is known as a color cast.

You’ve probably taken photos under ordinary incandescent lights that have a strong yellow-orange cast to them. When you took the photos the scene didn’t look yellow to the naked eye, but your camera showed you a yellow image. Is the camera to blame? No. The camera faithfully recorded what it was pointed at. Incandescent light really is that color.

When you’re surrounded by slightly off-color light, your brain is smart enough to compensate for the cast. You know what white is supposed to be and that is how you see the light. Your camera is less smart. It records what it’s told to record, so you have to tell it to record the scene a little differently.

Your camera will have a control or menu option called white balance. As well as an automatic setting, you should find settings for tungsten,^[2] fluorescent, sunlight, flash, cloudy, and shady conditions. When you’re surrounded by those lights, they might all look the same to your eyes, but there are some fairly major differences in color, as Figure 3, “The most common preset white balance options offset the most common sources of color casts” illustrates.

Figure 3. The most common preset white balance options offset the most common sources of color casts

Set your white balance to whichever setting most closely describes your light source before you start shooting. You can correct a color cast on your computer later, but it’s usually best if your camera does it while you shoot. If you’re unsure which setting is best for your lighting, try shooting test shots of an object you know is supposed to be white. Check the results on your LCD and pick the one in which the object is closest to white. You could set your white balance to automatic, but then you trust the camera to guess it right. If it guesses wrong, the color cast will be worse. Figure 4, “Having the wrong white balance will create color problems” illustrates the problem. The photo on the left is taken in daylight with the balance set to tungsten and the result is too blue; the photo on the right shows the exact opposite—shot under a tungsten bulb with the white balance set to sunlight—so the result is too yellow.

Figure 4. Having the wrong white balance will create color problems

As we covered in chapter 1, your camera’s lens covers the aperture: a hole that lets light into the camera. The aperture’s width is controlled by an adjustable diaphragm. Widening the aperture lets in more light; narrowing it lets in less. On cheaper compact cameras you might not have direct control over the aperture, though its settings may be affected by your choice of shooting mode. Even if you’re unable to control it directly, its effect on your image is important.

Aperture measurements may seem a little odd the first time you see them. They’re expressed in f-stops (as opposed to the plain old stops we’ve already covered), usually written either as f and a number (for example f16), sometimes with a slash (/) character between them. The number is not an absolute measure of the aperture’s size, but is the related focal length of the lens; therefore, an aperture of any given f-stop will let in the same amount of light regardless of what lens you use.

The first slightly tricky point to remember about apertures is that the smaller the f-stop, the larger the aperture, and the more light that enters the camera. To increase exposure, set your aperture to a smaller number. To decrease exposure, make the f-stop larger. It might help to think of the number as a measure of how much the hole is closed.

Another aspect of aperture that takes a bit of getting used to is the numbering. Standard lens apertures are usually a subset of the sequence f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, and so on. Each number is roughly 1.4 times the previous one. This is no huge joke played by math nerds on those of us who are more numerically challenged: 1.4 is the square root of 2, rounded off to fit on a camera’s display. If you multiply the width of the circular aperture by 1.4, you double the aperture’s area and let in twice as much light. You can see this demonstrated in Figure 5, “Each f-stop is half the area of the previous one”.

Figure 5. Each f-stop is half the area of the previous one

When you change the aperture from f/1.4 to f/2, you halve the amount of light the camera lets in. Go from f/8 to f/5.6 and you double it. You don’t need to understand the math, but it’s a very good idea to learn the sequence of f-stop numbers. It might seem counterintuitive that f/8 means half as much light as f/5.6, but knowing it well enough so that it’s second nature to you might one day save you missing a shot. Most cameras allow you to adjust apertures in half-stop steps; some allow steps of a third of a stop, but you’ll find some part of that sequence of f-stop numbers in your camera’s settings if you have manual control over the aperture.

The aperture’s primary effect is to control how much light passing through the lens reaches the sensor. As a secondary effect, the aperture controls depth of field.

As we saw in chapter 1, depth of field is the amount of scene that’s in focus in front of and behind the point you’ve focused on. A small depth of field means that only a part of your image will be in focus and the rest will be blurred, as illustrated by Figure 6, “The size of the depth of field”. A large depth of field gives you a greater distance in front of and behind your subject in focus. A wide aperture gives you a shallow depth of field; a narrow aperture gives you a deep one.

Figure 6. The size of the depth of field

A bit of blur can be a good thing. If you want to shoot one flower in a garden full of flowers, or a portrait in front of a busy background, limiting your depth of field so that only your subject is in focus will separate the subject from the background. Background details, which would be distracting if in focus, are blurred, leaving your subject standing against a more abstract background. In the following flower pictures, at f/22 (Figure 9, “And again at f/22”) the trees in the background clash with the flower stems in the foreground, while at f/1.8 (Figure 7, “Flowers on a bush at f/1.8”) it’s hard to tell that there are any trees. An aperture in between (Figure 8, “The same flowers at f/8”) shows all the flowers on the one plant yet still blurs the background. (Automatic portrait modes use depth of field in this way to isolate a subject.)

When shooting a landscape or a large group of people, you might wish to keep a number of objects in focus that are all at different distances from the camera. You’ll need a narrower aperture to do this, so if it’s necessary to increase the exposure, you’ll have to add more light (difficult for a landscape photograph) or adjust one of the other corners of the exposure triangle.

There are other factors involved in depth of field besides aperture. For example, the closer you are to the subject, the shallower the field at any given aperture. The previous flower examples were all shot from within a meter of the plant. Had the focus been on the trees in the background instead, the depth of field would likely have extended to infinity at f/22.

Figure 7. Flowers on a bush at f/1.8

Figure 8. The same flowers at f/8

Figure 9. And again at f/22

Shutter speed controls exposure by limiting the amount of time light has to shine on the sensor. That’s the primary effect of the shutter speed control. A fast shutter speed can freeze a bird in flight, as shown in Figure 10, “A fast shutter speed can freeze a bird in flight”. Its secondary effect that needs to be considered is motion blur. The longer the shutter is open, the more likely an object can move in that time. At slower shutter speeds, fast-moving objects are blurred while slow-moving objects remain sharp, as Figure 11, “At slower shutter speeds, fast-moving objects are blurred, while slow-moving objects remain sharp” shows. For most shots you’ll be working with shutter speeds measured in fractions of seconds.

Figure 10. A fast shutter speed can freeze a bird in flight

Figure 11. At slower shutter speeds, fast-moving objects are blurred, while slow-moving objects remain sharp

The more the subject or the camera moves, the more the image is blurred across pixels in the frame. If you’re holding the camera in your hands when you take pictures, it shakes; there really is no way to avoid it. So if you shoot at too slow a shutter speed, you run the risk of even the steadiest subject being blurred by the shaking of the camera. The movements are tiny—most of the time you never notice them. They will rarely move your camera more than a fraction of a degree between the opening and the closing of its shutter. The problem arises with zoom. When your angle of view is only a couple of degrees, a small shake makes a considerable difference. The longer the focal length of your lens, the more susceptible it is to the effects of shaking.

There are a number of ways to combat this, the best being to place the camera on a sturdy object that won’t move. A tripod is ideal, but putting it down on any solid surface is more stable than holding it in your hands. (We’ll look at tripods and the fun things you can do with them in chapter 3).

Sometimes the light is too poor for any combination of aperture and shutter speed. Scenes in very low light—where you need either a wide aperture or a fast shutter speed (or both), and you’re unable to use a flash—require a high ISO setting. For instance, you might want to use a fairly fast shutter speed to capture the blowing out of candles on the cake at a birthday party. Unfortunately, even the widest available apertures will prohibit you from shooting very fast by candlelight. A flash will wash out the scene and kill the golden glow.

When you need more light and are unable to change aperture or shutter speed, the last option left is to change the sensitivity of the sensor. The camera’s light sensitivity, referred to as ISO, is controlled by a less accessible means than the shutter and aperture, but you should be able to find it among your camera’s settings menu. This is partly a holdover from the old days when all photos were made with film (which wasn’t too long ago, mind.) The sensitivity of a film camera is a matter of the formulation of the light-sensitive chemicals on the film. To change the sensitivity you change the film; the decision is unable to be made on the spur of the moment. Now we can change the camera’s light sensitivity electronically by amplifying the sensor data.

The other reason some cameras’ ISO setting is buried away in a menu is that most photographers would prefer to avoid changing sensitivity if they can. As well as increasing the brightness of photos, increasing the sensitivity has the secondary effect of introducing and increasing noise—the visible graininess in shots. By amplifying the signal from the sensor, we also amplify the differences between pixel values. Increase the differences enough and they become visible. Transitions between colors and tones look rougher. At low ISO settings it might be unnoticeable, but as the ISO gets higher images take on a grainy appearance that’s generally considered unappealing. Detail is lost to noise, along with the smoothness of color and tone transitions.

We say generally, because ISO noise is not necessarily all bad. Sometimes you might want a rougher, noisier look to your images. We’ve all grown up seeing grainy news photos and film, usually of breaking stories shot as events unfold. Sometimes it’s a result of cheap cameras in the hands of eyewitnesses, sometimes it’s from seasoned photojournalists using very high ISO film because they’re in situations where speed is important. At the same time, most advertising and entertainment photographs are clean and noise-free. We’ve come to think of grainy images as being more real, while slick and polished images can seem fake.

That does not mean that you should crank the ISO up to 6400 every time you want an image to look spontaneous. Noise that looks just like high ISO can be very easily added in Photoshop, but real noise is difficult to remove. If you think you want grain in an image, you should seriously consider the option of adding it later.

While shutter and aperture priority modes are very common on digital cameras, ISO priority is not at the time of this writing. A couple of camera manufacturers have introduced ISO priority modes, but given the preference of most photographers to shoot at the lowest ISO setting available, it’s unlikely to become very popular.

Now that we’ve covered shutter speed, aperture, and ISO, let me introduce a useful technique. It’s unlikely that you’ll find yourself in a situation where you have no meter at all, but it’s always a good idea to be prepared to shoot quickly. If you preset your camera according to the following rule, chances are good that if you need to shoot quickly you’ll already be set for the shot.

A good guideline for estimating exposure without using a meter is the Sunny 16 rule. On a sunny day with your camera’s aperture set at f/16, setting your shutter speed to the same value as your ISO setting should give you a decent exposure. At ISO 100, shoot at 1/100th of a second. From that, you can adjust for different conditions. It’s best to keep your ISO as low as it can go, so all you need to do is balance any increase in shutter speed with a similar decrease in aperture. Double the shutter speed, halve the aperture, and so on.

Judging how bright is bright enough for the rule to apply is easy. Away from shade, cloud cover is about the only element that will diffuse the sunlight. Look at your shadow. Is it distinct and hard-edged? Can you see a shadow at all? Try the settings in Table 1, “Aperture guide” for a start; look at your preview, then adjust if necessary.

Now that you’ve learned a bit about exposure, aperture, and shutter speed, it’s time to put it into practice. Begin by downloading the sample PDF of Photography for the Web, which includes all the text of this article, along with the rest of Chapter 2: Photo Basics. When you’re ready to take your photography skills to the next level, buy the book and learn how to store, edit, and share your images online.