Exposure Digital Field Guide

Chapter 1:

Understanding Exposure

Photography is the art of capturing light. You use your camera’s lens to focus the light and the sensor to record the light, creating an exposure. As a photographer, your job is to decide how much light the sensor is allowed to record, how long the shutter is opened, and how big the opening is in the lens to let in light. You also get to decide how sensitive the sensor is to light. All these factors let you control the exposure. You need to understand the exposure modes and light metering to help get the proper exposures and how to use the histogram to check your exposures. Picking the right file type to store your image is also important because it can make a big difference if you need to adjust the exposure in post processing.

Using the proper settings enabled me to capture the light cat in the darker shadows without losing detail in either. 1/320 second, f/4.5, ISO 800.

Defining Exposure

The basic definition of exposure is very simple: The exposure is the amount of light that is allowed to reach the sensor in your camera to create a photograph. The amount of light that reaches the sensor is controlled by two main factors: the length of time the shutter is open (shutter speed) and the size of the opening through which it flows (aperture). A third factor — ISO — changes the sensitivity of the camera’s sensor to light, which can be used to modify the exposure. The question becomes how much light is needed to create the look you want. No matter what settings you use, the sensor in your camera records the light being reflected at the camera. If too much light reaches the sensor, the image is overexposed or too bright; if too little light reaches the sensor, the image is underexposed or too dark. The goal is to let through enough light so the bright areas in your image are not too bright and the dark areas in your image are not too dark.

1.1 These three images were taken around the same time. The left image shows the calla lily in good exposure (1/160 second, f/6.3, ISO 200), while the middle and right images show the same flower when overexposed and underexposed. Notice the lack of detail in the light areas when overexposed and the lack of detail in the dark areas when underexposed.

When discussing exposure, a set of standard terms are used that help you to define the exposure. Shutter speed is described using time, aperture is described using size, and ISO has a standard numerical value. You also need to understand what happens when you change the shutter speed, aperture, or ISO and the relationship among these different exposure factors. The basic unit used when describing this relationship is called a stop.

note1.eps You can also use the Exposure Value to describe the settings used to achieve a proper exposure.

Stop

A stop is the change between one shutter speed and the next, where the change is exactly double or half the original shutter speed. The difference between a 1-second and 2-second shutter speed is one stop because the amount of light that is let in during the 2-second shutter speed is twice as much as the light let in by the 1-second shutter speed. There is a stop difference between 1/60 second and 1/30 second as well since 1/60 second is half as much as 1/30 and lets in half as much light. A stop also describes the change in aperture where the new aperture is double or half the current aperture and it is the change in ISO between one value and a value that is either double or half the current ISO.

Each time you double or halve the ISO, it changes the sensitivity by one stop. For example, the difference between ISO 200 and ISO 400 is one stop, with the image taken at ISO 400 needing half as much light as an image taken at ISO 200 because the sensor is twice as sensitive to light.

A one-stop difference in the shutter speed, aperture, or ISO will either double or halve the exposure, but most modern cameras allow you to set the shutter speed, aperture, and ISO in 1/2 or 1/3 stop increments allowing for more choices.

You may hear a photographer say things like opening up a stop or stopping down. Opening up a stop means increasing the light entering the camera by a stop by increasing the size of the aperture, While stopping down is just the opposite: it is used to describe decreasing the amount of light reaching the sensor by decreasing the size of the aperture,. So, don’t let the fancy talk confuse you; using the word stop when talking about photography is just a way to describe adjusting the exposure.

Exposure value

Your camera indicates what the built-in light meter has determined to be the correct exposure. Cameras use a numerical value called the exposure value or EV to describe the exposure. The EV for a correctly exposed image has the value of 0. Negative values are scenes that the built-in light meter has determined are underexposed and need more light, while positive values are scenes that the built-in light meter has determined are overexposed and need less light. The values are measured in stops. When you look through your camera’s viewfinder, you see a readout showing the exposure value.

When you use the Program auto exposure mode, Shutter speed priority mode and Aperture priority mode have an EV of 0. When the camera is set to Manual mode, you can use EV to determine if the settings you have entered are close to what the camera considers the correct exposure. You can adjust your settings according to what the EV shows.

For example, if the EV value shows a –1, then according to the camera you are letting in too much light and overexposing the image by one stop. You can correct this by increasing the shutter speed by one stop, stopping down the aperture by one stop, or reducing the ISO by one stop.

Equivalent Exposures

Equivalent exposures are an important part of getting the best exposure for each situation. The idea behind equivalent exposures is that different combinations of shutter speed, aperture settings, and ISOs can create the same exposure. This is because the three work together to create an exposure. For example, if you use a fast shutter speed and a wide aperture, you can get the same exposure with a slower shutter speed and narrower aperture.

1.2 This flower was shot at 1/400 second, f/1.8, ISO 200.

1.3 This flower was shot at 1/50 second, f/5.6, ISO 200.

Think of the sensor in your camera as a glass and the light coming in as water. You want to get the perfect amount of water in the glass, so you turn on the faucet and the water flows into it. If the faucet is opened all the way, it can stay open for a short period of time to get the desired amount of water. If the faucet is opened only partway, it must be open for a much longer period of time for the same amount of water to fill the glass.

The easiest way to explain equivalent exposures is to show how it works. Say that you have an exposure setting of ISO 100 with a shutter speed of 1/125 second and an f-stop of f/16. If you increase the shutter speed by one stop, which halves the time the shutter is open, you must double the size of the aperture. So, for an ISO of 100 and a shutter speed of 1/250 second, the f-stop must be f/8. This works in reverse as well; if you leave the shutter open for twice as long, 1/60 second, you need to make the aperture smaller, letting in half as much light, which is f/32.

1.4 This flower was shot at 1/500 second, f/3.5, ISO 800.

1.5 This flower was shot at 1/3 second, f/22, ISO 200.

As you can see, the flower in figures 1.2, 1.3, 1.4 and 1.5 looks the same in all four images because the exposure is the same for all four images, yet the images are not identical. If you look at the background, it changes drastically from being completely out of focus to being easily identifiable depending on the combination of shutter speed, aperture and ISO used.

Exposure Modes

Today’s digital cameras come with a fully automatic mode, and most consumer cameras have scene modes to help you get the right exposure under specific circumstances. For example, both the Nikon D90 and the Canon EOS Rebel T1i have five scene modes: portrait, landscape, close-up, sports, and night portrait. These scene modes help photographers get the best results in specific situations, but the other exposure modes give you the maximum control over your images, and control is what it is all about. The modes I focus on in this book are Program auto mode, Shutter speed priority mode, Aperture priority mode, and Manual mode.

Program auto mode

When your camera is set to Program auto mode, the camera decides the shutter speed and aperture, and in some cases the ISO. But what separates this from a full auto mode is your ability as the photographer to adjust the shutter speed or the aperture and let the camera adjust accordingly to make a proper exposure. This is a great mode to learn with because the camera is basically picking the starting point for both the shutter speed and aperture.

1.6 For the image of the BMX rider flying through the air, I used a fast shutter speed of 1/2000 second to freeze the moment. Taken at 1/2000 second, f/2.8, ISO 100.

Shutter speed priority mode

This mode lets you pick the shutter speed and lets the camera pick the aperture to create a proper exposure. You use this mode when you want to control how long the shutter is open or when you want to control the motion in your images. Fast shutter speeds let in less light but will freeze motion while slow shutter speeds allow in more light but can cause blurring of moving objects in your image.

crossref1.eps I cover using shutter speed to control the exposure and what effect that can have on your images in Chapter 3.

Aperture priority mode

Aperture priority mode lets you select the aperture, and then the camera picks the shutter speed to achieve proper exposure. Controlling the opening or aperture in the lens that allows light to reach the sensor also controls the depth of field (which is the area that is in front of and behind what you are focusing on that is in acceptable focus). Because this mode involves setting the size of the opening in the lens, and by doing so lets the camera decide on the best shutter speed, it is a great way to get blurry images. When used at the right time it offers a great deal of creative control over your images.

crossref1.eps Depth of field is covered in much greater detail in Chapter 4.

1.7 I used a shallow depth of field in this image to blur the background so the flowers stayed the center of attention. Taken at 1/320 second, f/4.0, ISO 200.

Manual mode

In Manual mode, you get to set the shutter speed and aperture. This mode gives you the most control over your exposures. Being able to set both the shutter speed and aperture allows you to determine what the sensor captures, but it also is the easiest way to underexpose or overexpose your image because the camera won’t do anything to help you. The camera shows you what the built-in light meter believes is the correct exposure and whether your settings will produce an image that is lighter or darker than the camera’s choice.

1.8 This concert photo of musician Jackie Greene was taken in Manual mode to counteract the rapidly changing lights. Taken at 1/200 second, f/2.8, ISO 1600.

Metering Light

Metering light is simply measuring the brightness of the scene you want to capture. The best way to determine the amount of light needed to create a proper exposure is to use a light meter. All light meters work on the same basic principle: They convert the amount of light in a scene into a measurable form and then translate that information into a form useful to a photographer. The light meter uses that value to determine the shutter speed and aperture settings, given the ISO.

In the past, light meters were an extra piece of equipment that photographers used to get the correct exposure settings to set the camera manually, but all the DSLR and point-and-shoot cameras today have built-in light meters. The built-in light meter measures the light coming through the lens, the same light that reaches the sensor when the shutter is moved out of the way. This type of metering is called TTL or Through the Lens metering and can constantly adjust to the changes in the exposure. The entire process now happens in the camera.

1.9 Sekonic light meters used to meter light

Sometimes it pays to have a separate light meter, and shooting in the studio is one of those times. Using strobe lights that fire only when triggered by the camera doesn’t allow the built-in light meter to get an accurate measurement of the light. If you use a light meter that can trigger your studio strobes, you can get an accurate reading and input those settings into your camera using Manual mode. I use an older Sekonic L-358 along with a Pocket Wizard transmitter to trigger my studio strobes.

The light meter that is built into your camera is a reflected light meter; that is, it reads the light that is reflected from the subject back at the camera. The light meter then tries to determine the overall brightness of the scene, using the assumption that the typical subject reflects a middle or 18 percent gray. This averaging works for lots of situations but not all. When you take the darks, lights, and middle tones of an average scene, the 18 percent gray works well. Scenes that have lots of very light colors can cause the camera’s light meter to underexpose, causing everything to look too dark, while very dark scenes can cause the light meter to overexpose, making the image too light.

Each camera manufacturer has a slightly different method of metering light, and I believe that each camera model has a slightly different way of metering light. But that doesn’t really make any difference as long as you know what the camera is doing, which lets you adjust the settings to get the exposure you want for your photograph.

Camera Metering Modes

Camera manufacturers spend lots of time and money developing new and improved light meters for their cameras because an accurate light meter creates properly exposed images. Your camera has a very sophisticated light meter built right in, letting you set the camera on the fully automatic mode, so you can expect to get pretty good photos most of the time. The problem is that you won’t get great photos, and in some situations you won’t get anything like the image you wanted.

One of the first steps in taking control of your photos is to understand how the different metering modes in your camera work. You usually can choose from three methods of metering light: spot, center-weighted, and scene. It’s like having three separate light meters built into your camera, and knowing how each one works will enable you to get the best results.

Spot metering

Spot metering is when the built-in light meter uses only a small spot in the center of the scene to calculate exposure. Spot metering uses the smallest amount of light to determine the exposure. Because the spot meter measures light only in a very small part of the overall scene, it is less affected by the surrounding brightness when the scene has lots of very dark or very light colors. Some cameras use the area around the selected focus point as the area to take the light reading from. This lets you move the spot metering circle to whatever focus point is being used, ensuring that you are measuring the light of the most crucial element in your image. This is very useful when shooting a subject against a very bright or very dark background because those areas are not used in calculating the exposure.

1.10 I used spot metering when taking this photo of Nicole. I didn’t want the large expanse of bright sky to play any part in the metering. I photographed this at 1/125 second, f/5.6, ISO 200.

Center-weighted metering

Center-weighted metering uses light readings from the center spot-metering area and from the surrounding area, but it gives more importance to the center spot area. Some camera manufacturers allow the size of the center area to be changed to suit your needs. Center-weighted metering usually does not meter the top and bottom of the frame so that things like a bright ribbon of sky at the top or a very dark foreground are not taken into account when determining the exposure.

Center-weighted metering is useful when your subject is in the center of your frame and you are not very concerned with the outer edges.

Scene metering

Scene metering has different names depending on the camera manufacturer and camera model. For example, Nikon calls this Matrix metering, Sony calls it Multi-segment metering, and Canon calls it Evaluative metering, but these metering modes all do the same thing — try to be smarter light meters to get more accurate reading of the brightness in a scene. This type of metering has come a very long way in a relatively short period of time. The idea itself is quite simple, let the built-in light meter look at the scene and work out the best exposure by determining what you are photographing. This has become easier to do as computer processing and memory have become faster, smaller, and cheaper.

For example, the newest Nikon metering system uses a 1,005-segment metering sensor to read the light in the scene; it compares the data to a built-in database of images and tries to predict what you are photographing. This happens so fast that you never even know it’s going on. New technology starting to appear in cameras recognizes the subjects in your scene and adjusts the exposure accordingly. This is most noticeable in the new face recognition built into newer cameras. These smart scene-metering modes are getting better and better, causing fewer badly exposed images than ever before.

This mode generally does a fantastic job in just about all situations and is the mode I set all my cameras to by default. As new cameras are introduced, the camera manufacturers keep improving the metering capabilities of their cameras. But even with the best computers and biggest databases of images, yours is the only brain that really knows what is in your scene and what the subject really is.

1.11 The Sony A700 multi-metering zones is shown as an overlay. Sony multi-metering divides the scene into 40 different areas, 39 grouped in the center with the last one taking up the border of the image.