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Reciprocity and the Exposure Triangle

In the last post, I mentioned that a cameras automatic settings will usually give you a good exposure, but they may not give you the exposure you want. To understand why, it’s helpful to know how cameras determine exposure and how camera settings influence the exposure.

Camera Metering Systems

Cameras use reflective light meters, meaning they measure the amount of light bouncing off the scene in order to determine exposure. In the early days of photography, photographers discovered that most of the scenes we shoot reflect about 18% of the light falling on them. In the case of a black and white image, 18% reflectance comes out to a middle gray tone. This is why most camera metering systems are calibrated to recommend settings that will let in enough light to properly expose a subject with 18% reflectance. With a camera in Manual mode, the photographer makes adjustments to the exposure settings until the exposure meter indicates a proper exposure. In Auto mode, the camera uses an algorithm to choose a combination of settings that will achieve proper exposure.

Exposure Metering

The exposure meter (lower right corner of the LCD display) indicates when the scene is properly exposed. In the image on the top-left, not enough light is reaching the sensor and the meter indicates that the image is underexposed. In the bottom-left image, there is too much light and the image is overexposed. The image on the right properly exposed.

The Exposure Triangle

In order to achieve that 18% middle tone there are two basic approaches. The first approach is to change the amount of light falling onto the sensor. The more light the sensor receives the brighter the image will be. The less light the sensor receives, the darker the image. We can change the amount of light entering the camera by making the lens opening, known as the aperture, larger or smaller. We can also change the exposure by adjusting the amount of time that the sensor is exposed to light. This adjustment is known as the shutter speed. The other way to adjust an exposure is by adjusting the ISO setting. You can think of the ISO as a signal amplifier. More on this later. Aperture, shutter speed, and ISO make up what's known as the Exposure Triangle. The photographer can use the three elements of the Exposure Triangle to adjust the brightness and the visual characteristics of their photographs.

Aperture

The aperture is the adjustable opening located inside the lens, through which light enters the camera. This opening is made up of a set of overlapping leaves that can be adjusted in and out to make the aperture bigger or smaller. The aperture setting is designated in f/stop numbers, ranging from very large (F/1.8 or F/2) to very small (F/22). When it comes to aperture sizes, the numbering system seems counterintuitive, with larger openings having smaller numbers. This is because the f/stop number is a ratio of the focal length divided by the aperture diameter. As an example, below is a picture of a 50mm lens set to f/2. At this setting, it has an aperture measuring 25mm across. The lens at f/22 has an aperture 2.3mm across.

Focal length ÷ f/number = Aperture Size

50mm lens at f/22. The aperture is 2.3mm across. (50 ÷ 22=2.3)

50mm lens at f/2. The aperture is 25mm across. (50 ÷ 2=25)

The aperture setting of your lens is one of the most important settings for forensic work. This is because aperture affects not only how much light enters the camera, it also controls the depth of field of the image. Depth of field refers to how much of the photo will be sharp and in focus. With a small aperture, like f/22, the image will be sharply focused from foreground to background and everywhere in between. Taking a photo “wide open” ( f/1.8 or f/2) will result in a very blurred background. Photographers can use this blurred background effect, known as bokeh, to make the subject stand out from the background. You'll see this often in portraiture. In almost every case, we as forensic photographers, want to maximize the depth of field by using a smaller aperture. This ensures that we will have clear details throughout our images.

Depth of Field Examples

Toy fire engine shot at f/2. The bokeh causes the subject to stand out but all background detail is lost.

The same shot at f/22. Shooting with a small aperture gives us the most detailed image.

Shutter Speed

The shutter is a mechanical door, positioned behind the lens that opens to allow light from the scene to fall onto the sensor. The longer the shutter is open, the more light reaches the sensor. Shutter speeds are measured in fractions of a second, ie., 1/60 , 1/250, 1/500, etc. Shutter speed is often used by photographers for artistic effect. Keeping the shutter open for a long period can be used to blur the motion of objects, make moving water look smooth, or make stars appear to streak across the sky. A fast shutter speed will freeze motion, leaving a crisp image of the subject in the peak of action. As forensic photographers, our subjects rarely move and we’re not concerned with artistic effects. Our main concern when it comes to shutter speed is camera shake. If the shutter is open too long, the photographer may not be able to hold the camera steady enough to prevent motion blur due to camera movement. A good rule of thumb to ensure a fast enough shutter is to use a shutter speed that is the reciprocal of your focal length. For example, if you’re shooting with a 50mm lens you should shoot for a shutter speed at least 1/50 second to avoid motion blur from camera shake.

ISO

The surface of a camera’s sensor is covered with millions of light sensitive photosites. The photosites, also called pixels, collect the light and convert it into an electrical charge. The electric charge carries color and brightness information from each photosite to the camera's central processing unit (CPU). The CPU uses this information to construct the digital image. We call the light information coming from the sensor the signal. Within the light information there is a certain amount of digital noise. The noise is caused by random variations in the electrical charges coming from the sensor. Noise can appear in the image as overly bright or miscolored pixels, as graininess, or banding. If there is plenty of light in a scene the signal will be strong and noise won’t be noticeable in the resulting image.

In cases where there is not enough light for a good exposure, we can compensate by amplifying the signal coming from the sensor. We do this by raising the ISO setting. ISO levels are designated by whole numbers. Levels typically begin at a base ISO somewhere around 100-200 and can go up to ISO 64,000, or even higher, depending on the camera. The ability to boost the light signal is a great help to fire investigators, who often find themselves in poorly lit scenes. Increasing the ISO comes with a tradeoff, however. As we boost the signal, we also boost the noise. The poorer the initial signal the more noise we will see in our images.

Reciprocity

To achieve a proper exposure for a given scene a camera in auto mode adjusts the three parts of the Exposure Triangle until the combination of settings result in a middle tone for the scene. Each component, ISO, aperture, and shutter speed, can be adjusted in increments known as stops. Let's say for example that we meter a scene and the camera decides on an ISO setting of 200, an aperture of f/4, and a shutter speed of 1/500. But let’s say that for whatever reason, we want to stop down the aperture one stop to f/5.6. This one stop decrease in aperture cuts the amount of light entering the camera by one half. In order to maintain a proper exposure we need to slow the shutter speed by one stop to 1/250. The one stop decrease in the shutter speed doubles the amount of light reaching the sensor, balancing the exposure. Likewise, if we wanted to increase the aperture size one stop, to f/2.8, doubling the amount of light, we need to double the shutter speed to 1/1000 to maintain proper exposure. We can also change the exposure by increasing or decreasing the ISO. If we change the ISO by one stop, we would then need to offset the change in exposure by changing the shutter speed, the aperture, or both. For each scene there are hundreds of reciprocal settings that will achieve proper exposure. This principle is known as Reciprocity.

Why Does this Matter?

If any number of settings can achieve a proper exposure, why should the photographer care which combination they use? Why not let the camera’s Auto mode choose the exposure settings and be done with it? After all, properly exposed is properly exposed... Right?

Not really. As I mentioned previously in this post, the three elements of the Exposure Triangle affect more than just the brightness of the image. The combination of settings you use to capture an image can drastically affect the look, quality, and therefore the usability of that image. What are the qualities that we are looking for in a forensic photo? What camera settings can we use to achieve the result we want? We’ll address those issues in future posts!

Shutter Speed Examples

Frozen Mid-Jump. The fast shutter speed (1/2500 second) freezes the motion.

Merry-go- Round at 1/30 Second. The movement of the merry-go-round and the long shutter speed give blur to the background, creating a sense of motion.

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