Exposure Series — The Basics

Our cameras see light differently than our eyes. Where we simultaneously see detail in the deepest shadows and the brightest highlights, our camera can only see one or the other. That is, in a high contrast scene, the camera is not capable of capturing the full range of tones that our eyes can see. Sometimes this is a disadvantage but at other times it can be advantageous because it allows us to tell a story about our subject by deliberately overexposing or underexposing our images. Yet to be creative, we need to when and how to tell the camera to produce what we see in our mind.

To the camera meter, nature appears as a series of tones, from black to white. The camera’s light meter tries to make every scene a medium-toned (techincally 18%) gray, similar to the tonal value of a grassy lawn on a sunny day. This works well for most pictures. Yet there are many scenes in nature that are not medium-toned. Your camera, for instance, will take a scene of a polar bear in the snow and make it look like a gray bear on a fine gravel bar. Similarly, your camera will attempt to make a medium gray scene of a black bear on a dark rock outcrop. We can solve this problem quite easily with exposure compensation, which is a way of telling your camera that you are smarter than it is. Before we can use exposure compensation, though, we must understand a bit about the factors that control the camera’s exposure.

For the mathematically- or statistically-minded among you, consider an equation with three independent variables, two of which will actually be codependent at any time, that determine the value of a dependent variable, whose value is actually known in advance. That dependent variable is the total amount of light needed to expose a given scene, be it a sweeping landscape in Yosemite National Park, a tiny poison frog in the Costa Rican rainforest, or a picture of your favorite cousin at a backyard barbecue. There may be different tones within the scene, but you always need a certain and exact amount of light to expose the scene properly (note that the definition of a proper exposure depends on you, a concept which we’ll go into below). The independent variables are your camera aperture (aka f-stop), shutter speed, and ISO value (formerly film speed or ASA). The camera aperture is the size of the lens hole through which light passes. A smaller hole lets in less light, and a larger hole lets in more. The shutter speed is the length of time that the curtains open to leave your camera’s sensor exposed to light (formerly film). It is only during this time that light strikes the sensor. And the ISO value is the sensitivity of the sensor to light. A lower ISO value will absorb less light during a given amount of time while a higher ISO will absorb more. Among these three variables, one is constant, one is manipulated by you, and another is inversely related to your manipulation.

TABLE WITH THREE VARIABLE — LESS LIGHT/MORE LIGHT

Now let’s try a metaphor to help clear things up a bit. Say we have to fill a bucket with water. The bucket holds one gallon of water. Think of this as the amount of light needed to expose our scene. We have a hose that has a one inch diameter. Think of this as your aperture. Let’s say that with a one-inch hose, it takes us one minute to fill the bucket with one-gallon of water. This time, one minute, represents our shutter speed. Now imagine that we change to a hose with a two inch diameter. How long will it take to fill the bucket with one gallon of water? Since twice as much water now comes out of the hose in the same amount of time, we only have to leave the hose open half as long to get our one gallon of water. With our two inch hose, it now takes only 30 seconds to fill the bucket.

Now pretend that we can choose any hose diameter we want but we need to fill the bucket more quickly. Say we need to fill the bucket in just 15 seconds. We needed one minute to fill the bucket with a one inch hose and 30 seconds to fill the bucket with a two inch hose. So, we will need a four inch hose to fill the bucket in 15 seconds, half the time it took with the two inch hose. If for some reason, we wanted to spend two minutes filling the bucket, we would need to use a ½ inch hose.

Now let’s pretend that instead of a bucket we are putting water onto a sponge. Different sponges might absorb water at different rates. This is similar to our ISO value, whereby our camera’s digital sensor can be set to receive light at different rates. So, what happens if we have two sponges. Sponge A can absorb water twice as fast as Sponge B. Let’s say that Sponge A can absorb a gallon of water at the same rate as our bucket can be filled, in one minute with our one inch hose. But Sponge B can absorb water twice as fast. So, we can now fill our sponge to one gallon capacity with our one inch hose in half the time, 30 seconds. Going from Sponge A to Sponge B is the same as changing our ISO from 100 to 200.

There is one more factor that we need to consider to make our metaphor complete. This is the flow rate of the water. Great water pressure will mean that everything takes less time while a trickle obviously will mean more time. Think of the flow rate as the brightness of your scene. With lots of water pressure, you can have a slow-absorbing sponge and very small diameter hose but still get the job done pretty quickly. But with a slight trickle, you’ll need a lot of time to finish your task, even with a large hose and fast-absorbing sponge. Similarly, a photograph taken at noon on a bright sunny day will require a much faster shutter speed (even with a small aperture and a low ISO value) than will the same exact scene photographed at sunset or even by moonlight (even with a large aperture and a high ISO value).

So, we now understand that there are ways that we can affect how we obtain the total amount of light necessary for our photograph. But remember that the camera determines the total amount of light by trying to make everything medium-toned. With exposure compensation, we can tell our camera that we want to adjust the total amount of necessary light. For instance, if we are photographing a polar bear in the snow, we can tell the camera that we want it to let in twice as much light as it thinks we need. Or for our black bear, we tell the camera to let in only half as much light. This way our polar bear will be white and our black bear black.

Exposure compensation is a function easily accessible on digital SLRs and usually a bit more hidden on advanced point and shoot digital cameras. In either case, it consists of a scale that has the numbers -2, -1, 0, +1, +2 or decimal values (if you shoot Nikon) such as -0.7 or +1.3. Leaving the function set at zero tells the camera to make the scene medium gray, the camera’s default behavior. Setting it to +1 tells the camera to let in twice as much light. Setting it to +2 will let in four times as much light than the camera would have allowed on its own. Similarly, -1 lets in half as much light as 0 and -2 four times less light than 0. Turn your flash off and try taking a series of pictures of a white wall with your camera set at different exposure compensation values. You’ll quickly see the effects on exposure. Exposure compensation is available in aperture priority (AV), program (P), and shutter priority (TV) on most cameras. If you work in manual mode, you can set combinations of aperture, shutter speed, and ISO values that effectively underexpose or overexpose the scene.

Text and images by Greg Basco. Submit a comment or contact me with any questions.

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