Exposure: I. introduction

I’ve heard a number of people who have recently purchased digital SLRs say that they have no idea what the manual and semi-automatic settings do. They leave the camera in automatic mode and take their photos. There’s nothing wrong with this, but understanding what the settings do and how they affect the photo being taken can allow you to understand the camera’s limitations and work around them to get better photographs.

A lot of people think this is too much work and would prefer to avoid it. That’s fine. My concern is that the nuts and bolts of photography can appear so complicated that those who wouldn’t mind learning some fairly simple concepts are being scared off. This is a shame because these concepts can really increase your creative freedom as well as reducing the chance that the limitations of the camera’s automatic settings will leave you with a poorly exposed photograph. Its these people for which I’ve conceived this series of posts. I don’t know if they’ll successfully impart the concepts I hope to describe, but all we can do is try.

Light

Photographs are made with light, and photography is the act of capturing light. What you photograph is an entirely separate issue, but the mechanics behind getting a properly exposed image involve how much light you allow to enter the camera.

There are only three variables you can adjust to vary the amount of light entering the camera. The specific settings used are called exposure settings, probably because they together vary how the film/sensor is exposed to light.

Proper exposure makes the photograph look just like the original scene. Under-exposure leaves the photograph looking too dark. Over-exposure makes the photograph look too light.

Reciprocity

How these three exposure variables relate to each other is a key concept known as reciprocity. The Wikipedia defines the word in photographic context as:

the inverse relationship between the intensity and duration of light that determines exposure of light-sensitive material.

Let’s leave photography behind for a moment. Imagine getting $1 from 100 people and then getting $2 from 50 people. Comparing these two situations, you’re getting a different amount of money from a different number of people, but you’re left with the same result in both cases: $100. Getting the same result requires the doubling of one variable if the other is halved. This is the essence of reciprocity.

See? I told you it wasn’t rocket science.

While the Wikipedia definition reciprocity gets the basic idea across, it ignores how the ‘light-sensitive material’ is also part of the relationship. I’m guessing this definition came from the pre-digital days when film was the light-sensitive material we used, and you were stuck with the same ‘film speed’ until you finished the roll. With digital cameras, you can change the sensitivity of the sensor between photographs, bringing it fully into consideration.

Exposure triangle

So we’ve got three variables. The intensity of light, the duration of light, and the sensitivity of the film/sensor to the light. These are sometimes referred to as the exposure triangle.

Photographic reciprocity means that there is no single proper exposure. All three variables can be changed to create an entirely different exposure that’s also correct, as long as reciprocity is used to calculate how the three exposure variables are changed in relation to each other.

If you allow twice the light for half the time, the exposure is unchanged despite two of the variables being entirely different. This may sound vague, but it’s only because we haven’t yet talked about the details of each exposure variable and the units used to measure them.

In the next three Exposure posts I’ll describe the exposure variables in detail.

Image courtesy of PostSecret