What is “Exposure” in Photography?

In photography, “exposure” is a key concept.

The “exposure” is the amount of light that the photographer, or the camera in automatic modes, allows to fall on the sensor, at the moment of image capture.

A correct exposure , i.e. enough light hits the sensor, but not too much, is crucial to the creation of a quality image.

Exposure Control

The photographer may act upon the exposure via the controls  built into the camera’s body. The 3 available controls are:

1. Exposure time (e.g. how long is the shutter open) (see this article for details)
2. Lens aperture (e.g. how wide is the len’s opening?) (see this article for details)
3. The sensor’s sensitivity to light, called the ISO setting (see this article for details)

Click to enlarge: Illustration of three factors affecting the exposure. Image © Charles Martel 2015

The photographer can control the exposure settings completely manually. Or, he can choose to give more or less control to the various automation modes built into the camera. We will discuss the various modes of exposure automation e.g. P, A, S, M, etc., in another article.

Depending on the choices made by the photographer (or the camera), the end result will be one of the three following scenarios:

1. Correct exposure
2. Overexposure
3. Underexposure

Correct exposure

The right amount of light hit the sensor and the image is well exposed. We see enough details in both the dark and bright areas of the image. No area is too dark or too light.

Following is a sample correct exposure:

Click to enlarge: Correct exposure, image © 2015 Charles Martel

Overexposure

Overexposure is the result of too much light hitting the sensor. The image is too bright and some areas are “burnt out”, i.e. completely white, with no detail.

Here is an example of extreme overexposure:

Click to enlarge: Extreme overexposure. Image © Charles Martel 2015

Almost everything is burnt-out, i.e. completely white. Such an image is irretrievable!

Underexposure

Too little light has been captured and the image is too dark. We see a bit of detail in the highlight (bright)areas but nothing in the shadows.

Click to enlarge: Extreme Underexposure. Image © Charles Martel 2015

However, unlike the overexposed image, it could be probably be at least partially recovered in software, in programs such as Adobe Lightroom (article coming up on this). However, the resulting image will likely contain a lot of noise (article coming up!).

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