Exposure for Beginners.

0. TL/DR

Use P-Mode (or Auto Mode) with Auto ISO. Come back and read if and when that's no longer good enough.

1. What is exposure and why do I care?

Exposure is the amount of light per area that falls on the sensor, which is related to, but different from, how bright your photo appears on the screen or in print. The reason exposure matters is because the greater the exposure, the greater the amount of light that falls on the sensor, resulting in a less noisy photo.

Exposure also matters because if the amount of light per area falling on the sensor is more than the sensor (or portions thereof) can record, those portions of the photo will be blown (all white devoid of detail) or exhibit strange color artifacts if one color channel oversaturates (is blown) before the other other color channels.

2. How do I get more exposure?

There are three ways to increase the exposure:

• Add in more light (e.g. use flash)
• Use a wider aperture (lower f-ratio), keeping in mind that this will result in a more shallow DOF (depth-of-field -- the range from the focal plane where objects are in focus).
• Use a longer exposure time (slower shutter speed), keeping in mind that this may result in greater motion blur and/or camera shake.

3. What about the ISO setting on the camera?

The ISO setting indirectly affects the exposure that the camera selects in all AE (auto exposure) modes (i.e. modes other than M mode, which will not be discussed in this post) based on the scene luminance and the metering mode you are using.

For example, if f/4 ISO 100 is selected in Av mode without flash, the camera might choose 1/200 as the exposure time for a particular scene and metering mode. If the ISO setting is raised two stops to 400 leaving the relative aperture at f/4, the camera will select and exposure time of 1/800 -- a two stop faster exposure time concomitant with a two stop higher ISO setting. This will result in 1/4 as much light falling on the sensor, and thus your photo will be twice as noisy.

Similar changes to the exposure settings (aperture, exposure time, and flash power) will happen in other AE modes where every doubling of the ISO setting on the camera will result in half as much light falling on the sensor (as a result of changes the camera makes to the relative aperture, exposure time, and/or flash power), and thus a photo that is 41% more noisy.

So, in a very real sense, when you set the ISO on the camera in any mode but M mode, you are telling the camera that you want (or will settle for) a particular noise level, where the photo becomes 41% more noisy for each stop increase (doubling) of the ISO setting:

• ISO 200 = 41% more noisy than ISO 100.
• ISO 400 = 41% more noisy than ISO 200 = twice as noisy as ISO 100.
• ISO 800 = 41% more noisy than ISO 400 = twice as noisy as ISO 200.
• .
• .
• .

One might ask, then, why not always choose base ISO (the lowest ISO setting the camera offers)? The answer is simple: it will result in the camera choosing exposure settings (flash power, relative aperture, and/or exposure time) that may adversely affect the photo more than the greater noise adversely affects the photo.