Mastering Exposure: The Exposure Dial

Exposure Modes
Correct exposure is one of the essential ingredients of a successful photograph. If too much light reaches the sensor all the brightest parts of the scene may burn out into a stark white, with no visible details. This is called over-exposure. Conversely, an inadequate amount of light (under-exposure) will turn dark areas of the scene, such as shadows,
into a sea of blackness.

Determining the correct exposure is also about recording a scene or subject in the way you want it to appear in the final image (which may or may not be the way it appeared to the eye at the time). You may decide that you wnat a dark, moody effect, or a clean, bright one. Deciding what the important element in a scene is and setting an exposure that will record that focal point correctly will put you in creative control of your images.

Exposure is controlled by the camera in three ways: by the sensitivity of the sensor (its ISO); by the size of the hole in the lens through which light passes (the aperture); and by the duration for which the sensor is being exposed to light (the shutter speed).

There are numerous combinations of these that will provide a technically correct exposure, but perhaps only one that will record the scene in the way you want it to be seen in your photograph. This is why a knowledge and understanding of the relationships between all these factors is so important.

LANDSCAPE MODE
Here the camera sets a small aperture, and other parameters that lend themselves to good landscapes.

PORTRAIT MODE
The camera chooses parameters suitable for good portraits, incl. wide apertures.

AUTO MODE
Beginners will want to head here. The camera does everything and trusts the user with nothing.

PROGRAM MODE
The next step up from Auto, you can override some of the camera’s choices.

APERTURE PRIORITY
You choose the aperture you want, and the camera picks the right shutter speed to go with it.

SHUTTER PRIORITY
You choose the shutter speed, the camera matches it with the appropriate aperture.

MANUAL MODE
In this mode you control both shutter speed and aperture yourself, so you can set your own exposure.

SCENE MODE
Yes, Landscape, Portrait, etc are scene modes too, and on some cameras they’d be in here too. On this Olympus E500 there are modes for lots more special subjects.

LOWLIGHT MODE
An ISO boost, and an exposure that balances the flash with the ambient light.

ACTION MODE
Fast shutter speeds, continuous drive and servo focusing will help you catch the action.

MACRO MODE
For close-ups head here. A small aperture is among the camera’s priorities in this mode

 Above: Shot on a Canon EOS 350D, these Masai tribesmen are lit entirely by the campfire and some fill-in from the small built-in flash. A high ISO of 1600 was needed to record the campfire and night sky

ISO
Generally, the ISO is set as low as possible for the conditions, since unlike the other controls, increasing it has a detrimental effect on image quality. This is because raising the ISO is, in effect, boosting the signal to the sensor, and just as raising the recording level on a tape recorder will increase the background hiss, so raising the ISO will increase the appearance of ‘noise’, which most commonly appears as specks or grains (often multi-coloured) over the image. In bright conditions, the ISO is generally left at 100 (usually its lowest setting) and is raised to 400 when there isn’t enough light to get a sharp, hand-held exposure. There will be more on this later.