Microsoft 702 manual Water glass analogy, Understanding automatic exposure

Chapter 2: Making the Most of Your Camera

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The water glass analogy

When taking a photograph, your goal is to achieve a perfect exposure. To create the right exposure, you need to understand the relationship between the three exposure factors: aperture, shutter speed, and ISO rating. Achieving perfect exposure can be compared to filling a glass completely without spilling any of the water. For a perfect exposure, the glass should become completely full with no water spilling over. In this analogy, the tap symbolizes the aperture: the wider the tap is open, the faster the glass fills up. The time that the tap is open represents the shutter speed: leaving it open longer lets more water into the glass. To fill the glass to exactly the right level, the rate of flow must be set according to the time the tap is open.

The third factor, ISO rating, can be equated to the size of the water glass. A smaller glass, representing a faster ISO rating, fills up more quickly than a larger glass, representing a slow ISO.

Understanding automatic exposure

As a photographer, you will come across a wide range of lighting conditions, and each condition requires that you adjust your camera to different exposure settings. For example, shooting a photo on a beach on a sunny day calls for different exposure settings than shooting on the same beach on a cloudy day.

For many conditions, the camera’s automatic exposure setting gives you good or even excellent results. But for some situations, the automatic exposure does not perform as well.

Automatic exposure assumes that the scene you are photographing has a few bright spots, many midtones, and a few dark areas. As the camera’s meter reads the available light in your scene, it averages the light in the bright, middle, and dark areas, and then calculates the exposure necessary to bring the average level to a tone of medium brightness called middle gray.

Microsoft Picture It! Companion Guide