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Chapter 12: Taking Great Pictures
Microsoft Digital Image Standard User’s Manual
The water glass analogy
When taking a picture, 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, taking a photo on a beach on a sunny day calls for differ
-
ent exposure settings you would use 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
.
Automatic exposure does not work well if your scene is dominated by large
sections of very light or very dark colors. A bright field of snow, for example,
has so much bright light that the automatic exposure lowers the brightness until
the snow looks gray. To work around this shortcoming, you can use exposure
compensation
.