Nikon D5100 Experience Manual page 38

Nikon D5100 Experience
Shutter Speed is the amount of time that the shutter, which is the curtain in front of the sensor, is
open. If it is open a long time, a lot of light hits the sensor and if it is open a short time less light
hits the sensor. You can control the shutter speed in Shutter-Priority Auto mode (S), and in
Manual mode (M).
ISO is the sensitivity of the sensor, its ability to grab the light that lands on it. A low ISO setting
is not very sensitive to light and does not grab much (relatively), and a high ISO is very sensitive
to light and grabs all it can. ISO is considered the digital equivalent of film speed.
As you can see, they all relate to the light hitting the sensor. For a proper exposure, one that isn't
too light or too dark, the right amount of light needs to hit the sensor and it needs to be at the
right sensitivity to accept that light. So, for example, a small opening (aperture) will need more
time (shutter speed) to let in the necessary amount of light based on the current sensitivity of the
sensor (ISO). All three work together to control and accept the amount of light that hits the
sensor. In Auto Mode, the camera chooses what it thinks is the best combination of these three
settings and creates a proper exposure. But it doesn't know your image intentions and thus it
may not select the aperture and shutter speed settings that you desire.
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