This is the answer to our question in the last section about
what happens when there is not enough light and the camera
has maxed out the Maximum sensitivity level. Even though
you’ve selected a Minimum shutter speed, the camera will go
below the Minimum shutter speed when the Maximum
sensitivity ISO number has been reached and the light is still
too low for a good exposure.
In other words, in P – Programmed auto or A – Aperture
priority auto exposure modes, if you get into low light and try
to take pictures, the camera will try to keep the ISO
sensitivity as low as possible until the shutter speed drops to
your selected Minimum shutter speed. Once it hits the
selected Minimum shutter speed value—like the 1/30S shown
in Figure 3-52, image 3—the ISO sensitivity will begin to rise
up to your selected Maximum sensitivity value, like the ISO
400 shown in Figure 3-52, image 2.
Once the camera hits the Maximum sensitivity value, if there
still isn’t enough light for a good exposure, it won’t keep
raising the ISO sensitivity. Instead, the camera will now go
below your selected Minimum shutter speed, dropping below
the 1/30s shown in Figure 3-52, image 3. Be careful, because
if the light gets that low, your camera can go all the way
down to a shutter speed of 30 seconds to get a good exposure.
You better be on a tripod and have a static subject with
shutter speeds that low.
Look at the Minimum shutter speed value as the lowest “safe”
speed, after which you’ll put your camera on a tripod. Most
people can handhold a camera down to about 1/60s if they are
careful, and maybe 1/30s if they’re extra careful and brace
themselves. Below that, it’s blur city for your images. It’s
even worse with telephoto lenses. Camera movement is
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